tit, vt, cot, and cot

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Photodissociation measurements of bond dissociation energies: Tit, vt, Cot, and Cot

Larry M. Russon, Scott A. Heidecke, Michelle K. Birke, J. Conceicao, Michael D. Morse, and P. B. Armentrout Department of Chemistry, University of Utah, Salt Lake City, Utah 84112

(Received 26 October 1993; accepted 14 December 1993)

The bond dissociation energies of Tit, vt, Cot, and Cot have been measured from the sudden onset of predissociation in the photodissociation spectra of these molecules, yielding values of Do(Ti{)=2.435±0.002 eV, Do(Vt)==3.140±0.002 eV, Do(Cot) ==2.765 ±0.001 eV, and Do(Cot)=2.086±0.002 eV. These values are in good agreement with values previously determined from collision-induced dissociation experiments. General criteria for the interpretation of predissociation thresholds as bond dissociation energies and periodic trends in the bonding of the 3d transition metal diatomic neutrals and monocations are discussed.

I. INTRODUCTION

Investigations into the bonding of small transition metal clusters have been carried out for many years now. Part of the interest in this field arises from the complex interplay of opposing forces which are at work in these difficult electronic systems. On one hand, d-orbital contributions to the bonding can strengthen the bond. This effect lessens as one moves left to right across the periodic table because the radial size of the d orbitals decreases significantly relative to the radial size of the s orbitals. On the other hand, there is often an energetic price that must be paid to promote the constituent atoms to an electron configuration that diabatically correlates to ground state molecules. This promotion energy may weaken the adiabatic bond energy relative to the diabatic bond energy.

Significant studies of the electronic spectra of many neutral transition metal dimers have been performed by using resonant two-photon ionization (R2PI) spectroscopy. 1-5 From such studies molecular term symbols, vibrational and rotational constants, and bond lengths have been determined. Bond strengths have been measured by the observation of the sudden onset of pre dissociation in R2PI spectra.5- 7 Knudsen cell mass spectrometry8,9 has also provided bond strength information for many such molecules. Together, all of this information contributes to a better understanding of the bonding in these complicated systems.

With the advent of resonance-enhanced photodissociation (REPD) spectroscopy,10-12 similar studies of transition metal clusters with nonzero charge are now possible. As yet, relatively little rotationally resolved spectroscopy has been performed, severely limiting the types of information that can be obtained. Nevertheless, the bond strengths of charged species have been measured by the observation of predissociation thresholds. IO,I1,13 Such thermodynamic information is also accessible by collision-induced dissociation (CID)14-21 studies, although these two methods do not always agree. For example, Lessen and co-workers10 recorded a REPD spectrum of Crt and observed a predissociation threshold at 2.13 eV, while Su et al. measured the bond strength as Do(Cr{)=1.30±0.06 eV by cm of Crt with Xe.14 It is apparent from this difference that a predissociation threshold is

not always an accurate representation of the true bond strength, a fact also demonstrated by the observation that not all transition metal diatomics exhibit a sharp predissociation threshold.4

We report here the bond strengths of Tit, vt, Cot, and Cot measured from the sudden onset of pre dissociation in REPD spectra. These values are in good agreement with values obtained by Armentrout and co-workers,15-18 again using cm with Xe. From these studies and those performed on neutral species using R2PI, criteria for the assignment of bond strengths to predissociation thresholds have been determined.

The experimental apparatus used in this study is described in detail in Sec. II, and results of the studies of Tit, vt, Cot, and Cot are presented in Sec. III. In Sec. IV, we discuss criteria for the interpretation of a pre dissociation threshold as an accurate measure of the bond strength, and periodic trends in the bond strengths of homonuclear diatomic molecules and monocations in the 3d transition metal series are also examined. A brief summary is presented in Sec. V.

II. EXPERIMENT

This study was performed on a recently completed, jetcooled ion photo dissociation apparatus (Fig. 1). The ion source is a laser vaporization, supersonic expansion similar in design to that used in the R2PI apparatus of the Morse laboratories.22 A metal target disk is rotated and translated against a stainless steel vaporization block23 mounted on a magnetically actuated, double solenoid valve.24 Helium at -10 psig, seeded with a small amount of argon « 1 %), passes through a 3A molecular sieve trap maintained at -78°C (dry ice/isopropanol bath), and is used as the carrier gas. At the approximate peak of the gas pulse, 248 nm radiation (KrF) from an excimer laser (Lambda Physik, EMG 101 MSC, 20-40 mJ/pulse) is focused onto the metal disk by a 47 cm focal length lens. The resulting metal plasma is swept through a clustering region 2 mm in diameter and approximately 3 cm in length. Ion formation in this source is sufficient to preclude the necessity of any secondary ionization. The vapor then expands supersonically through a diverging

