J. Phys. Chem. 1994, 98, 12702-12707
12702
Charge Transfer Resonance Raman Process in Surface-Enhanced Raman Scattering from p-Aminothiophenol Adsorbed on Silver: Herzberg-Teller Contribution Masatoshi Osawa,*Naoki Matsuda; Katsumasa Yoshii, and Isamu Uchida Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tohoku University, Aoba, Aramaki, Aoba-Ku, Sendai 980- 77, Japan Received: June 11, 1994@
The surface-enhanced Raman scattering (SERS) of p-aminothiophenol adsorbed on silver measured with visible excitations is compared with the normal Raman and ultraviolet resonance Raman (UVRR) scattering of the free molecule. The SERS is dominated by four bZ and one al symmetry species of benzene ring vibrations (under an assumption of CZ,symmetry), whereas the normal Raman scattering at visible excitations only by a1 species. The bz modes corresponding to the strong SERS bands are observed for the free molecule only when the electronic transition from the ground state to the lowest unoccupied molecular orbital (LUMO, n* bz, Am= 300 nm) is excited. In electrochemical environments, the bz SERS bands show resonanceshaped intensity profiles as a function of applied potential. The intensity-potential profiles shift to positive potentials as the excitation energy increases. This is well interpreted by a resonance Raman-like process associated with the photon-induced charge transfer from the metal to an affinity level of the adsorbed molecule. The affinity level is assigned to LUMO on the basis of the UVRR experiments and a symmetry consideration. The bZ modes gain their intensities via a Herzberg-Teller (vibronic) term in both SERS and UVRR scattering. Due to the difference in the electronic levels vibronically coupled, however, the enhancement patterns are greatly different.
-
Introduction Considerableresearch has been directed toward understanding the origin of surface-enhanced Raman scattering (SERS). A number of excellent reviews have been published describing the progress made in this It is now generally accepted that two separate mechanisms must be involved in SERS. The first involves the enhancement of the optical fields at the surface. 1-5 Small surface irregularities allow surface plasmon resonance, resulting in strong local enhancement of the electric field of both incoming and scattered radiation. This electromagnetic (EM) model does not require specific bonds between adsorbates and metals and explains the observation of enhancement at some distance from the metal surface. The second mechanism is a resonance Raman-like process in which charge transfer (CT) between metal and adsorbate is the intermediate stage of the resonance Raman scattering.1-8 Direct proximity between the molecules and the surface is required for the CT enhancement. The following experimental results were taken as evidence of the CT mechanism to explain SERS: (i) Demuth and c o - w o r k e r ~found ~ ~ ~low ~ energy transfer bands at 2-2.5 eV for benzene, pyridine, and pyradine adsorbed on silver by electron energy loss spectroscopy. They assigned the bands to metal-to-molecule CT states. For pyridine, an affinity level has been identified at 2.6 eV above the Fermi level, Ef, of the metal by inverse photoemission measurements." (ii) When observed on an electrode surface, SERS spectra show resonance-shaped intensity profiles as a function of applied potential.12-18 The potential at which the intensity reaches a maximum (Emm)varies linearly with the excitation energy (ho). This approach is based on tuning of the CT excitation into and out of resonance by changing either Ef or ho.12-18 The molecules can be divided into two classes: those for which Em Permanent address: Inorganic Analytical Laboratory, Department of Analysis, National Institute of Material and Chemical Research, 1-1, Higashi, Tsukuba 305, Japan. @Abstractpublished in Advance ACS Absrrucfs, October 15, 1994.
