Surface Raman spectroscopy of a number of cyclic aromatic

Langmuir 1988,4,61-76

67

Surface Raman Spectroscopy of a Number of Cyclic Aromatic Molecules Adsorbed on Silver: Selection Rules and Molecular Reorientation M. Moskovits,* D. P. DiLella,? and K. J. Maynard Department of Chemistry, University of Toronto, Toronto, Ontario, M5S 1A1 Canada Received March 12, 1987. I n Final Form: June 26, 1987 SERS spectra of several aromatic molecules adsorbed on vacuum-deposited silver are reported, as are the spectroscopic changes observed following laser heating. The observed changes are interpreted as being due to molecular reorientation upon the surface. The appearance of normally Raman-forbidden bands in the spectra is attributed to the rapid spatial rate of change in the radiative electric field strength occurring near sharp surface features, causing vibrational modes belonging to the same representations as components of the dipole-quadrupole tensor to become Raman active. This model together with the application of appropriate surface selection rules accounts for the observed intensity distribution of SERS bands, from which the orientation of the adsorbed molecules with respect to the surface normal can be determined.

Introduction of 21 and 7 cm-’, re~pectively.~ An alternative suggestion that is not based on chemical Surface-enhanced Raman scattering (SERS) spectrosbonding was made to account for the presence of these copy is a well-established technique1 referring to the unmodes in the SERS spectrum.6 According to it, the usually intense Raman emissions which one obtains from normally silent modes derive their intensity through a term rough surfaces or aggregated small particles of certain metals. Its origin is understood broadly in terms of en- . in the expression for the induced dipole of the form ‘/,AVE where A is a third-rank tensor (sometimes called hanced electromagnetic fields that exist at the surface of the dipolequadrupole tensor) operating on the dyadic VE. these metal features augmenting both the intensity of the Far from a metal surface this term provides negligible luminous field striking the molecules adsorbed on the intensity since the rate of change of electric field strength surface and that of the Raman-scattered radiation. over a region of space as small as an average molecule is In addition to the electromagnetic enhancement, the low. Near a metal surface, however, the electric field surface Raman intensity may be modified by chemical strength drops rapidly since its value must go to zero deep influence which make the technique a useful probe for within the metal. (In all cases we are concerned with the chemistry at (appropriate) surfaces.l radiative field strength.) Hence, the above term may have The surface selection rules which operate in SERS are a nonnegligible value. This mechanism should operate also understood in a general way on the basis of the both on smooth as well as on rough surfaces. On rough electromagnetic model, at least for those modes which are surfaces, where features of small radius of curvature Raman allowed. In addition, one often observes intense abound, there is an additional contribution to the above Raman-forbidden modes.w The presence of these modes term arising from geometry alone. (These mechanisms will has been explained variously. Most straightforward is the be referred to,loosely, as the “first” and the “second”field suggestion that the molecule’s symmetry has been lowered gradient mechanisms.) This second mechanism can be through bonding to the surface. (More correctly, the shown to be important only when dealing with SERS-achigh-symmetry molecule has become a lower symmetry tive surfaces: surface complex.) While this interpretation is tempting Assume that a small surface asperity may be modeled since it holds out the promise of determining the symmetry as a small metal sphere of radius R. The potential of the surface site to which the molecule binds, it was (truncated at the dipolar level) describing the fields about rejected4because most SERS spectra containing prominent such a sphere, which is placed in a uniform field EopoRaman-forbidden modes were only slightly shifted in larized in the z-direction, is given in polar coordinates by frequency with respect to the analogous liquid spectra. This implies that the surface has perturbed the molecule V(r,B) = Eo(r - gR3/r2) cos e (1) only slightly. On the other hand, the large intensity of the forbidden modes suggests a very great perturbation of the where g = ( e - t o ) / ( € 2e0), E and eo being the dielectric molecule by the surface. When chemical binding is refunctions of the metal and its environment, respectively. sponsible for the resurrection of silent modes, high inThe electric field components at field point (r, 0) normal tensity and large frequency shifts normally go hand in and tangential to the surface of the sphere are obtained hand. Even then the normally silent modes are usually not very intense compared with the active modes. For example: in the Raman spectrum of the molecule Cr(1)(a) Surface Enhanced Raman Scattering; Furtak, T. E., Chang, (C,H,)(CO), vibrations derived from ull (a2,) and (e2,) R. K., Eds.;Plenum: New York, 1981. (b)Otto, A. In Light Scattering of benzene, both Raman silent, are seen only weakly, the in Solids ZV; Cardona, M., Ed.; Springer: Berlin, 1983. (c) Metiu, H.; more intense being about 10-fold weaker than ul, yet the Das, P. Annu. Reu. Phys. Chem. 1984,35,507.(d) Moskovita,M. Rev. Mod. Phys. 1985,57,783. frequency shifts are 130 and 22 cm-’, respectively. In the (2) Dorhaus, R.; Long, M. B.; Benner, R. E.; Chang, R. K. Surf. Sci. SERS spectrum, by contrast, these two modes are the 1980,93,240. second and third most intense bands in the spectrum (each (3)Moskovita, M.; DiLella, D. P. Chem. Phys. Lett. 1980, 73,500. (4)Moskovita, M.; DiLella, D. P. J. Chem. Phys. 1980,73,6068. about one-half the intensity of vl), with frequency shifts