J. Chern. Phys. 100 (7). 1 April 1994 0021-9606/94/100(7)/4747/9/$6.00 © 1994 American Institute of Physics 4747


4748 Russon at al.: Bond strengths of Ti;. V;. Co; • and Co;

D

F

F

B C

Source Chamber 2xH)"' Torr

10' Diff. Pump

FIG. 1. Schematic of the ion beam photodissociation spectrometer used in this study. Note that the two flight tubes are shortened to more easily illustrate the details. (A) gate valve, (B) 248 nm excimer laser radiation, (C) pulsed gas valve, (D) two-dimensional turning quadrupole, (E) WileyMcLaren acceleration assembly, (F) einzel lenses, (G) deflector plates, (H) dual microchannel plate detectors, (I) reflectron assembly, (J) tunable dye laser radiation.

nozzle into a low pressure region (=10-4 Torr, Varian VHS-10, 10 in. diffusion pump). The resulting cooling of the internal degrees of freedom is sufficient to produce cobalt atomic ions with several helium atoms attached (Fig. 2).

The metal vapor is collimated by a 6.5 mm conical skimmer as it enters a differentially pumped extraction chamber (-2X 10-6 Torr, 6 in. diffusion pump). Positive ions are extracted using a two-dimensional turning quadrupole.25 In this device, four stainless steel, quarter-circle rods (7.5 cm long, 3.8 cm radius) are placed at the comers of a square stainless steel box (11 cm square, 7.5 em high). Electrodes in opposite

f j '"

120

Mass (amu)

FIG. 2. Mass spectrum of laser-vaporized cobalt entrained in helium carrier gas. For this spectrum, no argon was seeded into the helium. Iron and water are incidental impurities.

comers are held at the same electrostatic potential while those in adjacent comers have opposite applied potentials, referenced to the potential of the surrounding box. Shim electrodes, positioned between the box and the circular electrodes, help to generate the requisite hyperbolic equipotential lines and carry potentials that are intermediate between that of the nearest rod electrode and the surrounding box.

As the various species in the source beam have been accelerated to nearly the full supersonic velocity of the carrier gas, they will be traveling with nearly the same velocity. Because the species have different masses, however, each will have a characteristic translational energy. The quadrupole, being an electrostatic device, affects ions of different energy differently, making it a mass-sensitive device in this application. To overcome this effect, the box surrounding the rods carries a negative voltage (- -180 V), thus accelerating the positive ions as they approach. Because the ions start with only a few electron volts of energy, the addition of 180 eV greatly narrows the relative energy difference between ions of different mass. Defining the potential on the box as Vo, the potentials on the circular electrodes are (1.0±0.8)Vo and those on the shim electrodes are (1.0±0.4)Vo. In this configuration, negative ions are repelled by the potential on the box, neutral species pass through largely unaffected, and positive ions are turned 90°, pass through a single-element, electrostatic lens (maintained at - -1350 V), and enter a Wiley-McLaren time-of-flight (TOF) acceleration region.26

The TOF source consists of two steel tubes 10 cm in diameter and 5 cm long. They have thin metal plates with 1.5 cm holes in the center attached to one end and are assembled with their open ends facing each other, separated by 1 mm. This assembly constitutes the repeller and draw-out plates in the Wiley-McLaren scheme. A third piece of 10 cm diam tube, 3.8 cm long, is left open on both ends and is mounted 1 mm away from the second tube. This shield electrode is held at ground potential. There is no ground plate per se but the first element of an einzel lens, which is kept at ground potential, is about 2.5 cm away from the final element of the TOF assembly. The repeller and draw-out plates are kept at ground potential until the ions fill the intervening region, and are then pulsed to 900 and 750 V, respectively, using a homebuilt circuit with rise times of 800 and 670 ns, respectively. This voltage is maintained for about 8 ms, which is sufficient for all of the ions to be accelerated out of this region. As the experiment is operated at 10 Hz, there is no problem in returning the plates to ground potential before the next experimental cycle. Following acceleration, the ion beam passes through two einzellenses 2.5 and 30.0 cm downstream from the TOF source. In each lens, the outer elements are 0.8 cm thick and the center element is 1.6 cm thick. The outer elements are separated from the inner element by 0.8 cm. The inside diameter of the outer elements is 3.2 cm and that of the inner element is 4.8 cm. The central element of both lenses is held at approximately -1100 V.

The combination of the Wiley-McLaren source and einzellenses allows the ion beam to be brought to a longitudinal and radial focus in the spectroscopy chamber, 1.73 m downstream from the center of the quadrupole. This chamber is pumped by a 6 in. diffusion pump (Edwards 160M diffstak),

J. Chern. Phys., Vol. 100, No.7, 1 April 1994


Russon et al.: Bond strengths of Ti;, V;, Co; , and Co~ 4749

which maintains a pressure of -9X 10-7 Torr. Here the ions are irradiated with the output of a tunable dye laser (Lumonics, HD-SOO) pumped by 308 nm excimer laser radiation (Lumonics, EX-700 running on XeCl). Typically the laser radiation counterpropagates along the axis of the ion beam. For higher resolution work it is necessary to overcome the Doppler broadening inherent in the system. This is accomplished by directing the laser radiation across the ion beam path at right angles. In this arrangement the residual Doppler width is below the laser linewidth of 0.03 cm -1.

