J. Phys. Chem. 1984,88, 1731-1735
1731
Surface-Enhanced Raman Spectroscopy of Benzene Adsorbed on Vapor-Deposited Sodium. Chemical Contribution to the Enhancement Mechanism P. A. Lund,* D. E. Tevault, and R. R. Smardzewski Chemistry Division, Code 61 70, Naval Research Laboratory, Washington, DC 20375 (Received: March 7, 1983)
Surface-enhanced Raman (SER) spectra of benzene on vapor-deposited sodium at 15 K have been obtained. Fifteen out of twenty normal modes of benzene have been observed, including a large number of Raman-forbidden lines. The vibrational frequencies closely match those of polycrystalline benzene. However, the relative intensities among the SER lines differ greatly from those of bulk benzene, the out-of-plane vibrations being more enhanced than the in-plane modes. This difference indicates that the SER-active molecules are lying flat on the sodium surface. The laser excitation spectra of all SER lines are nearly identical in shape with each other, exhibiting increasing intensity with increasing excitation wavelength. However, the intensities of some lines, most notably v,(alg), increase more rapidly than the others. This trend may be interpreted as evidence for a metal/molecule electronic charge-transfer transition below 1.8 eV, which contributes to the SER intensity of the various benzene vibrations in a selective manner. N
Introduction A fundamental question in the analysis of surface-enhanced Raman (SER) experiments’ is how to separate the contribution to the overall enhancement of the so-called “physical” classical field enhancement (CFE) effect2-I6from that of processes involving “chemical” interaction^.'^^'^-^^ One avenue that should prove fruitful is to compare the Raman spectra of adsorbates on different SER-active surfaces. We recently reported the observation of enhanced Raman spectra of benzene on vapor-deposited sodium surfaces.26 These spectra show significant differences from those of benzene on vapor-deposited silver.27 In the present article, we analyze the SER spectra of C6H6 on sodium and present evidence for the existence of a charge-transfer transition between sodium and benzene at wavelengths above 700 nm. For excitation wavelengths below 700 nm, the observed mode selective Raman enhancement can be explained by the classical electromagnetic enhancement theories.
sample was deposited on a SER-inactive surface. The SER-inactive sample required -250 times as much benzene to produce Raman scattering intensities comparable to the former. Assuming equal sticking coefficients for benzene in the two experiments and equal scattering contributions from all of the benzene layers in each experiment, one may estimate an enhancement factor of 250. Of course, if only the first monolayer, or less, contributes to the SER signal, the enhancement factor would be - 3 X lo4, or greater. There is also the possibility that some of the sodium surface may have been oxidized and thereby likely to have been rendered SER inactive, which would also increase the estimated enhancement factor. Although it is difficult to be quantitative about the absolute degree of enhancement, there are qualitative aspects of these ~~
(1) A recent compilation of experimental and theoretical work in SER
research may be found in ‘Surface Enhanced Raman Scattering”, R. K. Chang and T. E. Furtak, Eds., Plenum Press, New York, 1982. (2) M. Kerker and C. G. Blatchford, Phys. Rev. B: Condens. Maffer,26,
4052 (1982). Experimental Section (3) G. S. Agarwal and S. S. Jha, Phys. Rev.B: Condens. Matter, 26, 4013 The apparatus used in these experiments:* as well as the method (1982). (4) G. S. Agarwal, S. S. Jha, and J. C. Tsang, Phys. Rev. E : Condens. of sample preparation,26 has been described previously. The latter Matter, 25, 2089 (1982). consisted of depositing a shiny sodium mirror onto a copper block, (5) U. Laor and G. C. Schatz, J . Chem. Phys., 76, 2888 (1982). which was held at 15 K. Sodium deposition was halted simul(6) K. Arya, R. Zeyher, and A. A. Maradudin, Solid State Commun., 42, taneously with commencement of benzene vapor condensation on 461 (1982). (7) D.-S. Wang and M. Kerker, Phys. Rev. B Condens. Matter, 24, 1777 the mirror. Six discrete lines of a krypton ion laser were separately employed to excite the SER spectra. A bulk overlayer of ‘*02 (1981). (8) M. Kerker, D . 3 . Wang, and H. Cheu, Appl. Opt., 19, 4159 (1980). was deposited on the sample and used as an internal standard for (9) J . Gersten and A. Nitzan, J . Chem. Phys., 73, 3023 (1980). calibration of Raman intensities. (10) J. I. Gersten, J . Chem. Phys., 72, 5779 (1980).
