Visible Spectra of Lithium in InertGas MatricesLester Andrews and George C. Pimentel Citation: The Journal of Chemical Physics 47, 2905 (1967); doi: 10.1063/1.1712314 View online: http://dx.doi.org/10.1063/1.1712314 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/47/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Xenon excimer compounds with oxygen in inert-gas crystal matrices Low Temp. Phys. 27, 938 (2001); 10.1063/1.1421460 Ultraviolet Fluorescent and Absorption Spectra of S2 Isolated in InertGas Matrices J. Chem. Phys. 44, 3274 (1966); 10.1063/1.1727224 Infrared Spectra of the Lithium Halide Monomers and Dimers in Inert Matrices at Low Temperature J. Chem. Phys. 41, 463 (1964); 10.1063/1.1725890 Theory of InertGas Fluorides J. Chem. Phys. 38, 1783 (1963); 10.1063/1.1776953 InertGas Compounds Phys. Today 16, 94 (1963); 10.1063/1.3050838

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THE JOURNAL OF CHEMICAL PHYSICS VOLUME 47. NUMBER 8 15 OCTOBER 1967

Visible Spectra of Lithium in Inert-Gas Matrices

LESTER ANnREWS* AND GEORGE C. PIMENTEL

Chemistry Department. University of California, Berkeley, California

(Received 16 May 1967)

A careful study of the electronic spectrum of lithium atoms deposited in solid argon, krypton, and xenon showed that isolation of the incident lithium atoms improved with increasing matrix atomic weight, de­creasing lithium-atom concentration, and decreasing deposition temperature and that dimerization of the incident atoms proceeds to a major extent except in very dilute samples of xenon deposited at 4°K. The lithium atomic absorptions showed multiplet structure similar to those reported earlier for metal atoms in inert-gas matrices. The multiplet components varied in relative intensity on sample warming. Nonnearest­neighbor metal-metal atom interactions were shown to be likely contributors to the observed spectral features. The present work indicates the importance of thorough study of concentration and sample­temperature dependence before multiplet splittings can be interpreted.

INTRODUCTION

As necessary background information for the re­action of lithium atoms with reactive molecules in inert-gas matrices, a careful study of lithium atoms in inert-gas matrices was undertaken. The object of the present investigation was to determine the effects of lithium concentration, type of matrix, and deposi­tion temperature upon the isolation of lithium atoms in the matrix. A number of authors have observed com­plicated spectra for trapped atoms which they dis­cussed in terms of matrix perturbations (Na,1·2 K,1·2 Rb,1 Cs,1 Mg,3 Ht) and Jahn-Teller distortions (Na,5 Hg5). In this earlier work, less emphasis was placed on concentration-dependent effects and these changes, when observed, were not explained.

EXPERIMENTAL

The apparatus and experimental technique used have been described in detail by Andrews and Pimente1.6

The experimental conditions in ten experiments are tabulated in Table 1. Reagent-grade lithium (Lithium Corporation of America) was used in Expts. 1-8 and 99.99% 'lLi (ORNL) in Expts. 9 and 10, both without purification. Prior to use, argon (Linde, 99.99%) was passed over copper turnings at 450°C and through a coil at liquid-oxygen temperature at the rate of 10 cc·STP/min. Krypton (rare gas, 150 ppm Xe) and xenon (rare gas, 15 ppm Kr, 4 ppm N2, 3 ppm He) were transferred from cylinders through Pyrex fittings and used without purification.

With the deposition of matrix gas in progress, the Knudsen cell was warmed to the desired temperature,

and the shutter was lifted, allowing the lithium-vapor stream to be condensed along with the matrix gas. Spectra were recorded on a Cary 14 spectrophotometer during and after sample deposition. Deposition was stopped when the atomic absorptions reached an opti­cal density near one. The samples were highly scatter­ing so that one to six screens (average transmission of 32% per screen) were required in the spectrophotometer reference beam. The window temperature was moni­tored with a 10% gold-cobalt vs copper thermocouple. Spectral slitwidths were 5 A near 6500 A and 1 A near 5000A.

