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The ground state and excited d-hole states of CuAu Gregory A. Bishea, Jacqueline C. Pinegar, and Michael D. Morse Department of Chemistry, University of Utah, Salt Lake City, Utah 84112

(Received 25 March 1991; accepted 12 July 1991)

Resonant two-photon ionization spectroscopy has been applied to jet-cooled diatomic CuAu. Eight band systems have been observed, rotationally resolved, and analyzed. The ground state is X ‘Z + in symmetry, deriving from the 3d ,!!\ 5d h”, d molecular configuration. Its bond length has been determined as r, = 2.3302 f 0.0006 8, ( lo error limits). The first excited state is the LI 32 + state, which derives from the 3d ,!!!! 5d h”, o’dc’ molecular configuration. This state possesses a nominal bond order of zero, but is nevertheless bound by Do = 1157 + 150 cm - ’ and has a bond length of 2.428 f 0.028 A. It is observed through the a 38c-X ‘B+ transition which is very weak, with f=: 5 X 10 - ‘. Seven higher energy band systems are also observed, most of which correlate asymptotically to d-hole states described by the Cu 3d “4s’, *S + Au 5d 9Q, *&2, Cu 3d 94?, *D,,, + Au 5d “6s’, ‘S’, and Cu 3d 94?, *D3,* + Au 5d “69, *S separated atom limits. The D 0 + excited state, however, displays a very large oscillator strength in transitions with the ground state CfzO. 1 1 ), and it is argued that this state corresponds to the ion pair state Cu + Au -. Future experiments are planned to test this hypothesis by measuring the permanent electric dipole moment of this state.

I. INTRODUCTlON

As we attempt to understand the chemical bonding between transition metal atoms, a major unresolved question concerns the role of the d orbitals in the bonding of these species. To what extent do the d orbitals take part in the chemical bond, and how does the extent of their participation vary as one moves through the Periodic Table? Al- though such questions are of paramount importance if we are to understand the chemical bonding between transition metal atoms, as of yet we have rather little detailed experimental information which may be used to answer this question.

To evaluate the magnitude of d-orbital participation in the chemical bonding of the transition metal dimers, a standard of comparison is needed. The coinage metal diatomics Cu2, CuAg, CuAu, Ag,, AgAu, and Au, provide such a benchmark since the ground electronic state of all of these atoms is d l”sl, “S, and the filled d subshell may be considered to be inert for all practical purposes. As a result, the ground states of these molecules are all d i”d pa, ‘B;t,, , with a single cr bond. The d subshells are filled and are not available for chemical bonding, making the ground states of these molecules simple models for transition metals without any d-orbital contributions to the bond. By comparing NiPt’ to its filled d-subshell analog CuAu, for example, one may ascer- tain the degree to which the partially filled d subshells of Ni and Pt contribute to the chemical bonding. With the aim of firmly establishing these standards of comparison we have undertaken a program to investigate the coinage metal diatomics. In a previous paper we have reported a study of cu, ,* while the paper preceding the present work in this Journal describes a study of the mixed dimer CuAg.3 It also provides a fairly comprehensive set of references of previous spectroscopic work on the coinage metal diatomics,3 so this list will not be repeated here. In the present work we report the results of spectroscopic investigations of CuAu, while

results on AgAu and Au, are given in the next paper in this Joumal.4

Previous experimental and theoretical investigations of diatomic CuAu have been quite limited. The bond strength has been measured by high temperature mass spectrometric studies of the dimerization equilibrium by Ackerman, Staf- ford, and Drowart,’ and subsequently reinvestigated by Kingcade, Choudary, and Gingerich.6 Some early spectroscopic studies of thermally activated emission systems in a King furnace have been reported by Ruamps,7 who has identified four emission band systems associated with the CuAu molecule. Theoretical work on CuAu appears to consist of only two investigations by the NASA Ames group, which were only concerned with obtaining information about the ground electronic states of CuAu, CuAu -, and CuAu + , along with the other coinage metal dimers and trimers.8*9

II. EXPERIMENT

The spectroscopy of CuAu was investigated using the apparatus described in the previous paper in this Journal, where results on CuAg are reported.3 A metal target consisting of an equimolar alloy of Cu, Ag, and Au was prepared by melting the weighed metals in an electric arc. This triple mixture was prepared so that the same sample could be used in studies of CuAu, AgAu,4 Cu, Au, and CuAgAu. A sepa- rate sample, consisting of a 1: 1 mix of copper and silver and described in the previous paper,3 was used for studies of CuAg3 and Cu, Ag. After cooling, the yellow CuAgAu alloy was pressed flat and polished to give a disk-shaped sample approximately 2 mm in thickness and 2.5 cm in diameter. This was suitable for pulsed laser vaporization using a rotat- ing disk mount similar to that described by O’Brien et al.” Pulsed laser vaporization of the sample was achieved using the frequency-doubled output from a Q-switched Nd:YAG laser (532 nm, 15 mJ/pulse, focused to a diameter of approx-

5630 J. Chem. Phys. 95 (8), 15 October 1991 0021-9606/91/205630-16!$03.00 0 1991 American Institute of Physics Downloaded 02 Apr 2001 to 128.110.196.147. Redistribution subject to AIP copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp

Bishea, Pinegar, and Morse: d-hole states of CuAu 5631

imately 0.5 mm). The ejected atoms were then entrained in a pulsed flow of helium ( 120 psi ) , which carried them through a channel 2 mm in diameter and 2 cm in length. The same extension channel which was successfully used in previous studies of CuAg3 was used to lengthen this channel prior to the final supersonic expansion into vacuum, thereby improv- ing the production of diatomic CuAu. The small exit orifice also promoted excellent supersonic cooling, so that low rotational temperatures were achieved.

The resulting molecular beam entered a reflectron time- of-flight mass spectrometer through a 5 mm skimmer. Exci- tation of the CuAu diatomic molecule was accomplished with a pulsed dye laser counterpropagating along the molecular beam path. Following excitation, a pulsed excimer laser operating on ArF (193 nm, 6.42 eV) or F, (157 nm, 7.90 eV) crossed the molecular beam at right angles to ionize the excited molecules. Ions so generated were extracted and passed through a reflectron time-of-flight mass spectrometer. Detection was accomplished with a dual microchannel plate detector, the signal was digitized, and further signal processing was performed using a DEC LSI-1 l/73 micro- computer. The optical spectra of 63C~197A~ and ‘%!u’~~Au were then obtained by separately monitoring the ion signals at mass 260 and 262, respectively, as a function of dye laser frequency.

As in the previous study of CuAg, the output of a side- pumped Hiinsch configuration dye laser was narrowed to 0.03 cm - ’ by the insertion of an air-spaced intracavity etalon, which was then pressure scanned with SF,. Absolute line positions were obtained using the simultaneously recorded I, absorption spectrum in conjunction with the I, atlas of Gerstenkom and Luc.” For some bands this required the dye laser output to be Raman shifted in high- pressure H, ( ~500 psi) so that the first Stokes radiation would fall in the range of the I, atlas. Since the Raman shifting process occurs in a stimulated manner, it always occurs on the line with the highest Raman gain, which for room temperature H, is Q( 1) . This leads to a precisely determined Raman shift, which allows the useful calibration range of the I, atlas to be extended. In cases where Raman shifting was required to obtain a calibration spectrum, the dye laser fun-

63C~‘97A~ Vibronic Spectrum

damental radiation was used to obtain the spectrum of CuAu in high resolution, while the first Stokes radiation was used to simultaneously record the I, absorption spectrum at a frequency 4155.264 cm - ’ to the red of the fundamental dye radiation, according to the published spectroscopic constants of H, .12 Finally, a correction for the Doppler shift experienced by the CuAu molecules as they travel toward the radiation source at the beam velocity of helium ( 1.77X 10’ cm/s) was included to provide accurate absolute line positions for all of the rotationally resolved bands.

Excited state lifetimes were measured by the time-delayed resonant two-photon ionization method. The resulting decay curves were fitted to exponential decay functions by a nonlinear least-squares algorithm,r3 allowing the upper state lifetimes to be extracted. For short lived levels of the A ’ 1 and D 0 + excited states a convolution of a pure exponential decay with a Gaussian instrument function (FWHM of 15 ns) representing the convoluted dye and excimer laser pulses was included in the fitting function. This was required to obtain a reasonable fit to the data.

Ill. RESULTS

A . The a 3I;,+ +X ‘8+ system of CuAu

Figure 1 presents the low resolution spectrum of the lowest frequency band system of CuAu yet observed, recorded using dye laser radiation in conjunction with F, excimer radiation (157 nm, 7.98 eV) as the ionization laser. This system, which may be designated as the a-X system, has been briefly discussed in a previous paper. l4 The a-X system cannot be observed using ArF excimer radiation ( 193 nm, 6.42 eV), a fact which is used below to bracket the ionization potential of CuAu. This band system is extremely weak, as may be judged by the long lifetime of the upper electronic state, which exceeds 90~s and is too long to be reliably measured in the present experiments. Assuming that the decay is dominated by fluorescence to the ground state, such a lifetime implies an absorption oscillator strength of fs 5 X 10 - ‘, which is in any case an upper limit. Indeed, this electronic band system is only observable because it falls in the gain profile of rhodamine 6G laser dye, where a tremen-

I I I I I 1 I I I

900 a 3ET t X ‘Z+ A” a=1 t X ‘C+ r

I-o ‘2-o ‘3.0 ‘4-o ‘5-o o-o I 1-o

I I I I I I I I I I

17600 17800 18000 18200 18400 18600 18800 19000 19200 19400 15

Frequency (cm-‘)

FIG. 1. Low resolution scan of the II ‘2,+-X ‘2 + and A ” 1-X ‘H + systems of 63Cu’97Au, recorded using rhodamine 590, fluorescein 548, and coumarin 540A dye laser radiation in combination with an F, excimer photoionization laser for the a-X system and an ArF ex- timer photoionization laser for the A “- X system.

