Pure vibrational Raman spectra of simple molecular crystals: Ar-N2, Ar-02, Ar-CO, P-CO

H. KIEFTE, M. J . CLOUTER, N. H. RICH, A N D S. F. AHMAD Dep.l,trrtrnetlt qfPl~y.sics. Met,~o,-ic~l Utli~jc.r.siry of 'Ne~~~fourfcl lat~cl , St . Jolltl's, Njlrl.. Catzaclcr A I B 3x7

Received January 12. 1982

Intcrferometric techniques have been employed to study the polarized (pure) vibrational Raman spectra of N Z r 02, and CO as dilute solutions in single crystal matrices of Ar. The observation of a clearly rc<olved and concentration dependent spcctral structure is interpreted as arising from the clustering of solute molecules.

On a utilisC dcs techniques interf6romCtriques pour Ctudier les spectres Raman vibrationncls (purs) polaris6s de NZ, O2 et CO en solutions diluCes dans des matrices monocristallines de Ar. L'obscrvation d'une structure spcctrale clairement rCsolue et dkpendant de la concentration cst intcrprCtCe commc le rCsultat de la formation d'amas de rnol6cules de solutC.

[Tracluit par Ic journal] Can. J . Phys.. 60, 1201 (1982)

1. Introduct ion associated with 02-0? interactions, but no discrete In this paper we report on a continuing series of structure was observed in these first experiments. How-

experiments which employ the methods of Fabry-Perot ever, given the results which were subsequently obtain- interferometry to study the polarized vibrational Raman ed for Ar-N2, the spectrum of Ar-O2 was reexamined spectra of simple molecular solids. Attention is cur- and a further report is included under Sect. 3B. rently focussed on the diatomic systems 0 2 , N2, and C O , both in pure form and as dilute solutions in solid 2. Experimental details Ar. Spectra obtained from single-crystal samples of One of the conclusions derived froln 1 was that none Y-02, P-N?, and h c ~ Ar - 6 % 0 2 have been re~ol- ted in of the spectra depended significantly on the orientations a previous publication (1) (hereafter referred to as 1). the different single crystals involved. Although this while the present results were obtained using samples of conclusion could be considered to obviate the need to Ar-N2, Ar-O?, and Ar-CO alloys with (diatomic) produce single-crystal samples, every effort was never- solute ~0nCent ra t ion~ in the range from 0 .3% to 5 % . theless made to do so i n the present investigations, It The spectrum of pure P-CO was also investigated. was found that single crystals of Ar-N? and Ar-CO

As demonstrated in I the Raman Spectra of interest were more difficult to produce than A r - 0 2 , are characterized by widths' which are typically in the which i n turn was more difficult than ei- range 0 .03 cn1-I to 0 . 3 cln-I. While the accurate nlea- ther of the pure solids (Ar, N2, 0 2 , CO). Details of the surement of these widths (and shifts) is of interest in techniques employed, together with Some itself, because the information Can lead to a better Un- regarding the phase diagrams of A r - 0 2 and Ar-N?, are derstanding of the vibrational dephasing mechanisms presented i n a separate publication (2). involved, the intrinsic narrowness of the lines Can also The used to record the Rarnan spectra was reveal discrete spectral stltctule which would otherwise the same as that described in 1. In brief, the be unobservable. As an example (see 1) the spectrum of system consisted of (a) a single mode Ar ' laser (Spectra crystalline y-02 was found to consist of a doublet, with Physics 165-08) operated at 514.5 nm, (b) an actively a splitting of 1.08 C ~ I - I . which was associated with the stabilized, piezoelectrically scanned Fabry-Perot spec- presence of two inequivalent Ill~leCular sites in the trometer (Burleigh Instrunlents RC-I 10 plus DAS-I ) , known structure of this solid. Such a site splitting was and (,) a dual grating Raman spectronleter (Spex 1401). not expected to occur to P-N2? and none was observed. The spectronleter (b) was employed to examine the fine ath her, the single con~ponent spectrum of P-N, is be- details of the spectra while (c) was used to measure the lieved to be the narrowest (0.027 cm-l) vibrational RanIan shifts and, where possible, to confirm the order- . . . . . Raman spectrum SO far observed in any liquid or solid, ing of spectral components as observed via (b), The and corresponds to a dephasing tillle (Tz) of -200 Ps. ~ a b r y - p e r o t spectra were accumulated in the memory The considerably broader spectrum (0. I 1 cm-I) of 6 % . fa 1024 channel scaler over periods from 5 to 50 h, O? in hcp Ar exhibited a slight asymmetry which was and measurements could be made directly from a CRT

'The term tl~;.l,tlr/l i s used throughout the papel to refer to the display which was an integral part the data acquisi- ~f i r / / - \ t~ i . l , t / t /~ -o t - /~n/ f - t t~ . l , t~ .y i~ , ,~~~~ after appropriate corrections for tion system. Interference filters of 10 nln bandwidth instruniental effects. were used to isolate the Ratnan regions of interest.