RESULTS

Variation of Matrix Concentration and Deposition Temperature

The spectra of lithium atoms in the 4000-7500-A region in argon, krypton, and xenon matrices deposited at 4°K with MjR~1.8X104 are contrasted in Fig. 1 (M = moles matrix, R=moles lithium). No absorp­tions appear between 3000 and 4000 A, and the samples are opaque below 3000 A. The structures in the region 6800-6400 A are assigned to the 2p+-2S lithium atom transition which appears at 6708 A in the gas phase. The broader, weaker absorptions near 5000 A corre­spond to the lIIu+-l 2:g + lithium molecule transition at 4890 A in the gas phase. The observed absorptions and wavelength accuracies are listed in Table II.

Ratios of integrated absorption intensities for the lithium atom and molecule transitions are listed in Table I for various deposition conditions. All of the Knudsen-cell temperatures provide equilibrium-vapor

* Present address: Department of Chemistry, University of compositions containing less than one percent lithium Virginia, Charlottesville, Va. A490 molecule. In Expt. 1, a broad band peaking at 5150 A (1~~). Weyhmann and F. M. Pipkin, Phys. Rev. 137, was recorded, but only the 4980-A band was observed

2 B. Meyer, J. Chern. Phys. 43,2986 (1965). in other krypton experiments with higher MjR and '0. Schnepp, J. Phys. Chern. Solids 17,188 (1961). lower deposition temperature. 4 L. Brewer, B. Meyer, and G. D. Brabson, J. Chern. Phys. 43, Al h h h b' b d'

3973 (1965). t oug t e same a sorptlOns are 0 serve III 6 M. McCarty, Jr., and G. W. Robinson, J. Mol. Phys. 2, spectra of krypton, Expts. 3 and 7, deposited at 15° and

415 (1959). 4°K and MjR's of 3300 and 1.8XI04, respectively, the 8 W. L. S. Andrews and G. C. Pimentel, J. Chern. Phys.44,

2361 (1966). absorption contours differ markedly, as shown in Fig. 2. 2905

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2906 L. ANDREWS AND G. C. PIMENTEL

TABLE 1. Isolation of lithium atoms in inert-gas matrices; Tw is the window temperature, TK is the Knudsen-cell temperature.

Tw TK Expt. Matrix (OK) (OC)

1 Kr 20 460 2 Ar 15 430 3 Kr 15 430 4 Ar 4 430 5 Kr 4 430 6 Ar 4 390 7 Kr 4 390 8 Xe 4 367 9 Xe 4 319

10 Xe 4 279

a Kr deposited at 1.0 mM/h, other rates; Expts. 2-7, 4,5 mM/h; Expts. 8-10, 1.6 mM/h, These values of M/R are calculated assuming the rate of deposition is the same when codeposited with argon as when deposited alone (see Ref, 6), If argon scatters a fraction of the lithium, the values of M / R will be proportionately raised. This will, In turn, change the deduced

The M/R variation in xenon for Expts. 8, 9, and 10 had no apparent effect on the relative integrated ab­sorption intensities of the identical components of the multiplet.

Warming of Deposited Sample

Spectra were recorded over a 20 _30 temperature range as the sample slowly warmed from deposition temperature after the coolant evaporated. The argon matrix spectrum shown in Fig. 1 sharpened somewhat into a multiplet during warming. When the krypton sample in Expt. 7 was warmed to 47°K and recooled to 4 OK, the spectra illustrated in Fig. 3 were recorded. Aside from the general loss of intensity, note the dis­tinct loss during warming of multiplet components a and b while the components d and e became more clearly resolved and their intensities increased relative to Component c. Note also that recooling to 4°K did not restore the original structure but it did shift Com­ponents c and d toward the blue and Component e toward the red, as well as increasing slightly the in­tensity of the entire absorption.

The effect of sample warming on the spectra re­corded in Expt. 9 for Xe/Li~1.7X1()6 is shown in Fig. 4. Again there was a gradual reduction in the total band intensity. In addition, the broad shoulder, Com­ponent a, disappeared before the sample reached 300 K.

1.0

0.8

wO.6 u Z

~O.4 o <f>

4000

Area Concen tration M/R Li/Li2 Li/Lb

250· O.lh 0.0003 3300 0.2h 0.0006 3300 2.3 0.0069 3300 3.2 0.0096 3300 6.3 0.019

18000 11.8 0.035 18000 12.3 0.037 18000 100.0 0.30

170000 c 1000000 c

absorption coefficients but no other conclusions, including the last column In this table.

h Lit area includes band extending to longer and shorter wavelength from Li, absorption, probably due to Lb, Lit, etc.