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Bishea, Pinegar, and Morse: d-hole states of CuAu

400

300

# 200 8

100

Rotationally Resolved Spectrum of 63Cul97Au I I 1 I I

O-O Band of a 3Z’ c X ‘Cf System Q(J)

P(J) I I I I I IIlIIIlll

17800 17801 17802 17803 17804

Absolute Frequency (cm-‘)

FIG. 2. High resolution scan over the O-O band of the a ‘Z,+ -X ‘Z + system of 63Cu’9’Au. The spectrum was recorded with an absolute calibration established by the absorption spectrum of I,, as described in the text. The presence of a Q branch identifies the band system as R’ = 1 + a” = 0 in character, and the severe R- branch head shows that the bond length increases substantially upon electronic excitation.

dous laser intensity is available. Based on the weak intensity of the band system, it is evident that we are observing a transition which is nominally forbidden in some sense.

In addition to this observation, the vibrational frequency of the upper electronic state is far smaller than that found for any of the eight other electronic states observed in this work. Furthermore, a Birge-Sponer extrapolation of the vibrational levels of the upper state predicts convergence to the dissociation continuum at an energy 2.41 f 0.07 eV above the ground level of the molecule, where the error limit is estimated as l/2 of the energy difference between the highest observed vibrational level and the predicted dissociation limit. Given that previous high temperature Knudsen effusion mass spectrometric studies of CuAu place its dissociation energy at 2.31 f 0.08 eV (second-law method),6 2.38 f 0.02 eV (third-law method),6 and 2.36 * 0.10 eV (third-law method),5 it seems clear that the upper state of this transition must dissociate to ground state atoms. With this in mind, the only possibility for the upper state is the nominally repulsive, a 32 + state. This state results from the d lo d h”, ala*’ molecular orbital configuration, which has a focAal bond order of zero, thereby accounting for the low vibrational frequency which is observed. The long radiative lifetime is of course explained by the assignment of the transition to a triplet-singlet intercombination band system. This also explains Ruamps’ failure to observe this system in thermal emission from a King fumace.7

The u 38 + state may be expected to split into an R’ = 0 - and an R’ = 1 component in a heavy molecule such as this, through off-diagonal spin+rbit coupling with other states. Indeed, it is only through such interactions that the a 32 + state gains oscillator strength for transitions to the ground X ‘Z + state. Since transitions linking the ground a” = 0 + level to an excited a’ = 0 - level are rigorously forbidden under dipole selection rules, only the R’ = 1 component of the upper state is expected to be observed. High resolution studies of the O-O and 1-O bands of this band system show this to be the case, as is evident in Fig. 2, which displays a high resolution scan over the O-O band of this system. The presence of R (0) and a Q branch confirms that

the transition is R’ = 1 c R” = 0 in character, conclusively supporting the assignment of the system as a ‘Z,+ +-X ‘Z + .

Table I provides a listing of all of the observed vibronic bands of 63C~197A~, the measured isotope shifts of %I’~‘Au relative to the more abundant form, 63C~197A~, and the measured excited state lifetimes. Measured rotational line positions for the O-O and 1-O bands of the a 38 + (0’ = 1) +X ‘Z + band system are available through the Physics Auxiliary Publication Service (PAPS) of the American Institute of Physics15 or from the author (M. D. M.). The results of a least-squares fit of the data to the expression

v=vo +B’J’(J’+ 1) -B”J”(J” + 1) (3.1) are given in Table II. From this analysis it appears that the bond length of the molecule increases substantially upon excitation to the a 32 ,+ state, as is to be expected.

Although the possibility of significant spin-uncoupling interactions between the observed 0’ = 1 component and the unobserved a’ = O- component of the a 32 + electronic state could complicate the conversion from an effective rotational constant to a bond length for the a 38 + state, such interactions would only affect thefparity levels of the a ‘Z + (a’ = 1) state. l6 Since only the e parity levels of this state are accessed in the P and R branches of transitions from the ground X ‘B + state, fits of the rotational constant of the upper state employing only the P and R branches will give results which are free of any artifacts which might result from spin-uncoupling interactions with the R’ = 0 - component. The congested nature of our spectra in the region of the Q branch prohibits any careful study of the f parity com- ponents of the a 32 + (R’ = 1) state. As a result, our upper state rotational constant corresponds to that of the e parity levels, and can be inverted to give an accurate upper state bond length of ri (a “Z,+ ) = 2.428 + 0.023 A. Unfortunate- ly, the lack of information about the f parity levels prohibits any estimate of the a’ = 1-n’ = O- splitting, which could be derived if the difference in rotational constants of the e and f parity levels of the a 32 + state could be accurately measured. l6 A summary of the spectroscopic constants for

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TABLE I. Vibronic bands of 6’Cu’97Au.’

System Band Observed frequency (Cm-‘p

Isotope shift (cm-‘)’ Lifetime @sjd

a’I,‘-x2+

A’ I-X%+

A’ I-X’S +

AO’-X’E’

BO+-X'Z‘

Cl-X'Z'

D' I-X'Z +

DO'-X'Z'

o-o 1-o 2-o 3-O 44 5-o 04 1-O o-o 1-o 2-O 3-O 4-o 5-o o-a 1-o 2-O 3-O 44 O-0 1-o 2-O O-0 1-o 2-O 3-O 4-o c-l 2-l o-o 1-O 24 3-O 44 o-o 1-o 2-a 3-O 4-o 5-o 6-o 7-O 8-o 9-o

10-O 11-O 12-O 13-O 14-O 15-a 16-O o-1 l-l

‘17 803.0270(35) *I7 909.3532( - 78)

18 016.22(41) 18 119.67( - 4) 18 222.04(21) 18 322.00( - 16)

*19 154.0257 $19 392.4733 l 20 201.6376(4) ‘20 433.4812(24) 20 661.50( - 85) 20 889.68(76) 21 112.87( - 9) 21 334.38( - 9)

‘20 211.2736( - 56) 20 407.82( 172)

*20602.1310( - 181) 20 806.03(69) 21010.25( - 5) 20 650.0023

*20 905.0390 21 156.00

‘22 l&4.8870( - 43) 22 401.64( 184)

*22 630.7443( - 85) 22 859.42( - 127) 23087.81(71) 21916.59( - 42) 22 383.70(42)

‘23 308.8675( - 39) 23 529.04(85)

‘23 745.0667( - 20) 23 959.88( - 58) 24 174.12(33)

‘23 914.9356( - 16) 24094.32(g) 24 274.66( - 37) 24 457.38( 15) 24 641.59(98) 24 825.15(21) 25010.18(18) 25 194.06( - 148) 25 380.79( - 56) 25 568.25( 107) 25 752.04( - 78) 25 938.02( 1) 26 124.74(216) 26 303.52( - 271) 26 490.34( 157) 26 670.00(4) 26 849.19( - 38) 23 666.53( - 16) 23 845.92(8)

* + 0.7578(63) * -0.4864(117)

- 1.20 - 2.40 - 5.11 - 5.60

* + 0.0199(45) * - 2.7955(44) * +0.0814(59) * - 2.9182(50)

- 2.89 - 4.71 - 8.86 - 10.38

* +0.1440(31) - 2.68

* - 3.5380(26) - 6.94 - 9.85

* - 1.2079( 19) * - 3.8612(30)

* + 0.0022( 56) - 3.52

* - 5.3888(83) - 9.19

- 10.94 + 1.86 - 3.89

l + 0.1239(35) - 2.61

‘I - 4.9161(61) - 6.79

- 10.39 * + 0.3782(36) * - 1.7583(33)

- 2.80 - 5.82 - 8.76

- 10.68 - 12.55 - 14.40 - 17.80 - 19.96 - 20.62 - 25.04 - 28.06 - 24.57 - 30.81 - 31.83 - 27.84 . . .