As the ions exit the spectroscopy chamber, they pass through another einzel lens and horizontal and vertical deflecting plates before entering a reflectron flight tube. The reflectron is mounted 2.9 m downstream from the center of the quadrupole. The einzellens is identical in design to those previously described and is also maintained at about -1100 V. The stainless steel deflection plates are 2.SX3.2XO.6 cm with matching pairs mounted 6.3 cm apart. The voltage on each set of plates is centered around 0 V. The first set of deflector plates is maintained at about ±20 V in order to change the horizontal flight path of the ions for proper flight through a grid less reflectron assembly. The reflectron is centered in the second flight tube, offset from the first flight tube by 2.86 cm, so that the deflection angle is slight (1.6°). The second set of deflector plates changes the vertical flight path to adjust for any vertical misalignment of the apparatus and is usually operated in the range of ±5 V.

The reflectron consists of a stack of 19 stainless steel plates with an outside diameter of 13.3 cm, an inside diameter of 8.9 cm, and a separation of 5.0 mm. The first plate is 6.35 mm thick and is shorted to ground. The 2nd through 17th plates are 0.5 mm thick and are connected in series by 100 kn resistors. The 17th through 19th plates are shorted together with a mesh on the 17th plate to prevent penetration of ground potential into the reflectron. A potential of approximately 400 V is applied to the second plate and approximately 950 V is applied to plate 17 for reflection of the full ion beam. In order to separate undissociated parent ions from fragment ions, the voltages in the reflectron, on the third einzel lens and the deflector plates are reduced by the ratio of the mass of the fragment to the mass of the parent (e.g., in the case of a homonuclear dimer, 1/2). Fragment ions are then detected at the same time of flight as unfragmen ted parents would be at full voltage on the reflectron. Reflected ions are detected by a dual microchannel plate detector (Galileo, 3025). Any ions that are not reflected may be detected by another dual microchannel plate detector (Hamamatsu, F2221-21) mounted directly behind the reflectron. The signal is amplified (Pacific Instruments, model 2A50 video amplifier, X 100 gain, 150 MHz) and digitized by a 40 MHz digital oscilloscope (Markenrich Corp., WAAGII) mounted in a 386-based personal computer (Zeos, 386-20/8). The data are then summed and stored for later analysis.

Predissociation thresholds were obtained by monitoring fragment ion signal intensity as a function of dye laser frequency. The purpose of this study was to determine the bond strengths of the molecules investigated, so only those regions of the spectrum where a threshold was expected were

Tii- Ti+ + Ti

FIG. 3. The predissociation threshold ofTi; detected in a photodissociation action spectrum. This spectrum was obtained by scanning the dye laser using coumarin 500. The arrow marks where a congested spectrum rises abruptly out of background noise at 19640 cm-I, giving a bond strength of Do(Ti;)=2.435±O.002 eV. The line across the top of the arrow indicates the uncertainty.

scanned. In each case, a congested spectrum arose out of background noise apparently caused by collision-induced dissociation of the metal cluster ions with residual helium or pump oil molecules. The spectra were calibrated by collecting an 12 absorption spectrum, while simultaneously collecting the experimental data, and comparing it to the 12 atlas of Gerstenkom and LuC.27 Where the laser radiation was to the blue of the 12 atlas, a high-pressure H2 cell was used to Raman shift the light back into the range of the atlas. At 500 psig, H2 gives a precisely known Q(1) Raman shift of 4155.162 cm- I .28 As a final correction, the energy scales of the spectra were shifted to correct for the Doppler shift experienced by the ions as they approached the source of the radiation. This correction ranged from 1.68 cm- I for Cot to 3.33 em-I for vi.

III. RESULTS

A. The bond strength of TI~

Figure 3 shows a strong, abrupt increase in the Ti+ fragment signal at 19640 cm- t. This is assigned as the pre dissociation threshold with the uncertainty of ± 15 cm -1 arising from the difficulty of precisely determining the threshold. Weak, broad features observed below the threshold are the result of periodic fluctuations in the parent ion signal that are associated with the rotational period of the sample rotary drive mechanism. These source fluctuations contribute to an uncertainty in the measured threshold. The threshold of 19 640±15 cm- I (2.435±0.002 eV) is in good agreement with the CID bond strength measurement of Do(Tii)=2.37±0.07 ev,t5 and we therefore assign this threshold as the bond strength of this molecule.