Results Figure 1 shows the Raman spectrum of 100 monolayers of C6H6on a vapor-deposited sodium surface maintained at 15 K as well as the spectrum of bulk polycrystalline benzene at the same temperature. That the former spectrum is an example of the surface-enhanced Raman effect was demonstrated by depositing 100 monolayers of benzene onto the 15 K sample block with no prior deposition of sodium. This experiment produced no measurable benzene Raman spectrum. Thus, the presence of the sodium surface was clearly responsible for an enhancement of the Raman scattering. The exact degree of enhancement in the present experiments is not known. However, we obtained an estimate of the enhancement factor by preparing two samples of benzene with approximately equal Raman scattering intensities for v,, the ring-breathing vibration. One sample comprised 125 layers of benzene on a SER-active sodium mirror while the other benzene
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*Present address: Department of Chemistry, Howard University, Washington, DC 20059.
(11) C. Y . Cheu and E. Burstein, Phys. Rev. Lett., 45, 1287 (1980). (12) S. L. McCall, P. M. Platzman, and P. A. Wolff, Phys. Lett. A, 77A, 381 (1980). (13) J. C . Tsang, J. R. Kirtley, T. N. Theis, and S. S. Jha, Phys. Rev. E Condens. Mafter, 25, 5070 (1982). (14) M. Moskovits, Solid State Commun., 32, 59 (1979). (15) M. Moskovits, J . Chem. Phys., 69, 4159 (1978). (16) K. Arya and R. Zeyher, Phys. Rev. E : Condens. Matter, 24, 1852 (1981). (17) S. S. Jha, J. R. Kirtley, and J. C. Tsang, Phys. Reo. B: Condens. Matter, 22, 3973 (1980). (18) B. N. J. Person, Chem. Phys. Lett., 82, 561 (1981). (19) J. P. Devlin and K. Consani, J . Phys. Chem., 85, 2597 (1981). (20) H. Ueba, J . Chem. Phys., 73, 725 (1980). (21) J. Billmann, G. Kovacs, and A. Otto, Surf. Sci., 92, 153 (1980). (22) A. Otto, Surf. Sci., 92, 145 (1980). (23) J. I. Gersten, R. L. Birke, and J. R. Lombardi, Phys. Rev. Lett., 43, 147 (1979). (24) E. Burstein, Y . J. Chen, C. Y. Chen, S. Lundquist, and E. Tossati, Solid State Commun., 29, 567 (1979). (25) F. R. Aussenegg and M. G. Lippitsch, Chem. Phys. Lett., 59, 214 (1 978). (26) P. A. Lund, R. R. Smardzewski, and D. E. Tevault, Chem. Phys. Lett., 8Y, 508 (1982). (27) M. Moskovits and D. P. DiLella, J . Chem. Phys., 73, 6068 (1980). (28) D. E. Tevault, R. L. Mowery, R. A. DeMarco, and R. R. Smardzewski, J . Chem. Phys., 74, 4342 (1981).
This article not subject to U S . Copyright. Published 1984 by the American Chemical Society
1732 The Journal of Physical Chemistry, Vol. 88, No. 9, 1984
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Lund et al. TABLE I: Observed Vibrational Frequencies (in cm-I) of SER Lines of C, H, on Sodium and Their Assignments (Wilson's Numbering System)e mode SER fwhm gasC solidd 1 2 3 4
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Figure 1. Comparison of Raman spectra of (A) 100 monolayers of benzene on a vapor-deposited sodium mirror and (B) bulk, frozen benzene. Both samples were maintained at 15 K and excited by the 514.5 -nm line of an argon ion laser. The line marked with a star is the Raman signal from a bulk overlayer of
spectra that lead to definite conclusions concerning which enhancement mechanisms are dominant in the benzene/sodium system. First, we note that we observed 15 of the 20 benzene normal modes, whereas only 7 are symmetry allowed for benzene in D6h site symmetry. The Raman spectrum of bulk, polycrystalline benzene29also exhibits some of these extraneous lines, due to a lowered site symmetry, but only with relatively weak intensity. Some of the Raman-forbidden lines that appear in our SER spectra, e.g. v16(e2,),have intensites equal to or much greater than those of the allowed lines which have D6h symmetries alg, elg, and e2g (see Table 11). Second, there is a very close similarity between the frequencies of the SER lines of benzene on sodium and those of polycrystalline benzene. In fact, the frequencies correspond very closely to those of C6Hs diluted in isotopically substituted benzene hosts (which consequently have no translational symmetry for the C6H6 molecules and thus no factor group ~plittings).