+ 1.48

>90

7.10(16) 4.54(228) 0.664(25) 0.104(4) 0.037( 1) 0.011(l) O.oo’( 1) 0.011(l) 0.444(3)

0.319(12)

0.638(54) 0.742(68)

0.862(49)

0.869(41)

7.81(8)

6.18(99)

0.023( 1) 0.023 ( 1)

0.023 ( 1)

0.025 ( 1)

0.036(2)

0.032(9) 0.053(3)

‘Bands indicated by asterisks were measured in high resolution using the I, atlas for calibration. For the D-X 1-O band the I, atlas calibration was unsuccessful, but the isotope shift was accurately measured nevertheless. Vibronic bands for h”C~‘9’A~ were fitted to the formula Y = v, + r&I - o:x: ( Y’* + I/) - SAG ;;2 for U” = O,l, resulting in the following values, with errors given in parentheses corresponding to la in the least-squares fit: a-X vw = 17 802.68(52); o: = 109.23(58); m;x: = 0.890(93). A “-X v, = 19 154.0257; AG;,, = 238x4476(41). A’-X v,=20201.60(61); o: =234.18(68); 1~;x:=1.268(111). A-X v,=20211.83(176); o: = 190.71(257); 0:x: = - 1.781(500). B-X v, = 20 650.0023; o; = 259.11;~:~: = 2.038.GXv, = 22 165.32(118);0: = 23’.17(16O);w:x: = 1.345(330);AG;,, = 248.31(133).0’- X v, = 23 309.26(77); o: = 220.81(113); o:x: = 0.934(220). D-X v, = 23 915.09(98); o: = 177.28(63); w;x; = - 0.993(86); o;y: = - 0.03835(333); AG;,, = 248.40( 1). For this state only, the fortnula given above was aug- mented by the term + o;y:u’(~P + 3v’/2 + 3/4) to allow an accurate fit to be obtained for the vibrational levels U’ = O-16.

bFollowing each observed frequency in the table, the residual v, - v,,, is given in units ofO.O1 cm-’ in parentheses. The isotope shift is defined as v(~~CU’~‘AU) - v( “C~‘97A~). Where this has been measured in high resolution, the lg error limit of the result is given in parentheses.

d Errors reported for lifetimes correspond to lo in the nonlinear least-squares fit.

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5634 Bishea, Pinegar, and Morse: d-hole states of CuAu

TABLE II. Fitted constants for the a ‘2,+-X ‘Z + system of CuAu.

Band

o-o v,, = 17 803.0270(48) v, = 17 803.7848(42) B; = 0.059 12(74) B; = 0.058 88(62) B; = 0.065 65(64) B; = 0.065 08(54)

l-o v, = 17 909.3532(37) v, = 17 908.8668( 111) B; =0.05741(26) B; = 0.058 57(213) B; =0.065 31(28) B,” = 0.065 79(139)

‘All values are reported in wave numbers (cm-‘), with loerror limits given in parentheses.

the a 3X + state, along with those of the other observed electronic states of 63Cu’97Au is provided in Table III.

B. The A” 1 +X ‘B+ system of CuAu

Also shown in Fig. 1 is a more intense system slightly to the blue of the weak a “Z;C CX ‘B + band system, for which only two vibronic levels are observed. This extremely short vibrational progression may be accounted for by the Franck-Condon principle, provided the bond lengths in the upper and lower states are nearly the same. In high resolution work, described below, this proviso is shown to be quan- titatively correct, with the difference in measured bond lengths only amounting to 0.006 A. This short band system is designated as the A “-X system, so that the notation previously introduced for the band systems observed by Ruamps’ may be maintained. The A “-X band system may be observed using either F2 excimer radiation ( 157 nm, 7.98 eV) or ArF radiation ( 193 nm, 6.42 eV) as the second, ionizing photon. Together with the ionization behavior observed for the a-X system, this places the ionization potential at IP(CuAu) = 8.74 f 0.05 eV.

Figure 3 displays a high resolution scan over the O-O band of the A “-X system for 63C~‘97A~. The band is rather

TABLE III. Electronic states of 63Cu’9’Au.’

symmetric in structure, with the P and R branches showing roughly equal separations between consecutive lines. The lack of a bandhead implies nearly identical rotational constants (and bond lengths) for the upper and lower states, and the definite presence of a strong unshaded Q branch shows that theA ” state possesses a’ = 1. Precisely measured line positions for the O-O and 1-O bands are available through the Physics Auxiliary Publication Service (PAPS) of the American Institute of Physics” or from the author (M. D. M. ). The results of least-squares fits of the data to Eq. (3.1) are given in Table IV. The resulting values of B ;, and B ; have then been used to determine B : and cr: for the A” state, and B: has been inverted to obtain r; (A “) = 2.336 & 0.005 A, as is listed in Table III.

The lifetime of the U’ = 0 level of the A ” state was measured using the time-delayed two-photon ionization technique to be approximately 7 ps. This slow fluorescence rate probably allows the A ” state to be collisionally deactivated prior to emission under the conditions of Ruamps’ experiments, thereby explaining his failure to observe this system.’ The weak 1-O band displayed a similar upper state lifetime, although the poor Franck-Condon factor associated with this excitation led to substantially poorer signal quality, and correspondingly larger error limits. The lack of a significant difference in excited state lifetime for the u’ = 0 and 1 levels suggests that predissociation is not occurring in this system, and that higher v’ levels are unobserved simply because of poor Franck-Condon factors. Given the similarity in bond lengths of the X and A R states, this comes as no surprise. Assuming that the A ” state decays solely by fluorescence to the ground state, the measured lifetime of 7.1 ps corresponds to an absorption oscillator strength off= 5.8 x 10 - 4. This is approximately an order of magnitude greater than that found for the a “2: +X ‘Z + system, and is of the general size found for dc +d transitions in NiCu16 and CuAg.3 With this in mind the A “Xsystem is probably best described as a a* +d excitation.

DO+ D’ 1 Cl BO+ AOf A’1 A’1 a3Z+ (1) x%+(0+)

T,(cm-‘)

23 914.936(3) 23 308.867( 3) 22 164.887(4) 20 650.002 ( 1) 20 211.274(2) 20 201.638(5) 19 154.026(3) 17 803.027( 5)

O.OOU

w,(cm-‘) o,x,(cm-‘)

177.28(63) - 0.993( 86) 220.81(113) 0.934(220) 237.17( 160) 1.345(330) 259.11 2.038 190.71(257) - 1.781(500) 234.18(68) 1.268(111) AG;,, = 238.4476(41) 109.23(58) 0.8!?0(93) AG;,, = 248.35

B,(cm-‘) a,ccm-‘)

0.057 58(30) 0.000 82(25) 0.06145( 19) O.o0048(16) 0.061 75(27) 0.000 16( 17) 0.064 56( 12) 0.00027(13) 0.06403(15) O.o0026(8) 0.06041(38) - o.ooo ll(30) 0.064 75(26) O.OCNI 46(28) 0.059 98( 112) 0.00171(78) B; = 0.065 098(36)

r,(A) Lifetimeb (ps)

2.478(6) 2.398(4) 2.392(5) 2.340(2) 2.350(3)’ 2.419(8)’ 2.336( 5) 2.428(23) 2.3302(6)

0.023( 1) 7.2( 1) 0.87(3) 0.68(4) 0.38(5) 0.66(3)b 7.1(2) >90 . . .

*Uncertainties are given in parentheses as one standard deviation. D,,(CuAu) = 2.344( 19) eV; IP(CuAu) = 8.74(5) eV; D,(Cu+-Au) = l-33(6) eV; DO(Cu-Au’) = 2.83(6) eV (see the text for details).

bExcited state lifetimes and their uncertainties are obtained as a weighted average of the results measured for various excited state vibrational levels. An exception is the A ’ 1 state, for which the lifetime depends dramatically on d, giving 664(25) ns (u’ = 0); 104(4) ns (v’ = 1); 37( 1) ns (0’ = 2); 11(l) ns (0 = 3); 7( 1) ns (v’ = 4); and ll( 1) ns (u’ = 5). The,4 ’ 1 state is predissociated through its interaction with thea “P + (1) state, with the predisso&tion rate increasing strongly with v’. It is presently unknown whether the u’ = 0 level is subject to predissociation. For this reason the 660~s lifetime quoted above should not necessarily be taken as the radiative lifetime.

‘Owing to the possibility of L- and S-uncoupling interactions between the A ’ 1 and A 0 + states, it is possible that the bond lengths of these states may be overestimated and underestimated, respectively, perhaps by as much as 0.007 A. See the text for details.

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!m

II 600

zt 3 300

0

Rotationally Resolved Spectrum of a3Cu’97Au

I I I I I I I I I

o-0 Band of A” 1 c X ‘X? System Q(J)

P(J)

[Ib 2

NJ) 1111111

0 2 4 6

19152 19153 19154 19155 19156


FIG. 3. High resolution scan over the O-O band of the A ” 1-X ‘Z + system of 63Cu’9’Au. The spectrum was recorded with an absolute calibration established by the absorption spectrum of I,, as described in the text. The presence of a Q branch identifies the band system as R’ = 1 + a” = 0 in character, and the lack of a bandhead shows that the bond length of the molecule remains nearly the same upon electronic excitation.

C. The A’ 1 CX lx+ system of CuAu

Figure 4 displays the region from 19 600 to 21 600 cm-‘, where three band systems of CuAu are located. All have been observed using ArF radiation ( 193 nm, 6.42 eV> as the second, ionizing photon in the resonant two-photon ionization process. Two of these systems have been previously observed by Ruamps and have been designated as the A-X and B-X systems.’ The lowest frequency system, lying very close to the A-X system, however, has not been previously detected. It is proposed that this new system be designated as the A ‘-X system.