+

Present address: Department of Chemistry, George Washington University, Washington, D.C. 20052.

( 5 ) Schaefer, L.; Begun, G. M.; Cyvin, S. J. Spectrochim. Acta, Part A 1972, A B , 803. (6) Sass, J. K.; Neff, H.; Moskovita, M.; Holloway, S. J.Phys. Chem. 1981,85, 621.

0143-~463/88/24Q4-0067$01.5Q JQ 0 1988 American Chemical Society

68 Langmuir, Vol. 4,No. 1, 1988

Moskovits et al.

from eq 1 in the customary fashion, yielding

E, = -Eo(l

+ 2gR3/$) cos 8

E, = Eo(l - gR3/$) sin 8

(2)

Components of the dyadic VE may then be formed from eq 2. Specifically, the quantity dE,/dr will be dE,/dr = 6E,,g(R3/r4)cos 8 The SERS enhancement is often taken to be roughly proportional to the fourth power of the field enhancement.' Likewise, the magnitude of the term VE will depend on intensity terms such as ( pEn/drlz),where the average is over the surface of the sphere. At the surface of the sphere r = R, and the intensity of vibrational modes of molecules adsorbed on the sphere surface belonging to irreducible representations that span components of A will be proportional to IAij&/RI2,where A,jk is a component of, or an appropriate combination of, components of A. Hence the ratio of a forbidden mode to that of an allowed mode will Since the ratio Aljk/av be of the order of (AiJ&/(cuiJR)12. is of the order of a molecular dimension aM, the ratio of intensities of forbidden to allowed modes will be roughly given by I(forbidden) /I(allowed) = laMg/Rl2 Clearly this analysis will not be valid for spheres that are so small as to make both classical electromagnetism and the use of a bulk dielectric function in g inappropriate. Contrariwise, if R is too large, I(forbidden1 will approach zero. For the cold-evaporated films of the sort used in the present study R is estimated to be of the order of 100-200 A; hence I(forbidden) will be small unless g becomes large. This is only near the surface plasma resonance frequency where g can have a value in excess of 10 (for silver), making it possible for I(forbidden) to be close to I(allowed). On top of this one must consider the modification in Raman and SERS intensities brought about by the difference in the relative magnitudes of the normal and tangential electric field components resulting from the proximity of the metal surface. This point was discussed in a previous paper,' where it was shown that at and to the red of the surface plasmon frequency normally allowed modes which draw their intensity from azz (2being along the surface normal) will be most enhanced followed by ~ among vibrations depending on cyxz and a y Likewise, the normally forbidden vibrations the most enhanced vibrations are those which possess large values of Azzz followed by Azzx and AZzy. The preferences given to azz and Azzz vibrations are commonly referred to as "surface selection rules". These and analogous Raman surface selection rules for flat surfaces have been tested and verified for normally allowed vibrations.s There still exists considerable uncertainty, however, about the origin of the normally forbidden vibrations. In a recent surface Raman study of benzene adsorbed on various silver single-crystal surfaces,B Campion and co-workers conclude that the "first" field gradient mechanism is neither necessary nor sufficient to explain the spectra they obtain. This means, presumably, that in their case symmetry reduction seems to be the (7) Moskovits, M. J. Chem. Phys. 1982,77,4408.Moskovits, M.; Suh, J. S. J.Phys. Chem. 1984,88,5526. Hallmark, V. M.; Campion, A. J. Chem. Phys. 1986,84,2942. (8) Hallmark, V. M.; Campion, A. Chem. Phys. Lett. 1984,110, 561; J. Chem. Phys. 1986,84,2933.