~~ Our assignments of the SER lines may be made in a straightforward manner because of this negligible frequency shift. Table I lists the assignments in terms of D6h symmetry labels.30 Even though the vibrational frequencies are nearly identical, there are major differences in the relative intensities among lines in our SER spectra and the Raman spectrum of bulk benzene; i.e., the enhancement that we observe in the SER spectra exhibits a pronounced mode selectivity. For example, one may divide the observed SER lines into three groups: those that are Raman allowed in D6h symmetry, I R allowed (Raman forbidden due to the center of symmetry), and both Raman and IR silent. If one then compares the relative intensities of the lines within each group with the bulk spectrum,31one finds the following: All Ramanallowed lines are enhanced because of proximity to the sodium surface, but vlo(elg)is enhanced more than the others. In fact, the ratio of intensity of elg to eZgvibrations is 8-10 times greater in the SERS compared to that of the bulk spectrum. The ringbreathing mode, ul(alg),at 993 cm-' is enhanced much less than all the others, at least for laser excitation wavelengths less than 600 nm. However, this is not the case for excitation wavelengths greater then 700 nm, a point upon which we will elaborate. In the group of IR-allowed lines (a2,, and el, symmetries), the 692-cm-l (a2") SER line is selectively enhanced. It already is the most intense line in the IR spectrum of bulk benzene,31but its intensity relative to vl8(elU)is even greater in the SER spectra. (29) E. R. Bernstein, J . Chem. Phys., 50, 4842 (1969); E. R. Bernstein, S. D. Colson, R. Kopelman, and G.W. Robinson, ibid., 48, 5596 (1968). (30) E. B. Wilson, Phys. Reu., 45, 706 (1934). (31) J. L. Hollenberg and D. A. Dows, J . Chem. Phys., 39, 495 (1963); ibid.37, 1300 (1962).
16 17 18 19 20
alg alg a2g
b,, b2, eZg
e2g e2g
ezg elg
a2u bl,
bl, b,, b,,
e,, e2u
e,, e,, e,,
7
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1010 3057 1309 1146 398 967 1037 1482 3064
7 9 17 14
8 6 a
991 3063 1350 705 1005 606 3042 1594 1175 86 3 697 1011 1313 1147 405 978 1038 1478
Fwhm not measurable due to presence of overlapping lines. Lines not observed in SER spectra. Reference 54. Reference 29. e The values in the last column are for C, H, diluted in isotopically substituted hosts. a
TABLE 11: Relative Intensities of Some SER Lines of C, H, on Sodium as a Function of Laser Excitation Wavelengtha
laser excitation wavelength, nm
Raman shift
mode 16 (e,,, 1 1 (a:;) 10 (elg) 17 (e2,) 1 (alg) 18 (el,) 15 (b,,) 9 (ezg) 14 (b2,)b 8 (e2g) 8 (e2,Ib
cm-l
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403 692 857 976 993 1038 1148 1177 1313 1588
12 8.5 8.9 2.7 3.1 1.9 2.4
5.5 0.8 1.7
7.3 8.6 6.2 3.1 2.9 1.3 2.9 3.3 0.7 1.7
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1.0
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38 36 20 9.7 22 4.9 4.1 12 4.3 5.7
1.9
2.4
46 37 39 62 27 20 9.2 7.7 33 53 3.7 1.6 5.9 3.2 14 8.2 1.7 1.1 6.7 4.4 4.4 3.3
a Values are integrated intensities of the individual lines relative to the intensity of the Raman line of a bulk overlayer of ' * O , deposited on top of the sample. b These two lines result from Fermi resonance of u s (e,,) with v I (a, g) + v6 (e2,).