0008-4204182109 1204-05W .OO/O 01982 National Research Council of CanadaIConseil national de rechcrchcs du Canada

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KIEFTE ET AL. 1205

FIG. I. Pure vibrational Raman spectra of N? in crystalline Ar for four different concentrations (in mol%) of the diatomic solute. The corresponding temperatures decrease with in- creasing concentration in the range from 82 K to 78 K. Stokes Raman shift increases towards the right, and the intensity scales are arbitrary. The separation of the peaks labeled n and b is 0.12 cm- ' .

3. Results and discussion A . Ar-N,

Fabry-Perot spectra corresponding to a series of N? concentrations in solid Ar are shown in Fig. 1 . At the lowest concentration the spectrum consists pr~marily of a single peak, a , with a Raman shift of 2326.2 ? 0 . 2 c m - ' . A second peak, b, which is i l l defined for 0 . 3 % N?, becomes clearly observable on the high frequency2 side for 1.5% N?. As the concentration is further in- creased the second peak grows relative to the first until both are roughly the same intensity for 5% Nz. At the same time an additional feature becomes evident in the form of a shoulder at still higher frequencies. The width of the single peak observed for 0 .3% N? was determined to be 0.027 c m - ' , and this value is the same within experimental error (1070) as quoted for P-N2 in I. The two clearly resolved maxima were separated by 0 .12 c m - ' , and the separation was found to be independent of N? concentration.

h he term j i e q ~ l e r ~ ~ l is used synonymously with Stokes Rarnan shift: to avoid possible ambiguities no reference is made to absolute optical frequency anywhere in the text.

The spectrum as a whole was found to shift in the direction of higher Raman frequencies as the tem- perature of the samples was lowered. This occurred at a rate of -0.015 cni- ' K - ' for temperatures in the range from 78 K to 8 0 K, but it was not possible to determine whether the effect was intrinsic to the sarnple or wheth- er it might have resulted from strains induced by adhe- sion to the cell walls. In any case, spurious con- tributions to the spectral width could consequently arise if a large enough temperature gradient were allowed to develop in the sample, and appropriate precautions were taken to avoid this.

With the exception of the highest ( 5 % N?) concen- tration, all the samples represented in Fig. 1 were veri- fied by X-ray diffraction methods to be single crystals with fcc structure (2). By a f o r t ~ ~ n a t e coincidence. the sample with 5 % N? content was observed to undergo a partial transformation into the hcp phase so that a bi- crystalline sample resulted (2). Careful probing of all regions in this sample did not reveal any effects in the Raman spectra which could be correlated with the change in crystal structure.

The relative intensities of the two niost proniinent features (labelled n and 6 in Fig. I ) in the N, spectra were estimated using graphical analysis. It was found that the intensity of component b relative to that of n varied approximately linearly with N1 concentration, and it is partly on this basis that we consider it niost plausible to associate the observed structure with the clustering of N? molecules on substit~~tional sites in the Ar lattice. If one assumes that the feature labeled a arises from vibrational transitions involving single Nz molecules in a nearest-neighbor environment of Ar atoms, then one expects that (for the range of low con- centrations in question) the intensity of this feature should vary linearly with concentration (3). Siniilarly. if the feature labeled 6 is associated with vibrational transitions involving individual N2 molecules which are members of a nearest-neighbor pair (in an Ar environ- ment), its intensity should vary as the square of the concentration (3) . Hence the appropriate ratio would be linear in concentration, as observed. The association of the high frequency shoulder (3%, 5 % NN?) with higher order clusters is also consistent with this interpretation, but a further extension of the data analysis was not considered fruitful in this respect.