• No Li, observed.

With further warming, Component b decreased in intensity faster than Component c and both shifted toward the red. Concurrently, Component d increased with respect to Component c (note 52°K spectra) and shifted slightly toward the blue. Cooling the sample from 80° to 200 K sharpened and shifted the bands back to their original wavelengths. In subsequent warming, Component b decreased faster than Com­ponents c and d (note 84 OK spectra), and at 91 OK, the matrix began to vaporize. The spectra in Expts. 8 and 10 showed similar warming behavior.

Meyer2 observed warming behavior of sodium multi­plet components in xenon analogous to that of lithium

d 0.8

0.7

0.6

0.5

w U

~ 0.4

'" a: 0 :::0.3 <I:

0'°'-76':!:950~"""'--:6~7"",-.J.....J....,6"'5!"50:!:--'--6""3~5""'...L..--' WAVELENGTH (A)

FlO. 1. Spectra of lithium in argon, krypton, and xenon matrices FlO. 2. Spectra of lithium atoms in krypton showing the effect of at 4 OK and M / R "" 18 000. deposition temperature.

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SPECTRA OF LITHIUM IN INERT-GAS MATRICES 2907

TABLE II. Absorptions due to Li and Li2 in inert-gas matrices, in A (p in cm-1 shown parenthetically).

Matrix Ar

Li (a) 6780±25 (a) (14750±50)

(b) 6700±25 (b) (14925±50)

(c) 6540±25 (c) (15290±50)

(d) 6340±25 (d) (15770±50)

(e)

Li2 4840±40 (20 660±160)

in krypton. In contrast, Brewer et al.4 report for mer­cury in xenon that a triplet is observed whose relative intensities do not change during warming, although the total intensity decreases reversibly.

During the changes shown in Figs. 3 and 4, there was a slow decrease in the lithium molecule absorption

1.0

0.8 ff-1.8 xl0' DEPOSITED

AT 4°K

0.6

e

~ 0.4 Z <t: CD a: 0 (/) 0.2 CD <t:

0

FIG. 3. Effect of warming on the spectra of lithium atoms in krypton at 4°K. The absorbance scale applies to the 4°K spectra and to the other spectra after vertical adjustment.

Kr Xe

6840±15 (a) 6975±30 (14620±30) (14335±60)

6750±10 (b) 6810±20 (14815±20) (14685±40)

6610±10 (c) 6690±10 (15130±20) (14950±20)

6505±15 (d) 6545±10 (15375±30) (15280±20)

6325±10 (15810±20)

7225±25 (13840±50)

4980±20 5150±50 (20080±80) (19 420±200)

near 5000 A. During warming of lithium in krypton matrices, an additional band appears at 7225 A which we assign to the 11:,/~11:D+ lithium molecule transition at 7110 A in the gas phase.

DISCUSSION

The degree of isolation of lithium atoms, the matrix shifts, and the multiplet structures of the lithium atomic absorptions in the various inert-gas matrices are discussed.

b d

1.2

1.0

0.8

0.6

0.4

w u ~ 0.2

'" a: 0 (J) 0

'" <t:

FIG. 4. The effect of warming on the spectrum of lithium atoms in xenon at 4 OK. The absorbance scale applies to the 4 OK spectra and to the other spectra after vertical adjustment.

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2908 L. ANDREWS AND G. C. PIMENTEL

TABLE III. Matrix shifts (Vum,rlx-V ... ), in em-I.

Transition

NH(3II~32;)'

S2(g2;u-~2;.-)b

Li2(12; .. +~'2;. +) Li2(1II .. ~'2;.+)

Hg(3PI~lSO) • K (2 Pm, 112~SI12) d

Na(2P312, 1I2~SI12)e Li(2P3/2, 1I2~S1I2)

• Reference 5. b Reference 8. e Not observed. d Calculated from Ref.!' • Reference 2.