Lifetime measurements of the upper states of the A ‘-X system show a strong and systematic decrease in excited state lifetime as the vibrational quantum number is increased (see Table I). Such behavior is normally unexpected and indicates that the A ’ state is predissociated through its interaction with another electronic state. The only energetically accessible separated atom asymptote is the ground *S + ‘S limit, to which the X ‘Z + and a 32 + states correlate. Given the much longer bond length of the nominally repulsive a ‘2 + state, the A ’ state is almost certainly predissociated through its coupling to this triplet state. Although we have no way of determining whether the u’ = 0 level of the A ’ 1 state undergoes predissociation, its decay rate may be used to set an upper limit on the absorption oscillator strength of the A ‘-Xsystem off<O.O056. If the u’ = 0 level does undergo predissociation, its fluorescence quantum yield may have

TABLE IV. Fitted constants for the A * 1 c X ‘Z + system of CuAu.”

Band 6’C~‘v7A~ ‘%u’~‘Au

o-o v,, = 19 154.0257(32) v,, = 19 154.0058(32) B; =0.06452(15) B; =0.063 74(21) B," =0.06481(17) B,"=O.O6398(25)

1-O v, = 19 392.4733(26) v, = 19 389.6778( 36) B; =0.06406(24) B; =0.063 lO(24) B," =0.06488(30) B; =0.06369(24)

‘AUvaluesartreportedin wavenumbers (cm-‘), with laerror limits given in parentheses.

been too small for Ruamps to observe, thereby accounting for his failure to observe this system under the conditions of his experiment.

Figure 5 displays a high resolution scan over the O-O band of the A ‘-Xsystem. A strong Q branch is evident in this spectrum, thereby identifying the A ’ state as an a = 1 state. This is consistent with the predissociation behavior of this state, since the only other value of R’ which is accessible through electric dipole transitions from the ground ‘B + state is R’ = 0 + . An R’ = 0 + state, however, could only be predissociated through heterogeneous couplings to the a’ = 1 component of the a 38 + state.16 Such couplings would be rotationally dependent,16 and would be small for the low rotational levels populated in the jet-cooled molecular beam. The A ’ ( 0’ = 1) state, however, may be coupled to the KY = 1 component of the a 38 + state by homogeneous mechanisms, allowing predissociation to occur in a manner independent of J. I6 Such homogeneous mechanisms could be efficient even for the low values of J populated in the supersonic molecular beam.

Rotational line positions for the O-O and 1-O bands of the A ‘-X system of both 63C~197A~ and ‘%u’~‘Au are available from the Physics Auxiliary -Publication Service (PAPS) of the American Institute of Physics” or from the author (M. D. M.). Fitted values of the spectroscopic constants are given in Table V. The resulting values of B ,‘, and B ; have been evaluated to give B : and a:, and B : has been inverted to give r; (A ’ 1) = 2.419 -& 0.008 A. The nearby presence of the A 0 + state, however, introduces an additional element of uncertainty in the conversion from a rotational constant to a bond length. The A ’ 1 and A 0 + states may, in principle, perturb one another through either L-uncoupling interactions or S-uncoupling interactions. In either case, the matrix element coupling the two states takes the approximate form of

(A,v,JIH IA ‘,u’,J) z -BUS [J(J+ 1) I”*, (3.2)

where BuU. is the matrix element of - i?/2,uR * evaluated between the vibrational wave functions of the u vibrational level of the A state and the u’ level of the A ’ state, and C is a



63C~‘97A~ Vibronic Spectrum

I I i I I I I

An’=O+t X’c+ 10.~ BSL’=O+c Xk+

I I

900- II-0 2-O A’n’e-1 + X ‘c+ O-0 1 i-0 12;0 13-O I 14-o

o-o “I.0 ’ “2.0 ’ ‘j-0 ’ ‘4lO ‘5.0

~600-

a 3

!3 300- I

I I I I I I I I I I I 19600 19800 2oooO 20200 20400 20600 20800 21000 21200 21400 21600

Frequency (cm-‘)

small number, probably less than unity.16 In second order perturbation theory this off-diagonal matrix element will affect the apparent rotational constant of the A ’ 1 and A 0 + states, since the energy correction is of the form

Ec2’zB2 C2J(J+ l)/AE. ml I (3.3)

This causes a shift in the effective B value for the vibrational level of

MkB;,C2/AE, (3.4)

Assuming 1 C I< 1, we can estimate the resulting error in B for the A ’ 1 state, since the energy difference AE is known. The result shows that the effective B constant measured for the A ’ 1 state may underestimate the true B value by at most 0.6%, leading to an overestimate of the bond length in the A ’ 1 state of at most 0.3%, or 0.007 A. Likewise, the bond length of the A 0 + state (see below) may be underestimated by at most 0.007 A.

Rotationally Resolved Spectrum of 63C~‘97A~

a 600 en

Gi

5 300

I I I I I )-OBandofA’lc X’,Y+sysm~

P(J)

-F-F--Y

7

20201 20202


20203

FIG. 5. High resolution scan over theO-0 band of the A ’ 1-X ‘Z + system of 63C~‘97A~. The spectrum was recorded with an absolute calibration eatab- lished by the absorption spectrum of I,, as described in the text. The presence of a Q branch identifies the band system as a’ = 1 + nN = 0 in character.

FIG. 4. Low resolution scan of the A' l-X%‘, A O+-X'Z+, and B 0 +-X ‘2 + systems of %I’~‘Au, recorded using coumarins 500,480, and 460, in combination with an ArF excimer laser for photoionization.

D.TheA O++X18+ systemof CuAu

Slightly to the blue of the O-O band of the A ‘-X system one finds the O-O band of a somewhat more intense band system, which has been previously observed by Ruamps,’ and has been designated as the A-X system. The values of To and AG ;,2 measured in the present work (20 2 11.2736 and 196.55 cm - ‘, respectively) are in good agreement with the previous results of Ruamps (20 2 14 and 195.7 cm - ‘, respectively), confirming that we are dealing with the same band system. Measured lifetimes of the U’ = 0 and U’ = 2 levels are 444 and 319 ns, respectively. Although the error limits quoted in Table I would suggest that these lifetimes are different, it has been our experience that the error limits obtained in the nonlinear least-squares fitting procedure are artificially low. As a result we do not believe there is any conclusive evidence of predissociation in the A state of CuAu. Assuming that the A state decays solely by fluorescence to the ground electronic state of CuAu, these radiative lifetimes imply an absorption oscillator strength off=:O.Ol, making the A-X system the strongest band system discussed as of yet in this molecule. This is in accord with the fact that it is the first of the band systems discussed to have been observed in Ruamps’ early work on CuAu.’

Figure 6 displays a high resolution scan over the O-O band of the A-X system of 63Cu’97Au. The obvious lack of a Q branch confirms that the transition is a parallel transition,

TABLE V. Fitted constants for the A ’ 1 +X ‘Z + system of CuAu.’

Band

o-o v,, = 20 201.6376(45) v,, = 20 201X62(38) B; = o.oa 46(25) B; =0.05861(26) B: =0.06490(25) B," =0.06326(26)

1-O v,, = 20433.4812(31) v,, = 20 430.5631(40) B; =0.06057( 17) B; =0.05929(20) Bo"=0.06493(20) B," =0.06367(22)

*All values are reported in wave numbers (cm-‘), with loerror limits given in parentheses.


Rotationally Resolved Spectrum of “Cu’“Au Rotationally Rcsolvcd Spxlrum 01 W U WAu 500

20648 20649 2065 I 20652 20653

Absolute Frequency (cm’)

Absolute Frequency (cm’)

FIG. 6. High resolution scan over the O-O band of the A 0 + -X ‘Z + system of 63Cu’97Au. The spectrum was recorded with an absolute calibration established by the absorption spectrum of I,, as described in the text. The absence of a Q branch identifies the band system as an 0 = 0-W = 0 transition, and the lack of an observable bandhead shows that the bond length changes little upon electronic excitation.

corresponding to an a’ = 0 + -X ‘Z + excitation. Line positions for the O-O and 2-O bands of 63C~‘97A~ and 65C~‘97A~ are available from the Physics Auxiliary Publica- tion Service (PAPS) of the American Institute of Physics’5 or from the author (M. D. M .). The values of vo, B 6, B 6, and B ;, obtained through a fit of the line positions to Eq. (3.1) are given in Table VI. From these data an excited state bond length of r: (A Of ) = 2.350 f 0.003 A may be derived. As discussed in the previous subsection, however, this value may underest imate the actual bond length by as much as 0.007li.

E.TheBO++X%+systemofCuAu

The third band system observed in the low resolution spectrum of F ig. 4 is the B-X system, which has been previously observed by Ruamps in em ission studies of copper- gold vapors in a King furnace.’ The values of To, w:, and C&X; observed in the present work for 63C~‘97A~ (20 650.0023, 259.11, and 2.038 cm-‘, respectively) are again in close agreement with Ruamps’ results7 (20 652.3, 257, and 2.2 cm - ‘, respectively). Measured f luorescence lifetimes show no obvious dependence on vibrational quantum number (see Table I), and correspond to an absorpt ion oscillator strength off~0.005 assuming that the decay is dominated by f luorescence to the ground electronic state.

TABLE VI. Fitted constants for the A O+ -X 'Z + system of CuAu.’

FIG. 7. High resolution scan over the O-O band of the B 0 +-X ‘2 + system of 63C~‘9’A~. The spectrum was recorded with an absolute calibration established by the absorption spectrum of I,, as described in the text. The absence of a Q branch identifies the band system as an fl’ = O+fI” = 0 transition, and the lack of an observable bandhead shows that the bond length changes little upon electronic excitation.