In a spectroscopic investigation of Ti2, t DoverstiH et al. report a lower limit to the Ti2 bond strength as Do(Ti2);;;.1.349 eV. By using the thermochemical cycle,

D~(M2) + IE(M)=D~(Mi) + IE(M 2), (1)



4750 Russon et al.: Bond strengths of Ti~, V~, Co~ , and Co;

f J

FIG. 4. The predissociation threshold of V; detected in a photodissociation action spectrum. These data were collected while scanning the dye laser using exalite 398. The arrow indicates where the predissociation threshold arises at 25 326 cm-I, giving a bond strength of 3.140:t:0.OO2 eY. The line across the top of the arrow indicates the uncertainty.

this lower limit on the dititanium bond strength, in combination with our measured value of the Tii bond dissociation energy and the ionization energy of titanium, IE(Ti) =6.828 12±0.000 04 eV,29 provides a lower limit on the ionization energy ofTi2 as IE(Tiz)~5.742 eV. This may be combined with the upper limit on the ionization energy of Ti2 measured by Doverstal et al., IE(Ti:z)~6.125 eV, to provide IE(TI:z)=5.93±0.19 eY. The same thermochemical cycle then provides Do(Ti:z)=1.54±0.19 eV.

B. The bond strength of V~

The photodissociation spectrum of vi shown in Fig. 4 is relatively weak, making it difficult to pinpoint the predissociation threshold. Nevertheless, an abrupt increase in the V+ fragment signal is evident at 25326±15 cm-I, providing Do(Vi)=3.140±0.002 eY. This value is in excellent agreement with the cm measurement of Do(Vi)=3.13±0.12 e V.16 Given the bond strength of V 2 measured from a predissociation threshold in a R2PI spectrum, Do(V:z)=2.753±0.001 eV,6 and IE(V)=6.740±0.OO2 eV.30 Equation (1) provides an ionization energy for divanadium of IE(V:z)=6.353±0.003 eV. This compares very favorably with a recent direct measurement of IE(V2)=6.356±0.001 eV,31 providing strong evidence that predissociation occurs at the thermochemical threshold in V 2 and vi. Employing a more recent, highly precise value for the ionization energy of atomic vanadium, IE(V)=54411.7±0.1 cm-1 (6.74619 ±O.OOO 01 eV),32 the ionization energy predicted for V2 using Eq. (1) and the bond energies of V2 and vi is IE(V2)=6.359±0.002 eY, which remains within experimental error of the directly measured value.

C. The bond strength of Co~

In the predissociation threshold of Coi, 11 there is an abrupt rise in the Co + fragment ion signal at 22 300 cm -1 , and above this energy the spectrum is very congested, as shown elsewhere. ll Although most of the features above this energy are reproducible, no assignment was attempted be-

Co:) -Coi + Co

T f ,g

J

FIG. 5. A photodissociation spectrum of Co; collected by monitoring the Co; signal intensity while scanning the dye laser using a 7:3 mix of rhodamine 590 and rhodamine 610. The arrow marks the predissociation threshold at 16825 em-I, giving a bond strength of 2.086:t:0.002 eV. The line across the top of the arrow indicates the uncertainty.

cause of the complexity of the spectrum. The predissociation threshold was determined to lie at 22 300±5 cm-l, providing Do(Coi) =2.765 ±0.001 eV. The lower uncertainty limit in this example reflects a sharper, more precisely defined threshold. This bond energy again compares very favorably with the cm value of Do(Coi)=2.75±0.1O eVP

D. The bond strength of Cot

To illustrate the extension of this technique to larger clusters, the dissociation energy of Co; was also investigated. Figure 5 displays the pre dissociation threshold for Co; dissociating to Coi +Co. The fragment Coi ion signal measured in this experiment is slightly weaker than that for vi because of the lower Co; parent ion signal. A second difficulty in recording this predissociation threshold results because it occurs in a spectral region where no single dye lases efficiently. Figure 5 was obtained by scanning light output from a 7:3 mixture of rhodamine 590 and rhodamine 610 laser dyes, and as a result only covers a limited range. The threshold and bond strength are assigned as 16825±15 cm-1=2.086±0.002 eV. This again agrees well with the cm value of Dij(Coi -Co)=2.04±0.13 eV.lS

IV. DISCUSSION

A. Assignment of predissociatlon thresholds as bond strengths

In addition to the molecules presented here, the observation of predissociation thresholds in R2PI experiments have yielded the bond strengths of TiV,6 TiCo,6 V2,6 VNi,6 Ni2,7 NiPt,2 Pt2,3 and AlNi.33 Other transition metal systems, NiPd4 and PdPt4 for example, do not exhibit sharp predissociation thresholds. From this information, Spain and Morse6

proposed that in order to interpret a predissociation threshold as a measure of the true bond strength certain factors must hold. First, the molecule must have a large density of electronic states near the ground separated atom limit, and second, this separated atom limit must generate repulsive curves. Table I lists the number of relativistic, adiabatic



Russon et al.: Bond strengths of TI;, V;, Co; , and Co~ 4751

TABLE I. Number of adiabatic Hund's case (c) potential energy curves evolving from separated atom limits ;;;; 10 000 cm -I above the ground separated atom limit and bond strengths of selected diatomic transition metal molecules.