T o our knowledge, structure of this kind has been observed in the vibrational Rarnan spectra of only one other system, namely (pure) solid H? In this case the normal solid consists of a mixture of the two ortho and para species in their respective ground states specified by values of the rotational quantuni number J = 1 and 0. It is a characteristic of this system that mixing of rotational states is a minor effect so that the rotational wavefunction for a given para niolecule is spherical to

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RAMAN FREQUENCY ( c m - ' I

FIG. 2. Vibrational Raman spcct1.2 corresponding to thc orrl~o H. transition (1, = 0. .I = I ) + ( r 3 = I . J = I ) in thc prctlominantly pnrtc Hz solitl at 1.3 K . Orrho Hz conccn- twtions arc as shown. Thc spcctra wcrc scpl-oduccd from rct'. 4 courtcsy o f Prof. E. J . Allin.

a high degree of approximation, ancl all m ~ ~ l t i p o l c nio- ments can be consideretl absent. It is in this respect that an analogy can be drawn with the case 01' Ar. On the other hand, the (rotating) ortho molecules possess per- manent quaclrupole moments ancl their behavior as sub- stitutional impurities in tIiel)~it.c~ Hl lattice is determined to a significant extent by the influence of the ;~nisotropic quadsupole-quaclrupole interaction. Given that thc quadrupole moment of N, is also rcsponsiblc for the dominant anisotropic interaction in tlie Ar-N, case, one can reasonably expect that the vibrational Raman spectra of these two systems should display some clual- itative similarities.

Such similarities are incleed evident for the low orrho HI concentrations of Fig. 2, which is a reproduction of published spectra obtained by Prior and Allin (4). A significant difference between the two sets of spectra is also evident in that the doublet component which ex- hibits the strongest dependence on concentration is at lower Raman shifts for H,. while for Ar-N, it is at higher frequencies. In either case it s h o ~ ~ l d be noted that the component associated with ~ ~ n p a i r e d molecules is ~~nambiguously identified by its persistence at the low- est concentrations.

Analysis and theoretical calcul;~tions (3) pertaining to

the H' spectra of Fig. 2 show that the observed doublet ~~ndoubteclly arises because of quaclrupolc-q~~aclrupole interaction which pal-tially lifts tlie 9-fold rotational de- generacy in a pair of neighboring ot.tlzo nio lec~~les (3) . The extent of the resultant level splitting is determined by a quadrupolar coupling constant which (for given ~nterniolecular separation) is proportional to the product of the quadrupole moments for each member of the ortho pair. It is the depenclence of this coupling constant on vibrational quantum n~lnibcr (1,) which is of primary i m ~ o r t a n c e . and the observational reuuirement that one member of an ortho pnir be promoted from v = 0 to 1, = 1 leutls to an it~c~r.ense in this constant by a factor of 1.22 ( 5 ) . The consequent cnhancement of the level splitting is then quantitatively correlated with the ap- pearance of a second spectral component at frequencies lonvr than that for un~airecl ot.tho molecules (4).

It is clear that the p ~ ~ r c l y cluantum-mechanical treat- ment j ~ ~ s t o ~ ~ t l i n e d cannot be applied in the case of Ar-NI. It has been detcr~nined (6) , however, that under conclitions where attractive interactions are dominant, the N' vibrational Ranian frequency clecreases when the effect of these interactions is enhanced by an increase in density. One can consequently expect that in the present context there should be a corresponding correlation be- tween the observed Raman shift for N, pairs and the c~lzcltzge in the quadrupolar coupling constant which ac- companies thc transition. The occurrence of the higher frequency component 11, in Fig. 1 , therefore suggests that the coupling constant is nnn l io for a ( v = I , v = 0 ) pnir as compared with a (1, = 0, 11 = 0) p ~ ~ i r . This is in agreement with recent qu t l r~~pole moment calculations (7) for N, which predict that the coupling constant s h o ~ ~ l d indeed be smaller (by a factor of 0 .98) for the (v = 1, 1, = 0) pair. B . Ar-02

As previously mentioned, efforts were rnade in this case to attain better signal-to-noise ratios than in pre- vious experiments. These efforts were successful in that doublet spectra such as shown in Fig. 3 were consis- tently obtained with 0? concentlxtions of 5 % to 66%. As for Ar-N,. there is also evidence of n shoulder on the high frequency wing. The width of the spectrum (Fig. 3) was determined to be 0.097 cni-I: it is slightly less than the value (0.1 I c n i I ) given in I and is perhaps a reflection of tlie improved quality of the spectra. The doublet separation was 0.025 c m ' and the presence of the second, high-frequency component was further con- firmed by the observation that the spectrum for 1% 0, was structureless with width of 0.058 c n ~ I. The latter value should consequently be regarded as the dephasing linewiclth rather than 0.097 cm- ' or the value quoted in I.

It is apparent that the structure in the Ar-0 , spectra is analogous to that for Ar-N?, anel a similar inter-

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FIG. 3. Thc purc vibrational Raman spcctrum of 5% Oz in crystnllinc As at 81 K. Thc two maxima arc scparatcd by 0.025 cm ' I . Stokcs Rarnan shift incrcascs towards thc right and thc intensity scalc is arbitrary.