As was the case for the group of Raman-allowed lines, all the IR-allowed lines are enhanced to some degree and appear prominently in the SER spectra. By contrast, we did not detect all of the vibrations that are normally silent in D6h benzene (a2g,b2g, bl,, b2", and e2, symmetries). We found evidence only for the b2, modes, v14 and v15, and for the e2, modes, v16 and ~ 1 7 . Of these, V I 6 is selectively enhanced beyond the others. We therefore find that one vibration from each of the three categories is selectively enhanced, relative to others in the same category. These modes, vIo, v l l , and v16, have the one common feature that they are all out-of-plane vibrations. Modes 4 and 5 (b2J are the other out-of-plane vibrations and were not observed in our SER spectra. Consequently, of the out-of-plane modes that we do observe, three out of four are selectively enhanced to a greater extent than all the others and they are the only ones among all twenty normal modes of benzene that exhibit this selective enhancement. The overall enhancement of all the SER lines is strongly dependent upon the laser excitation wavelength. Table I1 lists the integrated intensities of the measurably intense SER lines relative The general trend of increased SER to a bulk overlayer of 1802. scattering toward longer excitation wavelengths is common to all the lines. However, two of the lines, vl(alg)and ull(azu),show
Raman Spectra of Benzene Adsorbed on Sodium 1
(A)
The Journal of Physical Chemistry, Vol. 88, No. 9, 1984 1733
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strongly increasing intensities even at the long-wavelength limit, while the remainder are leveling off or beginning to decrease. Figure 2 depicts SER spectra recorded at two different excitation wavelengths, and Figures 3-8 contain plots of relative SER in-
400 600 800 WAVELENGTH (NM) Figure 8. Same as Figure 3, but for mode 1.
Lund et al.
1734 The Journal of Physical Chemistry, Vol. 88, No. 9, 1984
this new mechanism also makes allowed the bl, vibrations. We have no evidence for these latter modes in the benzene/sodium SER spectra. For benzene, there is not much difference between the predictions of the low-symmetry and electric field gradient theories Discussion concerning the presence of the forbidden lines. We would prefer the gradient explanation because of the lack of large vibrational There are several theoretical models in the literature, proposing energy shifts in our SER spectra. Still, neither the reduced to account for the enhancement mechanism of SERS, which we symmetry nor the electric field gradient theory accounts for the might use to interpret our results. These models may be divided mode-selective enhancement that we observe, e.g., the greater into two sets: one group that considers the classical “physical” relative intensity of vlo(elg)compared to that of vl(alg) and us and enhancement of the electrical field of the incoming and outgoing v9(eZg). Published surface selection which are based on photons, by excitation of surface plasmons or collective electron the image-dipole formalism do not seem to be of any help here, resonances2-” and another set that requires a “chemical” intereither, if the benzene molecule is lying flat on the sodium surface, action between the metal surface and the substrate, which can as our data indicates. In that case, the eZgvibrations, which have modulate the surface dipole This latter group is polarizability components of x x , y y , and xy, should be favored restricted to consideration of molecules in the first monolayer or over the elg vibration, which has xz and y z components. so next to the metal, while the classical field enhancement A recent discussion of surface selection rules by Moskovits offers mechanism may involve molecules separated from the surface by a possible explanation of this observati~n.~~ There, a more detailed several tens of monolayers. In general, each of these types of evaluation of surface selection rules by consideration of the reenhancement can independently contribute to the SER spectra. flectivity of the metal was made for benzene-like molecules on It is not unreasonable to assume that there is a classical ennickel and silver surfaces, and the calculations indicated that if hancement due to the electric field near the sodium surface which the benzene molecule were lying flat on the surface, then the is responsible for at least part of the enhancement of the Raman Raman scattering for eZgmodes would be very weak. We repeated scattering that we observe. However, a primary aspect of our SER these calculations using published optical constants of vapor-despectra that we must explain is the observation of IR and silent posited sodium53and reached a similar conclusion. modes of D6,, benzene. By itself, enhancement by the photon The theory of Jha et al.” also considers the importance of the electric field near a metal surface does not account for their photon electric field gradient at metal surfaces. In that theory, activity. A concomitant reduction in the effective