Number of Molecule curves Do, eV

Ti; 5114 2.435" TtV 2186 2.068b

TiCo 1388 2.401b

V{ 4772 3.140" V2 2711 2.753b

VNi 1516 2.100b

Cr; 222 1.30<

"This work. bReference 6. <Reference 14. dReference 44.

[Hund's case (c)] potential energy curves that arise within 10 000 cm -I of ground state atoms for selected transition metal diatomic neutrals and cations. Clearly the ions that are the subject of this report have a very large number of lowlying electronic states, thus fulfilling the first requirement stated above.

From the ground states of Ti (4S23d2 ,3F) and Ti+ (4s3d 2,4F),34 588 curves are generated. Of those, 441 correlate diabatically to s(}';s(}'~d1C F)d~C F) attractive curves while the remaining 147 curves correlate to s(}'!s(}'~dleF)d~CF) repulsive curves. Thus, Ti; strictly follows both of the guidelines outlined above. In the case of V; , however, all of the 350 curves evolving from the lowest separated atom limit of V (4s 23d3, 4F) + V+(3d4, 5 D),3o correlate diabatically to s(}';d{(4F)d~CSD) attractive curves. This diabatic correlation for both Ti; and V; is based on guidelines set forth by Armentrout and Simons35 which consider the 4s bonding alone. It neglects any contributions of the d orbitals to the bond and is therefore most useful for predicting long range attractive or repulsive behavior. The more efficient mixing of wave functions among crossed curves as compared to nested curves might explain why the predissociation of V; does not show as sharp a threshold as does Ti; . However, Co; has a nearly identical pattern of diabatic correlations as V;' Ground state Co (4s23d7, 4F) and Co + (3 d8

, 3 F) 36 combine to generate 588 curves, all of which correlate to s(}'~dl (4 F)d~ C F) attractive surfaces. Within the first 10 000 cm -I above the ground state separated atom limit, there are fewer curves generated than for V; (see Table I), yet the Co; threshold is the strongest of those presented in this paper. Figure 6 displays a qualitative depiction of the predicted pattern of potential energy curves evolving from the first several separated atom limits of Co and Co + to form Co; . The number of curves is calculated for the various combinations of separated atom terms; the curves themselves are drawn to approach the lowest spinorbit level of each separated atom limit. The bands drawn represent the rich density of states formed from these atoms. These are drawn assuming no d-orbital interaction, as is appropriate for transition metal atoms from the right side of the 3d transition series.

The second rule developed by Spain and Morse6 con-

Number of Molecule curves Do, eV

Co{ 3458 2.765" Ni2 960 2.042d

NiPd 291 NiPt 630 2.798e

PdPt 159 Pt2 392 3.14f

AlNi 123 2.29g

eReference 2. fReference 3. gReference 33.

cerned repulsive curves, and was introduced to account for the lack of strong pre dissociation thresholds in the spectra of NiPd4 and PdPt.4 Like vt and Cot, these are molecules that generate only attractive curves from their lowest separated atom limits. As shown in Table I, NiPd and PdPt have a much lower density of states than Cot or V; . This occurs because the high stability of the ISO(d IO) ground term of palladium greatly limits the density of electronic states in NiPd and PdPt. The handicap of having no repulsive curves evolving from the ground separated atom limit may apparently be overcome in the cases of Co; and V; because there is a sufficient density of electronic states. One might ask whether repulsive curves are required at all because the NiPd and PdPt molecules may have lacked a sufficient density of states to fulfill the first requirement. However, another molecule studied by R2PI, AlNi,33 has fewer electronic states than either NiPd or PdPt (see Table I), but nevertheless displays a sharp predissociation threshold, presumably because of repulsive curves evolving from the Al (S2pI,2po)

u!d1d l - 294 curves Co + Co'

u!~d'd' - 588 curves __ 'F(s'd') + 'D(sOd')

~4::;;2e= :::::=::==-- 'F(s'd') +'F(s'd')

'F(s'd') +'F(sOd')

~F~====::::::::=---'F(S'd')+'F(s'd') ~~::Z:;;--·---__ ·F(s'd')+'F(sod·)

'F(s'd') + 'F(s'd')

~===""---'F(s'd')+3F(sOd')

';d'd' - 588 curves

FIG. 6. Qualitative potential energy curves for Co{ illustrating the enormous density of states. The shaded bands represent the great number of potential curves deriving from each separated atom asymptote. The different shading patterns are to distinguish states with an even number of s electrons from states with an odd number of s electrons. The 4F(sld8)+3F(sDd8) and the 4F(S2d7) +5F(Sld7) states are nearly, but not exactly, degenerate.