Gas (em-I)

29773 31690 14068 20439

39412 13020 16969 14908

Ar Kr

- 192 -264 - 340 -790

c -230 + 220 -360

+1281 +796 + 150 - 90 +1400 +600 + 360 +220

Isolation of Lithium Atoms

Xe

- 370 - 910

c -1020

+ 30 - 330 + 200 + 40

Table I shows that, as expected, isolation improves as M/R and matrix atomic weight increase and as depo­sition temperature decreases. Since Li2 account.s ~or less than one percent of the vapor stream, the majorIty of the Li2 observed spectroscopically is formed by re­action of diffusing lithium atoms in the matrix, pre­sumably near the surface of the matrix where the warm gases are being condensed.

To fix the degree of isolation, we attempted to deduce the extinction coefficients for the lithium atom and Li2 molecule. In Expts. 9 and 10 for xenon at very high M/R's, the Li2 equilibrium vapor concentration was less than 0.1% during deposition. Since no Li2 ab­sorption was observed, we assume that all of the de­posited lithium was is~lated as ato~ns. Fro~ the a.reas of the atomic absorptlOn (AX optlCal denSity umts) , in Expts. 9 and 10, the atom-extincti?n coefficient is found to be 58 and 69 nmole-I , respectlVely. In Expts. 5 and 7 the average of these two extinction coefficients was us;d to calculate the quantity of isolated lithium atoms. The remainder of the lithium deposited was assumed to be Li2, which gives extinction coefficients of 280 and 110 ).Lmole-I for Li2• The average shows that the oscillator strength of the atom is about 300 times greater than that for the molecule. With this factor, the Li/Li2 area ratio in Table I can be converted to the Li/Li2 concentration ratio. The presence of un­detected Lb in Expts. 9 and 10 would increase the ex­tinction coefficient of the lithium atom and reduce even more the Li/Li2-concentration ratios. These low isolation ratios show the difficulty of preventing diffu­sion of such a small atom as lithium.

The 5150-A absorption in Expt. 1 is probably due to higher aggregates, such as Lig, Li4, etc., since this absorption does not appear in any of t~e krypton e~­periments at higher M / R. The absorptlOns due to Ll2 listed in Table II appear as broad symmetric band~. No vibrational structure is observed although the Ll2

bandwidths are sufficiently broad (1000 cm-I ) to in­clude several vibrational spacings (v~346 cm-I for 7Li2).7 The presence of natural 7Li6Li and 6Li2 contri­butes to but cannot completely account for the band­width and lack of resolution.

These experiments show that only 2% of the lithium vapor is isolated as atoms in argon and krypton at M/R's~1.8X104 deposited at 4°K, whereas nearly 14% of the atoms are isolated in xenon under the same experimental conditions.

Matrix Shifts

Band-center displacements from the gas phase to the matrix are readily determined for the lithium mole­cule absorptions. Deciding upon the frequency shifts for the lithium atom is more ambiguous, unfortunately, because of the multiplet structure. We based our shifts upon the predominant component of the atomic multi­plet. The center of gravity of the entire band was not used because the band center can depend on experi­mental conditions (see Fig. 2). For argon and xenon matrices, the c components were used because of their intensities. For krypton, the original component c was selected because it was the last to be lost on warm­ing (see Fig. 3).

Matrix shifts thus determined for the lithium mo­lecular and atomic absorptions in argon, krypton, and xenon matrices are compared in Table lIP to values reported by other workers for other molecules and atoms. Although both red and blue shifts are recorded relative to the gas phase, the inert-gas matrix shifts all show the same trend. Relative to the argon matrix, krypton, and xenon frequencies are successively dis­placed to lower frequency in parallel with increasing matrix atom size and polarizability.

Multiplet Structures

It is difficult to explain how a perfect substitutional inert-gas matrix site of 0" symmetry could be distorted to a sufficiently low symmetry to remove the orbital degeneracy of the excited alkali atom. Schnepp9 ob­serves that the crystal field of a substitutional site less one nearest neighbor could not cause the observed triplet splittings; two nearest neighbors must be re­moved to lower the symmetry sufficiently. We might rationalize such lattice imperfections in terms of the rather large size of lithium atoms. In a one-electron approximation, the average radius of a lithium atom is near 2.5 A, somewhat larger than the crystal radii for argon, krypton, and xenon atoms (respectively, 1.88, 2.00, and 2.17 A). On this basis, it is possible that

7 G. Herzberg, SPectra of Diatomic Molecules (D. Van Nostrand, Co., Inc., New York, 1950), 2nd ed.

8 L. Brewer, G. D. Brabson, and B. Meyer, J. Chern. Phys. 42, 1385 (1965); L. Brewer and G. D. Brabson, J. Chern. Phys.44, 3274 (1966).