KIEFTE ET AL. 1207

TABLE I V~brattonal Ratnan data for O?, N,. and CO

Stokcs Raman Dcplias~ng Moleculc and phasc" \hlft (cm I ) ' ' wldtli (cm I)'

from 0.042 cm-I for 3+% to 0.03 1 cni- ' for I!%. A slight asymmetry in the wings of the spectrum was also apparent (as for Ar-O?). Although the results do not confirm the existence of a discrete structure in the spec- tra, it can at least be stated that the splitting associated with any such structure must be 7 0 . 0 1 cni-I. The vi- brational Raman shift was determined via the grating instrument to be 2138.4 r 0 .2 cni-I, with no detectable dependence on concentration.

D . P-CO The spectrum of pure P-CO at a temperature

(67.5 K ) just below the triple point was found to be highly symmetrical with a width of 0 .73 c n i ' . As veri- fied by direct cotnparison, this value was about 20% grecrter than for the liquid at the triple point. Although this behavior is qualitatively consistent with the ob- served increase in the liquid linewidth as the triple point is approached (6) , it is, on the other hand, distinctly different from the N, case where the width was ob- served to clec.r.ease by about 60% on solidification (see

0, gas 1556.2 -

O2 liquid 1551.7 0.23 I). The Raman shift was found to decrease by 0.13 cm- '

y-0, crystal 1552.5 ) l , ~ g 0.16 on going from liquid to solid at the triple point, and this

1551.4 0.09 is quantitatively consistent with the linear density Ar - 1 % 0, crvstal 1551.0 0.097 dependence of the shift as determined for the liquid N' gas Nz liquid P-N2 crystal Ar - 0.3% N2 crystal CO gas CO liquid P-CO crystal Ar - I+% CO crystal

"G:Is vi~lues arc t'rcc molecule values calculated from molecular constants given in ref. 8: othcr valucs wcre measured near the mclting points of the respcctivc ph;lscs. Mixture v;~lucs arc q~iotcd only for ~ninirnu~ii solule concentrations.

"Estimated crror in mcasurcd v;~lucs: 20 .2 cm ' . ' See footnote No. I : cstimatcd crror is t 10%.

pretation presumably applies. The difficulty of obser- vation in the 0? case is clearly linked to the ratio of the doublet splitting to the intrinsic (lowest concentration) linewidth; this ratio was 0 .43 : 1 for Ar-O2 as compared with 4 . 4 : 1 for Ar-N?.

C . AI--CO . . . . . . .

Spectra of this mixture were recorded at 81 K and

. . . 8 0 K with C O concentrations of If% and 34%, re- spectively. Attempts to produce acceptable samples with higher concentrations were unsuccessful. The

, o concentration showed some spectra for the higher (3'4 ) indication of a doublet structure similar to Ar-O?, but the evidence was not considered conclusive. The possi- ble presence of a second strong component is, however, consistent with an observed decrease in the linewidth

phase (6) .

4. Summary The present results mark the completion of a series of

measurements, performed near the respective melting points, on the polarized vibrational Ranian spectra of crystalline N?, O:, CO, and their dilute solutions in crystalline Ar. Table I presents the complete list of data obtained, including those already reported in I. Spectra of the Rayleigh wing in the range from 3 cni- ' to 150 c m - ' were also obtained for the same materials, and in most cases the same samples, referred to above. These results are currently being analyzed and will be the subject of a future publication.

5. Acknowledgements The authors are grateful to Professor E. 1. Allin for

granting permission to reproduce the published spectra of solid Hz which occur as Fig. 2 in this paper. The helpful comments of Professor J . D. Poll are also acknowledged.

I. H . KIEFTE. M. J . CLOUTER. N. H. RICH. and S. F. AHMAD. Chcm. Phys. Lctt. 70, 425 (1980).

2. S . F. AHMAD, H. KIEFTE, and M. J . CLOU.I.ER. J . Chcm. Phys. 75. 5848 (1981).

3. V. F. SEARS and J . VAN KRANENDONK. Can. J . Phys. 42, 980 (1964).

4. W. R. C. PRIOR ant1 E. J . ALLIN. Can. J . Pliys. 50, 1471 (1972).

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1208 CAN. J . PHYS. VOL. 60. 1982

5. G . KARLandJ. D. POLL. J. Chem. Phys. 46,2944(1967). 7. R. D. AMOS. Mol. Phys. 39. 1 (1980). 6. M. J . CLOUTER, H. KIEFTE. and R . K. JAIN. J. Chem. 8. G . HERZBERG. Spectra of diatomic molecules. Van Nos-

Phys. 73, 673 (1980). trand, New York. NY. 1959.

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