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4752 Russon at al.: Bond strengths of Ti~, V~, Co~ , and Co;

+ Ni( s 1 d9, 3D) limit. Repulsive curves evolving from the

ground state asymptote can allow efficient predissociation in those systems where the density of states is too low for predissociation among nested curves to occur to an appreciable degree.

In the case of Cr~ ,10 the predissociation threshold is not as abrupt as those presented here and there are distinct features in the signal above the threshold that suggest there may be Franck-Condon difficulties in either the excitation or the predissociation step. Additionally, assuming a 2I; ground state with an sO';,d lO configuration evolving from ground state Cr(4sl3d5, 7S)+Cr+(3d5,6S), the first strongly allowed optical transition in Cr~ is to a state that correlates to the Cr( 4S13d5, 5S) +Cr\3d5, 6S) excited separated atom limit, 0.94 e V37 above the ground separated atom limit. When the photodissociation threshold is corrected by this energy, it is in quantitative agreement with the cm result. l4

With these ideas in mind we revise and extend the Spain-Morse rules as follows. A predissociation threshold may be inferred to correspond to the bond dissociation energy if (1) The threshold is sharp and well defined, without evidence of Franck-Condon difficulties in either the excitation or pre dissociation step; (2) Dissociation can occur to the ground separated atom limit while preserving the good quantum numbers, such as 0, g/u, and ±; (3a) Either the ground separated atom limit generates a suitable number of molecular potential energy surfaces, some of which are repulsive; or (3b) The ground separated atom limit generates a sufficiently large number of attractive molecular potential energy surfaces to allow weaker predissociation processes to dominate.

B. Electronic configurations

1. Tit Ti2 is known by R2PI spectroscopyl to have a 3~g,r

ground state indicating that the dO' g orbital is mixed with the SO' g orbital to the extent that it lies higher in energy than the d 7T u orbitals, resulting in an S O'~d 7T~d O'!d o! configuration. The removal of one electron in order to form the ion leads to either a 2I;(sO'~d7T~dO'!) or a 2~g(sO'id7T~do!) ground state. Knight38 has performed electron spin resonance spectroscopy on a sample believed to contain Ti~ but no spectrum was observed, making 2I; an unlikely assignment for the ground state. This is consistent with the idea that in the transition metal cations the d orbitals are stabilized relative to the s orbital, enhancing the interaction between the sO' and dO' molecular orbitals to the point that the dO' g orbital lies above the dOg orbital in energy. Assuming a 2 ~g,r ground state for Ti; , the ground level will possess 0=3/2. Allowed electric dipole transitions can then access ungerade states with 0=1/2, 3/2, or 5/2, all of which may be produced by combining a ground state titanium atom e P z} with a ground state titanium ion (4p 3rz). As a result, Ti~ can dissociate to ground state atoms while preserving its value of 0 and g/u symmetry. The 3 P( 4sz3d2) and 4p( 4s3d2),34 ground states of Ti and Ti +, respectively, combine diabatically to form Ti; with a sO'isO'~d1ep)d1ep) configuration. On the other hand, combination of a ground state titanium atom, 3F(4sz3d2 ), with the 4F(3d3), excited state of Ti+, which

lies only 907.96 cm- l higher than the ground state of Ti+, should diabatically form Ti~ without an sO': antibonding electron, resulting in a stronger bond with an sO'id1ep)d~(4F) configuration and more d-orbital electrons available for additional bonding. The ground state of titanium dimer cation is therefore expected to arise from this excited separated atom limit.

2. vt From R2PI data5 the ground state of V 2 is known to be

3I; arising from an SO'~d7T~d~d8~ configuration. Because of s 0'-d 0' hybridization it is likely that the dO' g orbital lies between the d 7T u and the dOg orbitals in energy. Removal of one electron to form the cation will most likely result in a 4I; state, regardless of the relative energies of the dO' g and dOg orbitals, as long as the dO' g orbital lies above the d 7T u orbitals, which is very probable. Van Zee and Weltner39 have determined the ground state of the isoelectronic neutral molecule, TiV, to be 4I -, further supporting the 4I; ground state assignment for V~. From this state 0= 1/2g and 3/2g are generated, which can be excited to O:o;.;;5/2u states under dipole selection rules. The ground separated atom limit of V(4s23d3,4F3IZ)+V+(3d4,5Do)30 yields Hund's case (c) states of 0= 1/2u and 3/2u, allowing predissociation of 0= 1/2u and 3/2u states while preserving the values of 0 and g/u. The 0=5/2u states, which may be excited from the 4I;(3/2g) would be expected to provide a second predissociation threshold in this experiment. Our inability to observe a second threshold may indicate complete cooling to a ground, 4I;(l/2g) level. The ground state of vanadium atom is 4F( 4s 23 d3) and that for the vanadium atomic ion is 5D(3d4).30 These may combine to form an S~d~(4F)d~(5D) bond, plus contributions from the d-orbital electrons. This is consistent with the postulate of an sO'~d7T~dO'~d~, 4Ig ground state for V;.