F igure 7 displays a high resolution scan of the O-O band of the B-X system of 63Cu’97Au. As was the case for the A-X system, the obvious lack of a Q branch identifies the transi- t ionasanW=O+ t X ‘Z + excitation. The lack of a bandhead implies that the internuclear separat ion is nearly un- changed upon electronic excitation, as is confirmed by fitting of the observed rotational lines to Eq. (3.1). Line positions for the O-O and 1-O bands of the B-X system for both 63C~‘97A~ and 65C~‘97A~ are available from the Phys- ics Auxiliary Publication Service (PAPS) of the American Institute of Physics15 or from the author (M. D. M). These line positions have been analyzed to provide values of vo, B 6, B &, and B ;, which are given in Table VII. From these data values of B : and a: have been derived, and r; (B 0 + ) has been determined as 2.340 f 0.002 A. As expected from the lack of a bandhead and the short Franck-Condon progression observed in the B-X system, this is very similar to the ground state bond length of 2.330 A.

F. The C 1 +X ‘2+ system of CuAu At slightly higher frequencies Ruamps’ C-X band sys-

tern7 is observed. This system exhibits a vibrational progression extending up to the 4-O band, as is shown in the low resolution spectrum of F ig. 8. The values of To and w: found in the present investigation (22 164.8870 cm-’ and 237.17 cm-’ ) are again similar to those reported by Ruamps in his

TABLE VII. Fitted constants for the B 0 + 6X 'Z + system of CuAu.”


Band

o-o v, = 20 211.2736(20) v,, = 20 211.4175(23) B; = 0.063 90( 12) B; = 0.062 74( 18) B,” =0.065 lO(11) B;=O.O6382(15)

2-O v,, = 20 602.1310( 18) v, = 20 598.5930( 19) B; = 0.063 38( 10) B; =0.06229(13) B,"=0.06509(10) B&‘=0.06389(11)

‘All values are reported in wave numbers (cm- ’ ) , with lo error lim its given in parentheses.

Band “CU’~‘AU 65C~‘97A~

o-o v,, = 20 650.0023( 12) v, = 20 648.7944( 14) B; =0.06442(7) B; =0.06274(9) B; =0.065 16(6) B; =0.06347(9)

1-O v, = 20 905.0390( 22) v. = 20901.1778(20) B; =0.064 15(11) B; =0.06297(22) B;;=O.O6507(11) B; =0.063 94(27)

“All values are reported in wave numbers (cm - ‘), with lcerror lim its given in parentheses.



a3Cu’g7Au Vibronic Spectrum

I I I I I I I I I 900-

’ O-l CC?‘=1 t x’B+

O-O ’ I-O ’ 2-o ’ 3-o ’ 4-o 1 &il MKl-

;;3

1 300-

0 I

1 I I I I I I I I

21600 21800 22000 22200 22400 22600 22800 23ooO 23200 23400 23

Frequency (cm-‘)

previous study’ (22 167 and 23 1 cm - ’ ). Measured lifetimes ofthe U’ = 0 and u’ = 2 levels of the C state are nearly identical (see Table I), measuring 860 f 50 ns. Assuming the decay is again dominated by C-X fluorescence, this corresponds to an absorption oscillator strength offzO.0036.

A high resolution scan over the O-O band of the GX system of 63Cu’97Au is shown in Fig. 9. The P and R branches are observed to fan out nearly symmetrically from an intense, unresolved, Q branch in the center of the band. The presence of a Q branch immediately identifies the C-X system as an a = 1 tX ‘I: + transition. Line positions of the P and R branch lines for the O-O and 2-O bands of both a3Cu’97Au and ‘%u’~‘Au are available from the Physics Auxiliary Publication Service (PAPS) of the American In- stitute of Physics” or from the author (M. D. M). The results of a least-squares fit of the data to Eq. (3.1) are provided in Table VIII. From the values of B ; and B ;, B :, and ai have been calculated, and are listed in Table III. The value of B: has then been used to determine r;(Cl) =2.392~0.005A.

Rotationally Resolved Spectrum of 63Cu’“Au

300

0 22162 22163 22164 22165 22166

Absolute Frequency (cm-r)

FIG. 9. High resolution scan over the O-O band of the C 1-X ‘Z + system of @CU’~‘AU. The spectrum was recorded with an absolute calibration established by the absorption spectrum of I,, as described in the text. The presence of a Q branch identifies the band system as R’ = 1 c R” = 0 in character.

FIG. 8. Low resolution spectrum of the C 1-X ‘P + system and the begin- ning of the D' I-X’Z * system of 63C~‘WA~, recorded using coumarin 440 dye laser radiation in combination with an ArF excimer laser for photoionization.

Kl

G. The D 1 CX ‘8+ system of CuAu

Figure 10 displays the low resolution spectra of 63Cu’97Au in the highest energy range investigated in this work. Two band systems, along with several anomalous bands, are evident in this range. The lower frequency band system, with an origin band at 23 308.8675 f 0.0025 cm - ‘, is a new band system which was not observed by Ruamps.’ This system displays vibrational bands with intensity dimin- ishing rapidly as one proceeds from the O-O band through the 4-O band. Time-delayed resonant two-photon ionization studies of the lifetimes of the U’ = 0 and U’ = 2 levels of the upper state, designated as the D ’ state, show this state to be rather long lived, having a decay lifetime of 7.8p.s. No significant dependence of excited state lifetime on vibrational level was observed. Assuming that the decay is dominated by fluorescence to the ground electronic state, this lifetime corresponds to an absorption oscillator strength off=4 X 10 - 4. Without question it must have been this long fluorescence lifetime and the possibility of collisional deactivation that prevented Ruamps from observing the D ’ -+X emission system in a King furnace.

Figure 11 displays a high resolution scan over the C&O band of the D ‘-X system of 63Cu197Au. As in the case of the a-X, A “-X, A I-X, and C-X systems, the obvious presence of a Q branch identifies the D ‘-X system as an R’ = 1 c X ‘Z + system. Line positions for the Pand R branch lines observed in the O-O and 2-O bands of the D-X system for both

TABLE VIII. Fitted constants for the C 1 +-X ‘L: + system of CuAu.’

Band 6Qp7A” Wu’97Au

o-o v, = 22 W&8870(37) v,, = 22 W&8891(42) B; = 0.06167(21) I?; =0.06004(34) B:=0.06494(19) B;=O.O6334(28)

2-O v,, =22630.7443(49) v, =22625.3555(67) B; = 0.061 34(27) B; =0.05888(55) B; = 0.065 35(25) B;=O.O6293(46)

‘All values are reported in wave numbers (cm- ‘), with laerror limits given in parentheses.



“Cu’“Au Vibronic Spectrum

q , I I Dn-O+a’c X’f

1 I # , 9Qo Dn+lC X9’

!w ‘IO 1 20 I ‘3.0 114 I ‘co 2-o 3.0 4.0 5-o 6.0

i”- 3 MO-

O I A

1 0 I I 23200 23400 23600 23800 2u)o 24200 24400 24600 24800 2%l!l

Frequency (cm,‘) I 1 I 1 I , I t t I

Dnw4- x92 9% 174 180 194 ll0.0 Ill0 112.0 113-O IWO Il5.0 ,160

J My)- 2 m-

0-1, Ii7 b .’ ’ 1 I I I I I 8 I I I I I

252W 254W 25600 25Wl 2&X0 26200 26400 26600 26800 2looO

Frequency (cm.‘)

FIG. 10. Low resolution scan ofthe D’ 1-X ‘Z + and DO * -X ‘2 + systems of6’Cu’97Au, recorded usingstilbene420, andexalite 398,389,384, and 376 dye laser radiation, in combination with an ArFexcimer Iaser for photoioni- &ion. The upper panel shows the 23 100-25 100 cm-’ range while the lower panel shows the 25 100-27 100 cm - ’ range. As discussed in the text, the DO +-X ‘H + system is extremely intense. As a result, vibrational levels of lower lying R’ = 0 + states may pick up intensity quite readily by mixing with the D 0 + state, and this presumably accounts for the extra bands observed near the D-X 5-0,6-O, 7-0,8-O, IM, 12-0, and 13-O bands.

63C~‘97A~ and 65C~‘g7A~ are available from the Physics Auxiliary Publication Service (PAPS) of the American In- stitute of Physicsis or from the author (M. D. M). The resulting fitted values of vo, B 6, B ;, and B ; are given in Table IX. Values of B : and a: have been extracted from the values of B 6 and B ; , and B : has been inverted to obtain the bond length of the D’ state as r:(D’l) = 2.398 f 0.004 A.