mechanism operating, and if one were to invoke a field gradient mechanism to explain the forbidden modes made visible in SERS, it would have to be the "second", geometrical, mechanism which is operating. We cannot distinguish between the two field gradient models in SERS. Campion's conclusion, however, rests on the observation of very few and very weak bands. The most prominent normally forbidden band in his spectrum, vll (az,), is precisely the one predicted to be most prominent by the field gradient mechanism. Moreover, Campion and coworkers do not discuss why ull becomes so intense in their spectra in contrast with transition-metal complexes of benzene, where despite the greater strength of the metal-ligand bond vI1 is never very intense. Our conclusion is that the field gradient mechanism has not been convincingly excluded as an important contributor to the intensity of certain forbidden modes in molecules adsorbed even on flat metal surfaces. For strongly bonded molecules symmetry reduction can, obviously, be an important or even the most important contributor to the relaxation of selection rules. In this paper we address further the question of symmetry reduction versus the field gradient model as the origin of the appearance of forbidden vibrations in SERS.

Experimental Section The experimental apparatus has been described e l ~ e w h e r e . ~ Briefly, cold-deposited silver films were made by vaporizing 99.99% silver wire from a tantalum ribbon filament onto a polished, aluminum, receiving surface cooled to 15 K by means of an Air Products DISPLEX closed-cycle refrigerator. The base pressure was in the 104-Torr range. Approximately 200 langmuir of the desired molecule was condensed from a liquid reservoir. Liquids were degassed by the freeze-pump-thaw method and vacuum distilled prior to use. Benzene-1,3,5-d3(98 atom % D) and benzene-d (98 atom% D)were obtained from Merck Sharp & Dohme. All other chemicals were obtained from Aldrich. For the experiment in which pyrazine was studied as a function of coverage, a different doeing technique was used. A precalibrated small volume was filled with pyrazine vapor and then dosed onto the silver. In this way submonolayer coverages of pyrazine were obtained. Raman spectra were recorded on a SPEX 1403 double monochromator equipped with photon counting and interfaced either to a Tektronix 4091 or a Commodore computer. Approximately 9-cm-I band widths were used. Argon ion laser lines a t 488 or 514.5 nm (100 mW) excited the Raman spectra.