9M. Brith and O. Schnepp, J. Chern. Phys. 39,2714 (1963).

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SPECTRA OF LITHIUM IN INERT-GAS MATRICES 2909

TABLE IV. Shifts due to lithium-lithium interactions at nonnearest-neighbor distances.

Argon D=3.76 A

Site number, Interaction Site Calc Obs

Fig. 5 distance population shift shift

2 1M 6 1281

Krypton D=3.99 A

Calc Obs shift shift

1017

Xenon D=4.33 A

Calc Obs shift shift

725 3 INJ 24 544 480 (d-c) 402 680 (e-c) 257 330 (d-c) 4 D(4)1I2 12 5 D(5)1/" 24 6 D(6)!l2 8 7 D(7)1I2 48

253 127

67 37

178 85 43 23

245 (d-c) 105 47 23 12

_ .... _-- .----.-.---.--.'"-~-~ -~"-.----~----.. --~----.---- --- .--... ---~ .. -.---- -' -_._--"--- ---- _."----

some of the lithium atoms occupy sites with more than one adjacent matrix atom vacancies, splitting the upper-state degeneracy. This would be consistent with the observation that the triplet observed in xenon matrix persists even at the highest M / R.

This simple view is confused by the irreversible changes in the relative intensities of the multiplet components. The data force us to retreat to the multiple site interpretation. If the components of the multiplet are due to atoms in different types of environment, then the changes in relative intensities are plausibly connected with the irreversible disappearance and ap­pearance of different types of environmental sites. This view, which has been invoked by numerous authors to dispose of unexpected multiplets,l,2,5,lo is unsatis­fying because there is generally no clue to the nature of the various sites. Nevertheless, x-ray evidence gives substance to the explanation. For example, Peiserll

describes the irregularity of solids formed by rapid

X

FIG. S. Face-centered cubic lattice of the solid inert gases.

10 C. K. Jen, V. A. Bowers, E. L. Cochran, and S. N. Foner, Phys. Rev. 126, 1749 (1962).

II H. S. Peiser in Formation and Trapping of Free Radicals, A. M. Bass and H. P. Broida, Eds. (Academic Press Inc., New York, 1960).

condensation of a vapor at very low temperatures and the changes that accompany annealing. The difference in relative component intensities for samples deposited at 15° and 4°K (see Fig. 2) could be due to a higher number of irregular sites in the low-temperature de­posit. Furthermore, the behavior of the multiplet features on sample warming could be due to the growth and loss of different sites or lattke defects as the matrix is annealed. If so, the warming loss of Components a and b relative to c in all matrices studied requires that a and b are due to less stable sites which can be an­nealed away.

Nonnearest-Neighbor Lithium-Lithium Interactions

Many of those who propose multiple sites to explain multiplets are noncommittal concerning the nature of the several sites.l,2,5,12 Others have proposed lattice defects of various types.3 ,lO Surprisingly, no one has proposed or investigated the possibility that non­nearest neighbor atom-atom interactions account for the site differences. We can e-,,-plore the magnitudes of such interactions here since lithium provides a par­ticularly favorable case. The potential curves for ground and excited states are both well known.13

Figure 5 shows a unit cell for a face-centered cubic lattice, the crystal form of the inert gases. The nearest neighbors (1) which lead to lithium molecule forma· tion are noted along with nonnearest-neighbor positions (2)-(6). The population and distance from the origin of each of these nonnearest·neighbor sites are given in Table IV.

Morse potential curves for the l~g+ (D.=9407 C111-1, /3=0.827/ A)7.13 and lII,. (D.=3871 cm-t, /3=0.983/ A)7,13

electronic states14 of the lithium molecule are shown in Fig. 6. The 2P_2S atomic and lII1l_1~g+ molecular transitions are noted. The transition energy for an interacting pair of lithium atoms, occupying non­nearest-neighbor matrix sites, is calculated from the potential curves assuming the interacting pair of atoms

12 K. B. Harvey and J. F. Ogilvie, Can J. Chem.40, 85 (1961). 13 F. W. Loomis and R. E. Nausbaum, Phys. Rev. 38, 1447

(1931) . 14 {J is the coefficient of AT in the Morse-function exponent.