H. The DO+ CX ‘E+ system of CuAu

Figure 10 also displays a very extended band system, for which the O-O through 16-O bands are observed. This is an

extremely intense band system, and part of the reason why so many members of the progression are observed is that it was saturated under the conditions of our experiment. Lifetimes of the upper state show only slight variations with vibrational level (see Table I), with measurements of the u’ = 0, 1,5, 7,10,12, and 13 levels giving values in the range of 23-53 ns. The weighted average lifetime of 25 _+ 0.5 ns corresponds to an absorption oscillator strength off z 0.11, making this by far the most strongly allowed band system observed in this molecule. It is in the same vicinity as the D-X emission system observed by Ruamps,’ however he reports the values V W = 23,665 cm-’ and w: = 182 cm-‘, while our results clearly give vW = 23 914.9356 cm-’ and o: = 177.28 cm - ‘. It seems certain that Ruamps was observing this same system, since his assigned origin band falls at 23 665 cm - *, which is almost exactly where the O-l band is calculated from the results of Table III to occur (23 666.59 cm-‘). Perhaps his analysis was complicated by the presence of the B ‘(R’ = 1 u )-X ‘2: transition of Au,, which occurs in this same spectral region,4 and which was unknown at the time of his studies. In any case, Ruamps’ previous designation’ of this system as D-X is retained.

Figure 12 displays a high resolution scan over the O-O band of the D-X system for 63C~‘97A~. Unlike most of the other band systems observed in this molecule, the D-X system gives bandheads which arise rather abruptly in the R branch, indicating a considerable increase in the bond length upon electronic excitation. Although the returning R branch introduces some slight problems in observing the structure of the O-O band displayed in Fig. 12, it is nevertheless, clear than the Q branch is missing, allowing the transition to be identified as an fi’ = 0 + +-X ‘Z + excitation. Absolute line positions for the O-O band and relative line positions for the 1-O band of the D-X system are available from the Physics Auxiliary Publication Service (PAPS) of the American In- stitute of Physicsr5 or from the author (M. D. M.). The results of a least-squares fit of the data to Eq. (3.1) are given in Table X. As in the previous band systems, these data have been used to extract an equilibrium bond length of the D state, giving r: (D 0 + ) = 2.478 f 0.006 A. This is by far the largest internuclear separation of any of the states yet

Rotationally Resolved Spectrum of a3Cu’97Au

900 O-O Band of D’ 1 c- X ‘F System

Lo 3

s 300

0 23306 23307 23308 23309


23310 23311

FIG. 11. High resolution scan over the O-O band of the D’ 1-X ‘Z * system of ‘%u’~‘Au. The spectrum was recorded with an absolute calibration established by the absorption spectrum of I,, as described in the text. The presence of a Q branch identifies the band system as R’ = 1 +-a” = 0 in character.

J. Chem. Phvs.. Vol. 96. No. 8.15 Octohnr 1991 Downloaded 02 Apr 2001 to 128.110.196.147. Redistribution subject to AIP copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp


TABLE IX. Fitted constants for the D' 1 -X ‘2 + system of CuAu.” TABLE X. Fitted constants for the D 0 + -X 'Z r system of CuAu.”

Band ‘=CU’~‘AU s5C~‘97A~

o-o v, = 23 308.8675(25) v,, = 23 308.9914(25) B; = 0.061 21( 14) B; = 0.059 38(25) B;=o.o6490(13) B; = 0.062 98(20)

2-O v,, = 23 745.0667(41) v, = 23 740.1506(45) B; = 0.060 25(28) B; ~0.058 87(33) B;=O.O6449(35) B;; =0.06327(27)

“All values are reported in wave numbers (cm - ‘), with laerror limits given in parentheses.

observed in CuAu, which is somewhat surprising since the D state is evidently very strongly bound, as may be judged by its long vibrational progression and negative anharmonicity. These odd characteristics suggest that the D state possesses a rather different electronic character than the lower lying electronic states. A likely cause of this unusual behavior is discussed in Sec. IV below, where a model of the electronic character of the D 0 + state is proposed.

The low resolution spectrum shown in Fig. 10 also shows a number of unusual features in the region of the D-X system. These take the form of unassigned bands occurring near the 5-0,6-O, 7-0,8-O, 10-0, 12-0, and 13-O bands of the&Xsystem. These anomalous bands display an intensity pattern that is directly related to how close they are in frequency to bands of the D-X system. For example, the extra features lying very near to the 6-0, 8-0, 10-0, and 13-O bands have at least 50% of the intensity of their companion &Xband. The extra features lying close to the 5-0,7-O, and 12-O bands are somewhat further removed from their companion D-X bands, and are not nearly as intense as those features near the M, S-O, lM, and 13-O bands. This intensity pattern almost certainly arises because an underlying O+ state gains intensity by mixing with the D O+ state, which as we have seen has a huge oscillator strength for

Rotationally Resolved Spectrum of VU’~AU

I I I I , I I I c I

O-O Band of D O* + X ‘Z+ System R(J)

400 mT 0 2, I

1 iij 5 200

0 23912 23913 23914 23915


23916

FIG. 12. High resolution scan over theO-0 band ofthe DO +-X 'P + system of 63Cu’97Au. The spectrum was recorded with an absolute calibration established by the absorption spectrum of I,, as described in the text. The absence of a Q branch identities the band system as an 0’ = 0-a” = 0 transition, and the severe R-branch head shows that the bond length increases substantially upon electronic excitation.

Band 63C~‘9’A~ 65C~‘97A~

o-o v, = 23 914.9356(28) v. = 23 915.3148(23) B; =0.057 17(19) B; =0.05408(49) B; =0.06583(20) B," =0.06289(35)

l-Oh v,, = 5.2742(23)b v. = 3.5159(23)” B; =0.05635( 17) B; =0.05505(26) B;=O.O6526( 16) B&'=O.O6342(20)

“All values are reported in wave numbers (cm-‘), with loerror limits given in parentheses.

“An absolute calibration based on the I, atlas was not possible for the 1-O band; as a result only relative positions of the ‘%ZU’~‘AU and 6sC~‘97A~ band origins are given.

transitions with the ground state. In fact, the somewhat longer lifetimes measured for the u’ = 10 and 13 levels of the D state (36 & 2 ns and 53 f 3 ns, respectively, as listed in Table I) may be explained by strong mixing of the underlying “dark” 0 + state with the v’ = 10 and 13 levels of the D state. Complete mixing of a vibrational level of the underlying 0 + state with a particular vibrational level of the D state would lead to states with a 50-50 composition, increasing the lifetime by a factor of 2. The U’ = 13 level of the D state is apparently mixed in a nearly 50-50 manner based on its intensity in Fig. 10, and this is confirmed by its measured fluorescence lifetime, which is roughly double that of the short- lived vibrational levels of the D state. Presumably the dark 0 + levels which gain intensity in this manner correspond to high vibrational levels of the A 0 + and B 0 + states. Accurate measurements of the band origins (which would require rotationally resolved work) would presumably also show significant level shifts, particularly in the U’ = 6, 8, 10, and 13 levels of the D 0 + state.

IV. DISCUSSION

A. The X ‘X+ ground state of CuAu

As in all of the diatomic coinage metals, the X ‘B + ground state of CuAu arises from the interaction of the two atoms in their ground, d ‘OS’, ‘S,,, electronic states. This state is well isolated from any other electronic states, with the nearest state (the u 38 + state) lying more than 2 eV higher in energy.

The bond strength of the X ‘Z + state has been estimated by two studies employing high temperature Knudsen effusion mass spectrometry.5*6 In the first study by Ackerman, Stafford, and Drowax? only the results of a third-law calculation were reported. However, these authors assumed a bond length of 2.51 A and a vibrational frequency of 250 cm-’ in their evaluation of the partition function of CuAu at elevated temperatures. Correcting these values to 2.33 A and 248.35 cm-’ leads to a slightly revised bond strength of 0: (CuAu) = 2.341 + 0.095 eV. In the second investigation of this molecule, Kingcade, Choudary, and Gingerich6 report second- and third-law values of D z (CuAu) derived from measurements of the association equilibrium

Cu + Au = CuAu, (4.1)



as well as from measurements of the displacement equilibrium

CuAu+Au=Cu+Au,. (4.2) The latter measurements of course require a knowledge of the bond strength of Auz, for which the value D z (Au, ) = 2.293 f 0.033 eV was used. Second-law values of Do (CuAu) = 2.307 f 0.081 eV and 2.258 & 0.087 eV were obtained from the equilibria (4.1) and (4.2), respectively.6 In the third-law calculations the values of r,(CuAu) =2.51 A and o,(CuAu) = 250 cm-’ were again u~ed.~ After correcting these values, revised third-law values of 0: (CuAu) are obtained as 2.346 f 0.022 eV [from equilibrium (4.1) ] and 2.339 f 0.039 eV [from equilibrium (4.2) 1. Thus the weighted average of all mass spectrometric measurements of D z (CuAu) is 2.339 f 0,018 eV. Using only the more reliable third-law values, a bond strength of 2.344 f 0.019 eV is obtained. For comparison, our experiments place a rigorous lower limit on DE (CuAu) of 2.272 eV based on the frequency of the 5-O band of the u-X system, which must lie below the dissociation limit, since the CI 3I; + state dissociates to ground state atoms. Moreover, a Birge-Sponer extrapolation of the vibrational levels of the a ‘Z + state places the dissociation limit at 2.41 f 0.07 eV, where the error limit is taken as l/2 of the difference in energy between the highest observed vibrational level of the a ‘I: + state and the predicted dissociation limit. These results are in good agreement with the Knudsen effusion mass spectrometric data [except for the second law value of D 8 (CuAu) derived from equilibrium 4.2, which is certainly too low]. With this in mind, the average of the revised third- law values, 0: (CuAu) = 2.344 f 0.019 eV, is selected as the most reliable measurement of the CuAu bond strength.