0022-3654/94/2098-12702$04.50/0
has a positive slope with h o and those for which the slope is negative. Molecule-to-metal CT, in which an electron is transferred from the highest occupied molecular orbital to Ef, is associated with the negative slope in Emaxversus ho profiles. Conversely, when an electron is transferred from Ef to an affinity level of the molecule (metal-to-molecule CT), positive slope is observed. Despite great efforts since the discovery of SERS, the CT mechanism is still poorly understood compared with the EM mechanism. Firstly, the molecular electronic levels involved in the CT have been definitely assigned only for very limited adsorbate-metal Secondly, both Franck-Condon and Herzberg-Teller (vibronic) mechanisms are theoretically predicted for CT enhan~ement.'~Creightonlg showed that the Franck-Condon term plays a dominant role in the SERS from pyridine on silver and copper. Only totally symmetric modes are enhanced by this term. The observation of overtones and binary combination modes strongly suggests the contribution of this term.17J9 On the other hand, both totally and nontotally symmetric modes will be enhanced via the Herzberg-Teller term.17 However, the contribution of this term has not been definitely addressed experimentally. In the present paper, we report the SERS from p-aminothiophenol (PATP) adsorbed on silver and discuss the enhancement mechanism. It is well-known that thiols are irreversibly adsorbed on metal surfaces and form compact monolayers.20-22The so-called self-assembled monolayers have been receiving considerable attention because of possibilities of many applications.20 SERS is one of the most useful techniques to characterize the monolayers, especially in electrochemical environments. This research was motivated by our previous finding that the SERS from the self-assembled monolayer of PATP on silver is significantly different from the normal Raman spectrum of the molecule in the solid state.23 Interestingly, however, the SERS from thiophenol on silver is substantially the same as the normal Raman spectrum.21~22Since electronic states of thiophenol may be altered by the introduction 0 1994 American Chemical Society
J. Phys. Ckem., Vol. 98, No. 48, I994 12703
SERS of p-Aminothiophenol Adsorbed on Silver of an NHz group on the benzene ring, it was speculated that the dramatic alternation of the spectrum of PATP is due to the contribution of the CT mechanism.23 Here we show clear evidence of the contribution of the CT mechanism in the SERS of PATP. The CT is metal-to-molecule and the affinity level is definitely attributed to the lowest unoccupied molecular orbital (LUMO) on the basis of ultraviolet resonance Raman ( W R R ) spectra of the free molecule, potential and excitation energy dependencies of the SERS intensity, and symmetry consideration of the enhanced modes. It is also shown that the HerzbergTeller (vibronic) term plays a dominant role in the SERS in terms of the theoretical CT model of SERS proposed by Lombardi et a1.l' In the present investigation, we also employed surfaceenhanced infrared absorption (SEIRA) spectroscopy23-25for the detailed analysis of the vibrational modes and the orientation of the adsorbed molecule. SEIRA is a phenomenon where infrared absorption intensity of molecules adsorbed on or near some kind of rough metal surface is enhanced by a factor of 101-103. The mechanism of SEIRA has been described elsewhere.24 SEIRA spectroscopy enables us to obtain the infrared spectra of the adsorbed monolayer at a high sensitivity comparable to that of SERS.
Experimental Section Silver thin films evaporated on CaF2 were used as the silver substrates in both SERS and SEIRA measurements. The metal evaporation was performed from a tungsten basket by thermal heating under a pressure of 2 x Torr. The thickness of the silver film was monitored and controlled to 8 nm with a quartz microbalance. Scanning electron microscopic observations revealed that the silver films consist of small metal particles of 20-30 nm in diameter (island films). A polycrystalline silver plate was used as well for SERS measurements in electrochemical environments. It was successively polished with alumina powder down to 0.05 pm and was sonicated in distilled water. Roughening of the electrode surface by an oxidation-reduction cycle treatment for observing strong SERS26was not employed in the present experiments to avoid the formation of adatoms and surface c o m p l e ~ e and s~~~~ other complexities. Reagent-grade PATP purchased from Aldrich Chemical Co. was used without further purification. A 0.1 mM solution of PATP was prepared with triply distilled water. PATP was adsorbed onto the silver surfaces by dropping the solution onto the evaporated silver film or by immersing the silver electrode into the solution for 0.5- 1 h. The excess molecules deposited on the self-assembled monolayer were removed from the surface by washing the metal surfaces with acetone and then with water. Raman spectra were taken with three different Raman systems. The SERS measurements in the air and normal Raman measurements were carried out with a JASCO NR-1800 Raman spectrometer equipped with a photomultiplier tube (Hamamatsu, R649). The SERS measurements in electrochemical environments were carried out with a JASCO TRS-300 triple monochrometer equipped with an image-intensified photodiode array detector (Hamamatsu, M2482 and M2493). A He-Ne laser (NEC, G1G-5800) or an Ar+ laser (Coherent, INOVA-70) was used as the excitation source at an output power less than 50 mW for the above two Raman systems. UVRR spectra were taken with the UV Raman system reported previously.28 A H2 Raman shifted Nd:YAG laser system was used as the excitation source at an output power of about 1 mW. Infrared spectra were taken with a Bio-Rad FTS-7 Fourier transform infrared spectrometer operated at a resolution of 4
1800
1600
1400
1200
1000
800
600
400
200
WAVENUMBER I cm-l
Figure 1. SERS spectrum of P A P adsorbed on a 8-nm thick silver film evaporated on a CaF2 plate (a) and normal Raman spectrum of PATP in the solid state (b). 20 = 514.5 nm, 50 mW.
cm-'. The SEIRA spectra of PATP adsorbed on the thin silver films were measured by the conventional normal incidence transmission configuration. The single beam spectrum of the silver film taken before letting the molecule be adsorbed was used as the reference. The electrochemical cell used for the SERS measurements was a standard tree-electrode glass one. A platinum foil was used as the counter electrode. The potential was controlled with a potentiostat (Hokuto, HAB-151) against a saturated calomel electrode (SCE), to which all the potential is quoted. A p-polarized laser beam was focused on the electrode surface at an incident angle of 60", and the Raman-scattered light was collected with an achromatic lens 60" from the surface normal.