3. Cot

The ground state of Co~ has been determined from ESR spectroscopy40 to be 6I. Possible 0 values are 1/2, 3/2, or 5/2. An electric dipole transition from the ground state can then access states with 0=1/2, 3/2, 5/2, or 7/2. All of these o values are generated by the combination of a ground state cobalt atom (4S23d7, 4 F 912) with a ground state cobalt ion (3 dB, 3 F 4) so there is no O-based restriction to prevent the states reached by the optical excitation of ground state Co~ from predissociating to ground state atoms. Because cobalt occurs later in the transition metal series where the 3d orbitals are quite contracted, the 3d orbital contributions to the bonding in Co; should be greatly reduced as compared to Ti; and V; . This effect may be mitigated to some extent by the contraction of the s orbital, which provides greater s-electron density between the nuclei for bonding. The 4F( 4s23d7) and 3 P(3dB)36 ground states of Co and Co+, respectively, are perfectly set up for the formation of an s~d~CF)d~eF) bond by the donation of the two s electrons of Co into the empty s orbital of Co + .



Russon et al.: Bond strengths of Ti;, V;, Co; , and Co; 4753

C. Promotion energies

In order to understand the bonding among transition metal dimers, it is important to consider the expected extent of d-orbital interactions and the magnitude of the promotion energies involved. In many cases, the ground state configuration of the molecule does not correlate diabatically to ground state atoms. Such promotion effects often occur in the neutral species where the atomic ground state configuration is usually s2dn. Notable exceptions along the 3d transition metal series are Cr(sld5

) and Cu(sld IO ). Atoms with s 1 dn

+ 1 configurations are expected to form a strongly bound s O"~dn 1-1 dn + 1 molecular configuration. In the case of the monocations, the usual ground atomic configuration is sOdn+ 1 which will combine readily with the usual ground state neutral atom (sZd") to form an sO"~dn+ldn molecular configuration. Scandium, titanium, manganese, and iron are the exceptions among the 3d atomic cations, having sld" ground state configurations. In each of these metals, it is energetically more favorable to promote the ion to an sO dn + 1

configuration than to promote the neutral to an s 1 d" + 1 configuration. The homonuclear dimer cation with an SO"idn+ Idn configuration is then formed from M(S2dn ) + M+ (sodn+ I). For this discussion, we define the promotion energy as the energy required to promote each atom from the lowest J level of the ground state to the lowest J level of the lowest lying excited state having the appropri-

I fi ·.,.,· f 2dn + 1dn+1 ate e ectron con guratlOn lor lormatlOn 0 an sO" g

molecular configuration for neutrals, or an S O";dn + 1 dn molecular configuration for ions. The diabatic bond strength will accordingly be defined as the measured, adiabatic bond

TABLE II. Bond strengths and promotion energies of diatomic transition metal ions and neutrals. All values in eV.

Molecule Measured Do • E I.b p Diabatic Do a.b

Ti; 2.435(2)< 0.113d 2.548(2) Tiz 1.54(19)< 2XO.813d 3.17(19)

Vi 3.140(2)< 3.140(2)

V! 2.7530/ 2XO.262g 3.277(1) Cr; 1.30(6)h

Crz 1.443(56)i 1.443(56) Mn; ;;'1.4i

Mnz 0.3(3)k

Fe; 2.74(10)1 0.232m 2.97(10) Fcz 1.14(1)" 2XO.859m 2.86(1) Co; 2.765(1)< 2.765(1) CO2 0.7-1.4° 2XO.432P 1.6-2.3 N·· 12 2.08(7)Q 2.08(7) Niz 2.042(2), 2XO.025' 2.092(2) CU2 1.84(8)'

Cuz 2.03(2)" 2.03(2)

'Uncertainties in the final digits are ICReference 47. given in parentheses. IReference 19.

hSee the text for definition. mReference 48. '111is work. "Reference 49. dRefercnce 34. OSee the text. <See the text. PReference 36. 'Reference 6. QReference 20. 'Reference 30. 'Reference 44. bReference 14. 'Reference 50. 'Reference 45. 'Reference 51. IReference 46. "Reference 52.