This selected value of the bond strength of CuAu may be combined with our measured ionization potential of CuAu (8.74 f 0.05 eV) and the ionization potentials of atomic copper (7.726 380 f 0.000 012 eV) ‘* and atomic gold (9.2257 eV) I9 to evaluate the bond strength of the cation CuAu + as Dz(Cu+-Au) = 1.33 f0.06 eV and DE (Cu-Au + ) = 2.83 f 0.06 eV.

The measured properties of the ground state of CuAu may be compared to those calculated in recent theoretical studies by the NASA Ames group.8*9 The measured bond length of r, = 2.3302 f 0.0006 8, is somewhat shorter than that obtained theoretically (2.39 1 A) .8P9 Likewise, theory estimates the bond strength of CuAu as D g ( CuAu) = 2.145 eV,’ which is 8.5% less than the selected value obtained above of 2.344 f 0.019 eV. The vibrational frequency is theoretically estimated as 230 cm- ‘,* which is again about 8% below the measured value. A calculation of the ionization potential of CuAu gives 8.08 eV,9 which compares badly to our measured value of 8.74 f 0.05 eV. This improves substantially (to give 8.8 1 eV), however, when it is scaled by an empirical factor of 1.09, which is found to be the approximate correction required in similar calculations on the atoms.’ In general, it appears that the present level of ab initio electronic structure calculations on CuAu provides results which are valid to within 10% in vibrational frequencies, bond strengths, and ionization potentials for the ground

state of CuAu, and are correct to within 0.06 8, in predicting the bond length of this state. To our knowledge no calculations of the excited states of CuAu have yet been performed.

B. The 8 38+($k 1) state of CuAu

The a 38 + state of CuAu arises from the interaction of two d lo? atoms to form a d &\d p”a’dC’, 38 + molecular configuration and would not ordinarily be expected to be bound. However, the observation of the O-O band of the a-X system at 17 803.03 cm-’ together with the selected bond strength of D 8 (CuAu) = 2.344 f 0.019 eV places the well depthofthea 32+(0 = 1) stateatD, = 1157 f 150cm-‘. This is very comparable to the well depth estimated for the u ‘2: state of Cu, ( 1250 f 250 cm - ’ ), which has been observed in emission in matrix-isolated Cu, .” The nominally repulsive 38& states of both of these molecules presumably interact with the d ‘, d gdc+’ excited states, which lie only a few thousand wave numbers higher in energy; as a result the 32,& states are lowered in energy, becoming bound by over 1000 cm - ’ in the process. In the case of CuAu some singlet character is gained by this mechanism, making the u-X transition optically accessible in the present experiments. In Cu, such singlet-triplet mixing is much less pro- nounced, with the a ‘Z,+ state of Cu, exhibiting a phospho- rescence lifetime of 27 ms as an isolated molecule in a neon matrix.20

C. The d E, d 1: I?%*’ and d L$ d i, US.*’ excited states of CuAu

The remaining excited states observed in this work all correlate to excited separated atom (or ion pair) limits. Ta- ble XI provides a list of the low-lying separated atom limits which are appropriate for consideration. The three lowest excited separated atom limits correspond to d 9.?, 2D configurations on either the copper or gold atom, and lie 9161, 11 202, and 13 245 cm - 1 above ground state atoms. From each limit only three states arise which are optically accessible by electric dipole transitions from the ground electronic state. These consist of one a’ = 0 + state and two a’ = 1 states. Energetic considerations would suggest that the A 0 + state correlates to the lowest of the excited separated atom limits, which is Cu 3d “4s’, 2S + Au 5d 96s2, ‘D,,, . The B O+ state is then expected to asymptotically correlate to the next separated atom limit, Cu 3d 94?, 2D5,2 + Au 5d “6s’, ‘S. Finally, the D 0 + state probably correlates adiabatically to the Cu 3d 94.?, 2D3,2 + Au 5d loti, 2S limit. As discussed below, however, we believe that it diabatically correlates to the ion pair state Cu + Au -.

The situation for the 0’ = 1 upper states is not so clear, because we should be able to observe six such states deriving from these lowest limits, but have thus far only observed four. It is likely that additional a’ = 1 states lie to the blue of the D 0 + state where we have not scanned. With this possibility in mind, we believe that the A c 1 and A ’ 1 states correlate to the lowest excited separated atom limit of Cu 3d ’04s1, ‘S j- Au Sd 96s”, 2Ds,z. The C 1 and D ’ 1 states should then be correlated to the Cu 3d 94?, *OS,* + Au 5d “6.?, *S limit


5642

TABLE XI. Separated atom limits in CuAu.

Bishea, Pinegar, and Morse: d-hole states of CuAu

Separated atom limit

%-vr Gold Energy’ (cm-‘) case (c) state2 Case (a) states’

al-(‘s,) Cuf(‘S,) d “p’,*P;,, d ‘“p’,2P:,2 d’~,=D,,, d’?,=D,,, d %=,=D J/2 d92,=D,,, d 9’ 9 d iOsl’2S

l/2

d%=‘D,;2= d ‘~,*D,,, d ‘%‘,?$,,

Au+ C’S,, Au-(‘&) d “s’,~S,,~ d ‘OS’ ‘S d Y,;o:n” d?? =D 3/2 d 9s=:2D,,2 d ‘?,=D,,, d’s=,=D,,, d92,=D s/t d ‘OS’ *S d ‘Os”2s

l/2 9 I/2

d ‘OS’,% 1/2

64506 0* 43 69-P 0+ 30 783.69 o*,o-,1(2),2 30 535.30 o+,o-,l 34 680.72 O* (2),0- (2),1(3),2(2),3 32 637.87 0’ (2),0- (2),1(4),2(3),3(2),4 22 406.72 0+ (2),0- (2),1(4),2(3),3(2),4 20 363.86 0+ (3),0- (3),1(5),2(4),3(3),4(2),5 21 435.30 o+,o-,1(2),2

9161.30 O*,O-,1(2),2(2),3 13 245.42 o+,o-,1(2),2 11 202.56 O+,O-,lO),W)J

0.00 o+,o-,l

‘z+ ‘z+ ‘Lz+,Yz+, ‘II, ‘II

1

‘Z+ (3),‘X- (2),‘II(4), w3),w2),‘r, ‘Z + (3),)X - (2),%(4), ‘A(3),%‘(2),T

1 ‘Z+,3x+,‘n, ‘JII, 'A,'A 'x+,'Z+,'n, 'II,'A,'A 'Is+, 5+

*From Ref. 19. b From Ref. 2 1. ’ From Ref. 19 and 22. d From Ref. 18 and 22.

on the assumption that two as yet unobserved Sz’ = 1 states exist which correlate asymptotically to the Cu 3d 94S2, 2D3,2 + Au 5d ‘O&r’, *S limit. With this in mind the experimentally measured values of ri and o: have been used to construct the potential energy curves of Fig. 13.

To test these ideas and provide a more definite assignment of states to the 3d & 5d pUddr’ and 3d ,!!\ 5d i, do*’ molecular configurations, we have considered the possibility of electronic isotope effects. As discussed in the preceding paper, it has previously been observed that excitations changing the number of 3d electrons on a copper atom lead to rather large electronic isotope shifts ( ~0.07 cm-‘) in both the free atom2V23-27 and in Cu, .* The observation of a significant electronic isotope effect may therefore be used as evidence that the electronic transition involves sed promotion on the copper atom. In the preceding paper we have derived the expression for the electronic isotope shift in terms of the measured band origins, v,, _ “,, , vibrational frequencies, w, , and anharmonicities, w,x, , as

+J i) e - ly = v,, _ “II (i) -v,,-“,,(I) + (u” + 1/2)w:‘(i)

x (1 -pii) - (v” + 1/2)%:x:(i)

X(1 -pi) - (v’+ 1/2)w:(i)(l --PO)

+ (u’+ 1/2)*w:x:(i)(l -p;,, (4.3)

wherepq = (pi//L] ) “*, and ,LL~ and pj represent the reduced masses of the iandjisotopic combinations, respectively. Ap- plying this formula to the bands of CuAu which have been accurately investigated at high resolution leads to the calculated electronic isotope effects listed in Table XII, where the la errors in the calculated values of vii) - ~2’ are given in parentheses, calculated on the assumption that the errors in each term of Eq. (4.3) are uncorrelated.

Our original hope had been that the results in Table XII would fall into two categories: states with electronic isotope

effects below 0.01 cm-’ (corresponding to the 3d &$5d 1, c?dri molecular configuration) and states with electronic isotope effects larger than 0.03 cm- ’ (corresponding to the 3d & 5d &lda*’ molecular configuration).

Electronic States of CuAu 6

01 I \/ , I I

1 2 3 4 5 6 7 Internuclear Separation (A)

FIG. 13. Qualitative potential energy curves for the experimentally known electronic states of CuAu. Bond lengths, vibrational frequencies, and electronic energies are taken from Table XII. Potential curves are plotted as Morse potentials, with anharmonicities adjusted to force dissociation to the appropriate separated atom limits. The ion pair curves for Cu + Au - and Cu- Au+ are plotted as dashed lines, without any corrections to account for the polar&ability of one ion in the field of the other, or for Pauli repul- sion at short distances. Owing to its intensity in absorption, its n = 0 + character, its long bond length, and its negative anharmonicity, the D 0 + state is thought to correspond to the ion pair state Cu + Au - , at least on the outer limb of its potential curve. On the inner limb its character may well be covalent, because of possible avoided crossings with the 0 + states arising from the 3d & 5d i\ o%+’ and 3d 2’: 5d 1. ddc’ molecular configurations.