Results SERS spectra for C6F6,1,3,5trideuteriobenzene,1,3,5trifluorobenzene, mesitylene (1,3,5-trimethylbeneene), s-triazine, pyrazine, perdeuteriopyrazine, pyridine, CGHSD, and C6H,Cl are shown in Figures 1-14. ,ReferenceRaman spectra of thick polycrystalline layers of the same molecules are shown in Figures 1-4,6,8,9, and 12-14. Figures 5, 7, and 11 show the progress of the SERS spectra of s-triazine, pyrazine, and pyridine with laser irradiation for durations indicated in the captions. Figure 10 shows the change in the SERS spectrum of pyrazine at two coverage values. Finally, the SERS spectrum of pyridine adsorbed on cold-evaporated films is compared with that obtained with aggregated silver sol in Figure 12. A summary of the observed frequencies and assignments is given in Tables I-IV. Wilson's notation is used to label benzene vibrat i o n ~ . The ~ spectra of the azabenzenes are sufficiently similar to that of benzene that the same nomenclature is used for them, with the exception, of course, that when the 6-fold symmetry is reduced below 3-fold, degenerate vi(9) Wilson, E. B. Phys. Reo. 1934,45,706.

SERS Selection Rules and Molecular Reorientation

Langmuir, Vol. 4, No. 1, 1988 69

A

J

F

I,l l B

I

I

500 I 200

,

I 600

I

I 1000

I

I I400

I

I I800

A Q (cm-') Figure 1. SERS spectrum of -200 langmuir of hexafluorobenzene (top) and Raman spectrum of a thick polycrystalline

1000

1

1500

1

ZOO0

I

2500

I

3000

AQ (crn-')

Figure 3. SERS and Raman spectra of 1,3,5-trifluorobenzene as in Figure 1.

sample of the same molecule (bottom),both deposited on silver at 15 K.

I

500

I 1000

I

I

1500 Zoo0 Ad (cm-')

I

I

2500

3000

Figure 4. SERS and Raman spectra of mesitylene a~ in Figure 1.

were assigned on the basis of ref 15. A? (ern-0

Figure 2. SERS and Raman spectra of 1,3,5-trideuteriobenzene as in Figure 1.

brations are split into two components. Assignments for the chlorobenzene and benzene-dl are based on the data of Varsanyi.'O Assignments for 1,3,btrifluorobenzenewere derived from ref 11, for heduorobenzene from ref 12, and for mesitylene from ref 13. Additional assignments for chlorobenzene were taken from ref 14, and the azabenzenes (10) Varsanyi, G. Vibrational Spectra of Benzene Deriuatiues; Academic: New York, 1969. (11)Eaton, V. J.; Steele, D. J. Mol. Spectrosc. 1973,48,446. Pearce, R. A. R.; Steele, D.; Radcliffe, K. J. Mol. Struct. 1973,15,409. Torok, F.; Hegedus, A.; Kosa, K.; Pulay, P. J. Mol. Struct. 1976, 32, 93. (12) Steele, D.; Whiffen, D. H. Trans. Faraday SOC.1959, 55, 369. Long, D. A.; Steele, D. Specrochim. Acta 1963,19,1947. Eaton, V. J.; Pea~ce,R A. R.: Steele, D.: Tindle. J. W.SDectrochirn. Acta. Part A 1976. 32A, 663. (13) Farot, J. Can. J . Spectrosc. 1978, 23, 177. Farrot, J.; Forel, M. T. Can. J . Spectrosc. 1978, 21, 144. (14)Whiffen, D. H. J. Chem. SOC.1956, 1350.

Discussion Molecules with 3-fold symmetry and higher were especially chosen for this study becahse for them the Raman selection rule is quite restrictive, providing the greatest opportunity for studying the appearance of forbidden bands. In principle one can distinguish unequivocally between the symmetry-lowering and field gradient mechanisms if it can be shown that one or the other predicts that at least one vibration that is observed should have been silent. For example, it was shown previously4 that the SERS spectrum of benzene could be made consistent with the symmetry-lowering model if one assumes that the symmetry of the molecule is lowered to C3,such that the three vertical reflection planes bisect three alternate sides of the molecule. (This cauld be accomplished, for example, by adsorbing the molecule on a 3-fold site such that three (15) (a) Innes, K. K.; Byme, J. P.; Ross, I. G. J.Mol. Spectrosc. 1967, 22, 125. (b) DiLella, D. P.; Stidham, H. D. J. Raman Spectrosc. 1979, 8, 180; 1980, 9, 90, 239, 247. (c) Sbrana, G.; Schettini, V.; Righini, R. J. Chem. Phys. 1973,59,2441. (d) Zarembowitch, J.; Bokobka-Sebaugh,L. Spectrochirn. Acta, Part A 1976, 32A, 605.