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2~10 1. ANDREWs AND G. C. PiMENTEL

2.67 2.93

6.91 7.98

Kr Dfl D-v'4

,_ FIG. 6. Lithium molecule potential curves.

to be a weakly bound molecule with an internuclear distance equal to the distance of the nonnearest-neigh­bor sites in the matrix. The shift from the isolated atom in the gas phase to a pair of interacting atoms in the gas phase is

[De-U(r)J~-[De-U(r) Jrr

(see Fig. 6). In the same manner the shift from the transition of an isolated atom in a matrix to that of a nonnearest-neighbor interacting pair of atoms is ap­proximately the above shift multiplied by a correction due to the dielectric matrix medium. This correction or matrix shift is only 1 %-5% for the observed lithium atom and molecule transitions. Since the corresponding effect for the nonnearest-neighbor interacting pair is expected to lie intermediate between these values, it is neglected in this order-of-magnitude calculation. The calculated blue shifts from an isolated atom transition to an interacting atom pair transition are listed in Table IV. The observed blue displacements from Component c of Components d for argon and xenon and d and e for krypton are contrasted to the calculated blue shifts. The components a and b to the red of Com­ponent c are not considered here, since these compon­ents are lost as diffusion of the trapped species occurs. [Of course it is possible that all of the differences should have been referred to a and that good isolation is quite rare.]

We find rough agreement between the observed blue shifts and the calculated ones. There is no doubt that nonnearest neighbors could be present and that they would cause shifts of the order of those calculated from the Morse curves. From a purely statistical point of view, if there are 68 potentially detectable nonnearest-

neighbor sites [Sites (3)-(6)JI5 which could cause a shift large enough to detect (see Table IV), the random probability of such a spacing is near one percent for M/R-::::::;6800 and 10% for M/R-::::::;680. That this is a gross underestimation is shown by the lithium molecule formation, which is very much greater than the ex­pectation. The seriousness of this consideration is most acute at the moderate dilutions used in some of the earlier work. For example, in those studies based upon M/R down to 2()()2-4 and to M/R-::::::;l00/o the statistical probabilities of nonnearest-neighbor interactions are, respectively, 34% and 68%. We believe that M/R-::::::; 100 000 is needed to escape the possibility of non­nearest-neighbor solute-solute interactions.

In actuality, our own lithium atom spectra are more likely to be influenced by nonnearest-neighbor lithium molecule perturbations since most of the lithium is in that form (see Table I). However, we expect atom­molecule perturbations to be comparable in magnitude to atom-atom perturbations, hence evident in the spectra.

It cannot be said to be verified that the observed multiplets are due to nonnearest-neighbor perturba­tions. In fact, the persistence of splitting even to ex­tremely high M/R implies they cannot be the sole cause of splittings. It is clear, however, that the split­tings are comparable in magnitude to the measured intervals and they are likely contributors to the con­centration and diffusion-dependent changes.16

CONCLUSIONS

The present work indicates the importance of careful study of the concentration and diffusional dependence of matrix-isolated atomic spectra prior to interpreta­tions of multiplet splittings. However, the fact that nonnearest-neighbor interactions may be measurable adds to the promise of such matrix studies. Once the effects of aggregation and site perturbation have been segregated and identified, there is real promise that the spectra will reveal the energetics of growth of metallic agglegates. This could contribute impOl tantly by closing somewhat the gap between diatomic molecules and the metallic state.

ACKNOWLEDGMENT

We gratefully acknowledge research support from the U.S. Office of Naval Research.

10 Site 2 was ignored since no splittings of this magnitUde were observed, undoubtedly because valence forces could attract the atom in this site to a nearest-neighbor site (1) leading to formation of a lithium molecule.

16 L. Meyer and co-workers have found that argon containing certain impurities forms a hexagonal crystal rather than cubic U. Chern. Phys. 41, 1078 (1964)]. Even if this were the case for lithium-argon mixtures, the qualitative conclusion would still be applicable: the calculated splittings would still be comparable to the measured intervals.

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