TABLE XII. Measured electronic isotope shifts in CuAu (cm-‘).”

System Band ~3..197 _ p-109 c c System Band p-197 _ p-197

c e

aJZ,'-x5+ o-o 0.0680( 118) l30+-XII:+ o-o 1.1614( 132) 1-O 0.0725( 187) 1-O 0.8705(385) Average 0.0693 ( 100) Average Meaningless

R”l-XII+ O-O 0.0355( 122) cl-x%+ O-O 0.0759( 145) 1-O 0.0833( 386) 2-O 0.0914(683) Average 0.0398(116) Average 0.0766( 142)

R’l-X’S+ O-O 0.0138( 110) D’l-X’Z+ O-O 0.0478( 100) 1-O 0.3258( 121) 2-o 0.0385(341) Average Meaningless Average 0.0471(96)

AO’-Xl);’ o-o 0.1884( 182) DO+-X’I:+ O-O 0.0377( 106) 2-O - 0.8527( 1053) 1-O 0.0528( 155) Average Meaningless Average 0.0425(87)

‘Calculated from the data of Tables I-X and Eq. (4.3), assuming o:I = 249.957 cm-’ and 0:x: = 0.804 cm-‘. These values reproduced the experimental value AG 1;2 = 248.35 cm-’ and give the correct bond strength when used in the expression De = &(&,x,), which is valid for the Morse potential.

This would allow a clean assignment of states to one or the other parentage. This is apparently not the case, since all of the excited electronic states which display electronic isotope effects which are consistent between the two rotationally analyzed bands show rather large electronic isotope effects (0.0398 $- 0.0116 cm - ’ or greater). This suggests that all of the observed electronic states retain significant character of the d-hole states of copper. Presumably the states arising from the 3d & 5d i,&Y’ and 3d c, 5d FUdd*’ molecular configurations are strongly mixed, so that all of the observed states possess significant d-hole character on copper. The resultsfortheA’l-X’Z’,AO+-X’H+,andBO+-X’Z+ systems show unexpected erratic behavior which is outside the error associated with the calculation. Either the assumption of uncorrelated errors in the individual terms of Eq. (4.3) is incorrect for these examples, or isotopically dependent perturbations, arising from high vibrational levels of lower electronic states, must be invoked to explain these per- plexing results. In any case, it seems clear that significant mixing of the 3d &\ 5d ‘,,ad”’ and 3d $5d hO,ddL’ molecular configurations is probably occurring throughout this energy range.

The inability to classify states as uniquely arising from either the 3d &\ 5d “,, 0-W’ or 3d & 5d fU ddc’ molecular configurations suggests that the d-based molecular orbitals of CuAu are delocalized. This in turn implies that, despite the small size of the 3d orbitals in copper, the d orbitals of this system are nevertheless split into bonding and antibonding orbitals. This conclusion explains the near identical bond lengths obtained for the ground state and some of the d-hole excited states of CuAu, since the excited states may be generated by promotion of an antibonding d electron to the sa* antibonding orbital. Such an sdc cd * promotion would tend to preserve the net bond order of the CuAu molecule, and might be expected to leave the bond length relatively un- changed. The possibility that the d orbitals of CuAu may be split into bonding and antibonding orbitals is consistent with recent work on the related diatomic, NiPt, which possesses a bond strength of D “0 (NiPt) = 2.798 eV (0.458 eV greater than CuAu) and a bond length of r;l = 2.208 f 0.002 A

(0.122 A less than r&’ for CuAu) .’ These results clearly dem- onstrate that the d orbitals of NiPt contribute toward the bonding in this molecule, making it likely that the d orbitals in the CuAu analog are split into bonding and antibonding orbitals as well.

D. Ion pair states of CuAu

In the preceding paper we made the case that the B state of CuAg is probably best described as a combination of ion pair states of the form Cu + + Ag - and Cu - + Ag + . Evi- dence for this assignment primarily consisted of the great intensity of the B-X transition, which would be a charge- transfer transition and would possess great intensity if the excited state were accurately described as a combination of these ion pair states. Energetic considerations were used to buttress the argument, since the ion pair potential curves are expected to extend into the energy range where the B state is located.

In the present case of CuAu it is the D 0 + state which exhibits great intensity cf~O.11) in its transitions with the ground state, and accordingly it is this state which should be considered as a candidate for the ion pair state. In addition to the potential curves calculated for the observed electronic states of CuAu, Fig. 13 also displays dashed curves which correspond to the Coulomb attraction of the ground state ions Cu + + Au - and Cu - + Au + . These curves are obtained by adding a - e*/R attractive term to the energy of the separated ion limit. No corrections to include additional attractive interactions resulting from polarization of the ions, or repulsive interactions due to Pauli repulsions of the filled orbitals have been included. As a result the dashed curves display the unphysical behavior of approaching - 00 as R -+ 0, while in reality they would turn up and become repulsive at some internuclear separation. Nevertheless, they do provide an idea of the energetic range in which the ion pair states may be expected to occur. Owing to the high electron affinity and high ionization potential of gold, the Cu + + Au - ion pair curve is considerably lower in energy than is the Cu - + Au + curve. Indeed, it clearly falls in the



region of the observed excited states and must be considered if these states are to be fully understood.

In all of the coinage metal dimers the lowest ion pair limit corresponds to the interaction of a d l’s”, ‘So cation with ad “3, ‘So anion. From this limit only a single ‘B + [or 0 + , in Hund’s case (c) notation] molecular state can arise. Thus the ion pair states should make their presence known through the 0 + states of the coinage metal dimers, but should leave the a = 1 states unaffected. It is no accident that it is the D 0 +-X ‘Z + transition which possesses by far the greatest oscillator strength of all of the observed transitions; it would be greatly surprising to find an R’ = 1 state with comparable oscillator strength in its transitions to the ground state. With the great intensity of the D-X system in mind, it seems clear that the D state should be identified with the ion pair state Cu + Au-. At short internuclear separations, however, it is possible that the ion pair state undergoes an avoided crossing with a state of 0 + symmetry deriving from one of the *D + *S separated atom asymptotes. This would then confer some 3d 5, character on the D 0 + state, thereby accounting for the significant electronic isotope effect observed in the D-X band system. Such an avoided curve crossing could also explain the negative anharmonicity ( - 0.993 f 0.086 cm-‘) and large equilibrium internuclear separation (2.478 f 0.006 A) which are observed for this state. The possibility that the D O+ state possesses significant Cu + Au - character may be tested by measurements of the permanent electric dipole moment of the D 0 + state. We plan to undertake such measurements in the near future.

also possesses a long bond length (2.478 f 0.006 A> and an unusual negative anharmonicity. It is suggested that this state is best described as the ion pair state associated with Cu + Au -, but that an avoided crossing with one of the lower a’ = 0 + states leads to a distortion of the potential energy surface. This then gives a long bond length and a negative anharmonicity. Experiments testing this hypothesis by measuring the dipole moment of the D 0 + state are planned for the near future.

ACKNOWLEDGMENTS

We thank Professor William H. Breckenridge for the use of the intracavity etalon employed in the high resolution studies, and we thank Jeff Bright for his expert help in pre- paring the CuAgAu alloy employed in these studies. We gratefully acknowledge research support from the National Science Foundation under Grant No. CHE-8912673. Ac- knowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.

V. SUMMARY Using the technique of resonant two-photon ionization

spectroscopy, eight band systems of CuAu have been observed and analyzed. The ground state has been shown to be of ‘x+ symmetry, deriving from the interaction of two ground state d ‘OS’, *S atoms to give a 3d &t Sd h”, 2 molecular state with r, = 2.3302 f 0.0006 A and AG r,* = 248.35 cm-‘. The first excited state is the nominally repulsive a 32 + state, which also correlates to ground state atoms, for which the extremely weak a 381+ +X ‘Z + transition has been observed. Analysis of the spectra show the a 32’+ state to possess r, = 2.428 f 0.023 A, w, = 109.23 f 0.58 cm-‘, and a radiative lifetime in excess of 9Ops. Above this state lie four more states with s1’ = 1 and three with R’ = 0 + . Most of these correlate to the 3d ,!!!! 5d%&P’ and 3d ‘& 5d aO,a2dc’ molecular configurations, although it has not been possible to assign definite parentage to the observed molecular states. This suggests that these molecular configurations are strongly mixed in the molecular states, which may imply that the d-based molecular orbitals of this system are significantly split into bonding and antibonding orbitals. This possibility is in line with the results of a recent study of NiPt,’ which shows a significantly greater bond strength and a considerably smaller ground state bond length than found for CuAu.

Finally, the highest energy state observed exhibits a very large oscillator strength for transitions to the ground state. It

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J. Chem. Phvs.. Vol. 95, No. 8,15 October 1991 Downloaded 02 Apr 2001 to 128.110.196.147. Redistribution subject to AIP copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp

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