70 Langmuir, Vol. 4, No. 1, 1988

Moskovits et al.

I

I

1

loo0 Ld (ern-')

500

I

1500

500

Figure 5. SERS spectra of -200 langmuir of s-triazine (A) aa deposited, (B)after 21 min, and (C)after 68 min of 100-mW, 488-nm Ar+ laser irradiation.

1

I

I

500

IO00

1500

I

2000 (cm-')

I

I

2500

3000

Figure 6. SERS spectrum (top)of -200 langmuir of s-triazine after 210 min of 100-mW, 488-nmAr+ laser irradiation; Raman spectrum (bottom) of a thick polycrystalline sample of the molecule.

sides, as opposed to corners, of the benzene molecule are directed toward the three metal atoms.) If the benzene molecule is triply substituted as in 1,3,5-trifluorobenzene, the same manner of bonding would lower the symmetry of the molecule upon the surface to C,. In this point group all species are Raman active. In terms of the field gradient mechanism, on the other hand, vibrations belonging to the a? representation are predicted to be silent. The substituted benzenes do not have a vibration belonging to allr however, and we have not been able to identify a molecule for which this strategy would work. Consequently, we must base our conclusions on less direct evidence, such as the correlation between the

1

IO00

I

I

1500 2000 A+ (cm-')

I

I

2500

3332

Figure 7. SERS spectra of -200 langmuir of pyrazine after (A) 13min, (B) 92 min, and (C) 150 min of lOO-mW, 488-nm Ar+laser

irradiation.

A Q (crn-')

Figure 8. SERS and Raman spectra of pyrazine as in Figure 1.

observed band intensity and the symmetry species of the vibration. In the introduction we referred to a hierarchy of relative enhancement according to the involvement of components of the Raman polarizability tensor in a given vibration. Let us see how consistent our observations are with these predictions. For benzene and hexafluorobenzene, alg modes are expected to be the most intense among the Raman-allowed vibrations, followed by those of elg symmetry; a2uand e2uvibrations should be the most intense vibrations among the Raman-forbidden modes, in that order. (Comparing intensities between the two groups is difficult.) And indeed, the four most intense modes in the SERS spectrum of benzene are VI (alg),v l l (a& v16 (e2J, and vl0 (elg)in that order. Moreover, modes of symmetry a2 and b2g, which should be silent, are not observed. Litewise, the two most intense bands in the SERS spec-

SERS Selection Rules and Molecular Reorientation

Langmuir, Vol. 4, No. 1, 1988 71

Table I. Raman Spectra (cm-’) of Benzene, Benzene-1 ,3,6-dr,and Benzene-d, ~

benzene bulk 990 vs 3059 s (1346) (703) (989) 606 m

SERS 982 vs 3060 w a a b 606 w

3046 s

b

2275 m

a

1596 m

1587 m

1574 s

1569 m

1178m

1174w

e’

1105 m

1103 w

10 elg

849m

864 m

e”

717 m

711 m

11 8% 12 b,, 13 bi, 14 bzu 15 bz, 16 e2,

(670) (1008) (3062) (1309) (1149) (404)

697 m a

b 1311 w 1149 w 397 m

a; a{ a,‘ a2/ a2/ e’‘

545vw 1004s 2284s (1322) (911) 378 w

544m 1002 s 2285 vw 1324 vw b 374 m

17 e2,

(966)

970 vw

e’’

932 w

921 w

18 el,

(1036)

1032 vw

e’

834 m

835 w

19 el,

(1479)