site symmetry Raman transitions are excited through modulation of the surface of the benzene molecules, however, will allow certain of the dipole moment of the molecule/metal system. Vibrational motion forbidden vibrations to become active. Moskovits and DiLella2’ perpendicular to the metal surface should be preferentially enfound that they were able to explain the SERS of benzene on hanced if the electron density of the metal can tunnel onto the vapor-deposited silver by assuming a reduction in symmetry to molecule where it can interact with the vibrating dipoles. If one C3”(ud), i.e., with the benzene lying flat on the surface with a assumes that the benzene ring is lying flat on the sodium surface, particular orientation of the ring relative to the silver atoms. An then the out-of-plane modes are predicted to be enhanced more identical explanation is adequate to account for the vibrations that than the others, as we observe. Our spectra, therefore, appear we observe in the benzene/sodium system, since we observe to be consistent with this enhancement mechanism. precisely the same modes as in the silver experiments. A lowAn additional mechanism that might be expected to contribute symmetry perturbation of sufficient magnitude to induce the to the SER intensity for benzene on vapor-deposited sodium appearance of forbidden vibrations with intensities comparable involves excitation of an electronic charge-transfer transition to those of the normally allowed modes would seem to imply a between the metal and the benzene molecule. This mechanism reasonably strong interaction between the benzene ring and the has been discussed by several authors, and evidence for a metal atoms. We would expect such an interaction to produce charge-transfer transition has been found in a number of SERsome measurable perturbation of the vibrational frequencies. active ~ y s t e m s . 4It~ is ~ ~not unreasonable to expect benzene, with Indeed, in the cases of benzene on various transition-metal surfaces, frequency shifts of tens of wavenumbers are c o m m ~ n . ~Our ~ - ~ ~ its 9-electron orbitals, and an alkali metal to show such a charge-transfer transition at low energy. SER spectra are almost identical, in terms of vibrational freOne line of reasoning predicts that the selection rules for this quencies, with those of polycrystalline benzene, which argues event should be similar to those observed in the shape resonances against a reduction of the effective site symmetry to C3”. that occur in the electron bombardment of gas-phase b e n ~ e n e . ~ ~ - ~ l An alternative explanation was put forth to account for the These are governed by the symmetry of the accepting orbital on benzene/silver r e s ~ l t and s ~ may ~ ~ ~be~expected to apply to our benzene, which is eZu,if the direction of charge transfer is metal system as well. It involves modulation of the molecular dipole to molecule. The active vibrations in this case should be of alg produced by interaction of the molecular quadrupole polarizability and e2gsymmetry. We see evidence for such a transition with and the photon electric field gradient. The photon electric field laser excitation wavelengths longer than 700 nm (see Figures 7 gradient is expected to be large within molecular distances from a metal surface, and consequently, this source of Raman intensity can become comparable to that produced by interaction of the (40) N. V. Richardson and J. K. Sass, Chem. Phys. Letr., 62, 267 (1979). dipole polarizability and the photon electric field. The point here (41) R. M. Hexter and M. G. Albrecht, Spectrochim. Acta, Parr A , 35A, 233 (1979). is that the site symmetry may remain high, but the new selection (42) H. Nichols and R. M. Hexter, J . Chem. Phys., 75, 3126 (1981). rules introduced by this gradient mechanism will allow additional (43) D. Schmeisser, J. E. Demuth, and Ph. Avouris, Chem. Phys. Lett., modes to be observed. In fact, the spectrum that is expected is 87, 324 (1982). identical with that for benzene in C,, (ud)symmetry, except that (44) H. Ueba, S. Ichimura, and H. Yamada, Surf: Sci., 119,433 (1982). tensity vs. laser excitation wavelength for several of the vibrational modes. Figures 7 and 8 contain the excitation profiles of the two modes that exhibit the additional intensity at the longer excitation wavelengths.
(45) J. E. Demuth and P. N. Sanda, Phys. Reo. Lett., 47, 57 (1981); ibid., 47, 61 (1981).
(32) D. M. Haaland, Surf. Sei., 111, 555 (1981); ibid., 102, 405 (1981). (33) H. Jobic and A. Renouprez, Surf. Sei., 111, 53 (1981). (34) H. Jobic, J. Tomkinson, J. P. Candy, P. Fouilloux, and A. J. Renouprez, Surf:Sci., 95, 496 (1980). (35) J. C. Bertolini and J. Rousseau, Surf. Sei., 89, 467 (1978). (36) S. Lehwald, H. Ibach, and J. E. Demuth, Surf. Sci., 78, 577 (1978). (37) H. F. Efner, W. B. Fox, R. R. Smardzewski, and D. E. Tevault, Inorg. Chim. Acza, 24, L93 (1977). (38) J. C. Bertolini, G. Dalmai-Imelik, and J. Rousseau, Surf: Sci., 67, 478 (1977). (39) J. K. Sass, H. Neff, M. Moskovits, and S. Holloway, J . Phys. Chem., 85, 621 (1981).