Ti VCr MnFe Co Ni Cu

Metals

FIG. 7. Measured (open symbols) and diabatic (filled symbols) bond strengths of neutral (squares) and cationic (triangles) homonuclear diatomic transition metal molecules from the data listed in Table II. See the text for definition of diabatic bond strength.

strength plus the promotion energy. These concepts are discussed further in a review of CID data by Armentrout et al. 41

Table II lists the adiabatic and diabatic bond strengths and promotion energies, E p , of all the homonuclear diatomic neutrals and mono cations in the 3d transition metal series from Ti to Cu. These data are also represented graphically in Fig. 7. The diabatic bond strengths of Cr;, Mn;, Mnz, and Cur are not represented because they are not expected to have sO";sO"~ ground state configurations. Instead, sO";sO"! and sO";sO"~ ground configurations are expected for Mnz and Mn2, respectively, and an SO"i ground configuration is expected for Cri 14 and Cui. While this configuration for Cri has not been proven conclusively, it does correctly predict the observed predissociation threshold for Crt reported by Lessen et aI., 10 as previously discussed. 14

D. Periodic trends

Reviewing Table II and Fig. 7 there are several observations concerning the bonding that can be made. (A discussion including larger clusters in the 3d series, and Nb and Ta can be found in the review by Armentrout et al.)41 The diabatic bond strengths of Tii and Ti2 are lower than those for V; and V 2, suggesting an increase in the bonding among the d orbitals in the vanadium dimers, which place more d electrons in bonding orbitals than do the titanium dimers. The lower diabatic bond strength of titanium and vanadium cationic dimers compared to their neutral counterparts is the result of the removal of an electron from a bonding d molecular orbital. This phenomenon is reversed for the iron and cobalt dimers, where the electron is removed from an antibonding d molecular orbital.

The diabatic bond strength of CO2 plotted on Fig. 7, taken as the midpoint of the range of values listed in Table II, seems anomalously low. The lower limit of this range is the lower limit of a bond strength for CO2 calculated by Shim and Gingerich.42 The upper limit is derived from an upper limit to the ionization energy of Co2 , IE(Coz):O:;;6.42 eV



4754 Russon et al.: Bond strengths of li;, V;, Co; , and Co;

which was established by noting that COz is readily ionized with 193 nm ArF laser radiation.43 The COz bond strength is thus easily calculated through Eq. (1), using IE(Co)=7.864 eV.36 Based on a comparison to similar molecules as shown in Fig. 7, it appears that the true bond strength of COz is probably closer to the upper limit and that the true ionization energy of COz is probably close to the 6.42 e V upper limit.

The diabatic bond strengths of the Fez, Fet, and Cot molecules are higher than those of Ni2, Ni;, and CU2' Copper dimer neutral, having a full 3d subshell, is representative of a transition metal diatomic molecule with no d-orbital contributions to the bonding. As the nickel dimer species have similar bond strengths, they also appear to have no d -orbital bonding. The higher diabatic bond strengths for Fe;, Fez, and Cot seem to indicate that d-orbital interactions may be occurring in these species. The diabatic bond strengths of Fez and Fei are slightly higher than those of COz and Coi, consistent with both greater d-orbital interactions and higher d orbital bond orders in the iron species.

v. SUMMARY

The sudden onset of pre dissociation in the resonanceenhanced photodissociation spectra of Tit, vi, Coi, and Co; has been observed and is interpreted as providing the thermochemical bond strength of these molecules. These values, Do(Tii)=2.435±O.002 eV, Do(Vi)=3.140±O.002 eV, Do(Coi)=2.765±O.OOl eV, and Do(Co;)=2.086±O.002 eV, are in good agreement with values obtained from collisioninduced dissociation experiments. The present measurements are the most precise currently available for these molecules. Combined with auxiliary data for Ti2 and V2, these results give Do(Tiz}=1.54±O.19 eV, IE(Tiz)=5.93±O.19 eV, and IE(V2)=6.359±O.002 eV. This last result is in excellent agreement with a recent direct measurement of IE(V2)=6.356±O.OOl eV.31 Together with the bond strengths of several neutral diatomic species measured from predissociation thresholds in resonant two-photon ionization spectroscopy, the data presented here illustrate the general applicability of this technique to transition metal cluster cations that meet certain criteria which are discussed in detail. The addition of the triatomic metal cluster cation, Co; , demonstrates the extension of this technique to larger clusters. Comparisons of diabatic bond strengths for homo nuclear diatomic molecules and monocations in the 3d transition metal series from Ti to Cu are made. Periodic trends in the metal dimer bond energies show that d-orbital interactions play major roles in the bonding of the early transition metal diatomics, with the importance of these roles decreasing later in the series.

ACKNOWLEDGMENTS

This research is funded by the Department of Energy, Office of Basic Energy Sciences (P.B.A.); the National Science Foundation under Grant No. CHE-9215193 (M.D.M.); and partial support is provided by the Donors of the Petroleum Research Fund, administered by the American Chemical Society (M.D.M.). Funds used to purchase the excimer-

pumped dye laser system employed in these experiments were provided by the National Science Foundation under Grant No. CHE-8917980 (P.B.A./M.D.M.).

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J. Chern. Phvs .. Vol. 100. No.7. 1 April 1994

tit, vt, cot, and cot

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