1473 w

e’

1413 w

1410 w

20 e,,

(3073)

b

e’

3061 m

a

species

9ezs

sym

benzene-1,3,5-da bulk SERS 956 w 952 s 3044 s 3046 vw 1265 vw 1267 vw 696 w, ah (697) 911 vw b 594 m 594 w

svm

bi a, b, a2 bz b2 a, a1 b, bl a2 b2 a2 b2 a, bi a, b, a, bl

benzene-dl bulk SERS 979 vs 972 s 3062 s 3057 w 1299 w 1302 w 703 w 697 m b (978) 600 m 603 w 600 m 603 w 2265 m 2273 vw b b 1589 m 1581 m 1572 m 1573m 1171 m 1176m 1076 w 1068 w 854m 859m 787 w 790 m 622 w 619m 1005 m 1005 wm b b 1320 w a 1156m 1161 w 405vw 396 m 382 vw 384m 970vw b 935w 929m 1032 w 1028 w (858) b 1475 w 1475 vw 1449 w 1449 w 3048 s b b

~ _ _ _ _ _ _

chlorobenzene bulk SERS 1082 m 1080 w 3064 1275 w 1275 w (682) b 989vw b 700 m 697 m 612 m 612 w b 416 m 3064 1582 m 1570 w 1572 w 1555 m 1172 w 1180w 292 w 300 w 837 w 850 w 204 m 202 w 745 w 752 w 1000 s 995 8 3064 1327 w 1318 w 1155 w 1150w (400) 403 m 470 w 463 m 965vw a 905 w 902 m 1022 m 1035 w 1067 vw a 1477 vw a 1445 a 3064 3064

“Absent in the SERS spectrum. bPossible overlap with other modes prevents unambiguous determination of the presence of this mode. Frequencies in parentheses are IR or calculated frequencies.

Table 11. Raman SDectra (ern-') of Pyridine and s-Triazine mecies 1 a, 2 a, 3 b2 4 bl 5 b, 6a, bz 7 bz 8al

b2 9al loaz bl llb, 12al 13al 14b2 15bz 16a2 b, 17 a2

pyridine bulk SERS 989vs 1000s 3054 s 3055 m 1230vw a 748vw 747 vw (1007) b 620m 602 w 650 m 656 w C

C

3035 vw 1581 m 1572 m 1218m

b 1589m 1566 w 1215m

C

C

886vw 856 945 vw 950 vw 711 vw 712 w 1029vs 1032m 3054 s 3055 m 1357 vw 1355vw 1146 w 1147 w 380 vw 382 m 408 w 410 m (980) b C

18al bz 19al bz 20 a, b2

1068 w 1056 vw 1481 vw 1 4 3 8 3054 s (3079)

s-triazine s v m bulk SERS a{ 99Ovs 999s a,’ 3040 m 3040 w a2/ a; 731 vw 721 m a? 936 vw 944 m e’ 673 m 682 693 e’ (3050) b e’ e’

1545m 1555s 1551 m 1575 1172 m 1164m

e”

1040 w

1041 vw

C

C

a,’

1122vs 1125s C

C

C

c

336 m

344 s

1299 vw

a2/ e”

C

C

a 1055 w 1479 w ~1434~ 3055 m b

A

I 5 00

C C

C

e’

C C

1407 m 1380 w 1413m 1410m C

“Absent in the SERS spectrum. bPossible overlap with other modes prevents unambiguous determination of the presence of this mode. Frequencies in parentheses are IR or calculated frequencies. No corresponding mode for this molecule.

trum of hexafluorobenzene belong to the aIgrepresentation (vl and v2 in Figure 1).

I

I

I

1000

1500

2000

A Q (cm-’)

Figure 9. SERS and Raman spectra of pyrazine-d, as in Figure 1.

In the molecules of D3h symmetry the agreement is also good. The hierarchy in enhancement for the allowed modes is a