Ph. Avouris and J. E. Demuth, J . Chem. Phys., 75, 4783 (1981). A. Otto, J. Electron Speetrosc. Relat. Phenom., 29, 329 (1983). W. Domcke and L. S.Cederbaum, Phys. Rev. A , 16, 1465 (1977). S. F. Wong and G. J . Schulz, Phys. Rev. Lett., 35, 1429 (1975). (SO) I. Nenner and G. J. Schulz, J . Chem. Phys., 62, 1747 (1975). (51) L. Sanche and G . J. Schulz, J . Chem. Phys., 58, 479 (1973). (52) M. Moskovits, J . Chem. Phys., 77, 4408 (1982). (53) T. Inagaki, L. C. Emerson, E. T. Arakawa, and M . W. Williams, Phys. Reo. B Condens. Matter, 13, 2305 (1976). (54) T. Shimanouchi, “Tables of Molecular Vibrational Frequencies, Consolidated Volume I”, National Standard Reference Data Series-National Bureau of Standards, 39, 1972. (46) (47) (48) (49)
J . Phys. Chem. 1984, 88, 1735-1740 and 8) for vll(aZu)and vl(alg). Since the irreducible representation a2ucorrelates with a l if the symmetry is lowered to C,,, which would be the case if the benzene were lying flat on the surface and not interacting strongly with the sodium, mode 11 as well as mode 1 is predicted to accompany the charge-transfer transition. Furthermore, modes 1 and 11 are the only modes that exhibit the long-wavelength excitation maximum. The result that remains to be accounted for by this analysis is the lack of e2gmode participation in the charge-transfer enhancement mechanism. One possibility is that the cross sections for the eZgmodes are simply much smaller than those for the elg modes. In fact, the observed shape resonances of gas-phase benzene are dominated by the 992-cm-I alg mode, although the ezgmode, Vs, does appear with measurable i n t e n ~ i t y .However, ~ ~ ~ ~ ~we observe no evidence of the charge-transfer contribution to the SER scattering of vg(ezP). This line is a doublet produced by Fermi resonance of vs and vl(alg) v6(eZg). Therefore, one might expect to see at least a small indication of the charge-transfer mechanism due to the admixture of Y,, even if vs by itself were not strongly active. Since we observe no indication of charge-transfer activity for us, we find it more likely that there is a cancellation of scattering due to our assumed orientation of benzene, Le., lying flat on the sodium surface.
+
Conclusions We have observed surface-enhanced Raman spectra of benzene on vapor-deposited sodium surfaces at 15 K. Our best estimate of the enhancement factor is about lo4, making sodium an only slightly less favorable SER substrate than silver, as has been p r e d i ~ t e d . ~The ~ spectra exhibit marked mode selective enhancement but almost no vibrational energy perturbation. The ( 5 5 ) R. Ruppin, Solid State Commun., 39, 903 (1981).
1735
modes that show the greatest enhancement are all out-of-plane vibrations. We therefore conclude that the large majority of SER activity arises from benzene molecules that are in the first monolayer next to the sodium surface and lying flat. A very weak interaction of these molecules with the surfaces results in a local site symmetry of C6".Modes 1 (al,) and 11 (az,) have nearly identical excitation profiles with rapidly rising intensity above 700 nm, which we interpret as being associated with a metal/molecule electronic charge-transfer contribution to the over11 SER intensity above 700 nm. The SER activity at shorter excitation wavelengths is explained by a mechanism that involves modulation of the surface dipole moment by the vibrating benzene molecules but does not require a reduction in symmetry to C,, or lower. The selective enhancement of the out-of-plane vibrations implies that the benzene is lying flat on the surface. In summary, we have reported the observation of SERS of benzene on a vapor-deposited sodium surface and have presented evidence for two different mechanisms of enhancement of the Raman spectra. These are differentiated by their excitation wavelength dependence and identified by comparison of their predicted selection rules and the observed mode selective enhancement. Additional experiments with lower symmetry substrates will be useful in further distinguishing the chemical and classical electromagnetic contributions to SERS. Acknowledgment. P.A.L. gratefully acknowledges support from a National Research Council/Naval Research Laboratory Research Associateship. We also acknowledge helpful discussions of our work with Professor M. Moskovits, University of Toronto, and Drs. D. Ladouceur and D. DiLella of NRL. Registry No. Benzene, 71-43-2; sodium, 7440-23-5.
Sorption of Argon, Oxygen, Nitrogen, Nitric Oxide, and Carbon Monoxide by Magnesium, Calcium, and Barium Mordenites Shozo Furuyama* and Michiko Nagato Department of Chemistry, Faculty of Science, Okayama Uniuersity, Okayama 700, Japan (Received: March 9, 1983)
Sorption experiments over a temperature range of -50 to 150 OC find sorption affinities in the order Ar < O2