Results and Discussion SERS and SEIRA Spectra of PATP on Ag Island Film. Figure l a shows the SERS spectrum of a self-assembled monolayer of PATP on a silver island film (8 nm in mass thickness) evaporated on CaF2. The excitation wavelength is 514.5 nm, but the same results were obtained with other excitations in the visible region. For comparison the normal Raman spectrum of the molecule (powder) is also shown by trace b in the figure. The two spectra are significantly different. Five strong bands are observed at 1573, 1440, 1391, 1142, and 1077 cm-' in the SERS spectrum, whereas strong and mediumstrong bands are observed at 1595, 1206, 1173, and 1089 cm-' in the corresponding range of the normal Raman spectrum. In addition, the medium-strong bands at 799, 634, and 463 cm" in the normal Raman spectrum are completely missing in the SERS spectrum. It was also observed that the SH stretch at 2564 cm-' in the normal Raman spectrum was missing in the SERS spectrum (not shown). Combined with the appearance of the 213-cm-' band assignable to the Ag-S stretching21,22in the SERS spectrum, this indicates that PATP is adsorbed onto the silver surface through the sulfur atom by the rupture of the SH bonding. No overtones and combination modes were observed above 1800 cm-' in the SERS spectrum. We carried out the SERS measurements with PATP purchased from several chemical companies, but the results were very reproducible both in peak frequencies and relative intensities. The same SERS spectrum as in Figure l a was obtained when a self-assembled monolayer of p-nitrothiophenol adsorbed on silver electrode was electrochemically reduced to PATP.23
12704 J. Phys. Chem., Vol. 98, No. 48, 1994
Osawa et al.
I
TABLE 1: Raman and IR Band Frequencies of p-Aminothiophenol bulk
Ra"
on silver surface
IR
SERS
SEIRA
3335 Sb 3060 w
w
3036 s 2565w 2564m (omit 1650-2500 region) 1620 vw 1620 s 1595 s 1595 s 1572 c 1490 w 1493 s 1480 w -1480 sh 1445 c 1423 w 1403 w 1400 w
Y a
8
-
I
1800
1600
1400
1200
1000
800
WAVENUMBER / cm-1
Figure 2. Infrared spectra of P A P adsorbed on a silver surface (a, the same sample as in Figure la) and in a KBr pellet (b).
Therefore, the strong SERS bands are not likely to be attributed to impurities. Although the changes in peak positions and relative intensities of vibrational modes are observed in general when molecules are adsorbed on metal surfaces, the difference between the SERS and normal Raman spectra is surprising. To investigate the vibrational properties of the molecule adsorbed on silver in more detail, we measured the transmission infrared spectrum of the same sample plate used in Figure la. The result is shown in Figure 2a together with the infrared spectrum of the molecule in the solid state (a KBr pellet). The figure reveals that the peak frequencies are not altered greatly by the adsorption onto the silver surface. No infrared bands corresponding to the strong SERS bands were observed in the SEIRA spectrum except the 1080-cm-l band, which is observed at 1077 cm-' in the SERS. The Raman and IR bands observed in Figures 1 and 2 are tabulated in Table 1 together with their assignments. The modes observed in the UVRR spectra of the molecule in methanol described later are also given, in which the modes observed only with UV excitations are denoted by asterisks. The band assignment was simply made by using that for thiophen01~~ and p-disubstituted benzenes as a g~ide.~O.~l The vibrations characteristic of p-disubstituted benzenes are given by notations of the corresponding vibrations of benzene (Wilson's notation32) together with the symmetry species under an assumption of C2, symmetry, where the axes are chosen such that the molecule is in the yz plane and z is the C2 axis. The benzene ring vibrations are classified as al, az, bl, and bz species. The a1 and bZ species are in-plane modes, and a2 and bl species out-of-plane modes. All the modes are Raman active, and all but a2 are infrared active. We assign the strong and medium-strong SERS bands at 1573, 1440, 1391, and 1142 cm-' to the fundamental benzene ring vibrations (%3b, ~19b,113, and Y9b, respectively). The medium-strong band at 1206 cm-' in the solid Raman spectrum (Figure lb) was missing in the normal Raman spectrum of a methanol solution, suggesting that this band is not a fundamental benzene ring mode. It should be noted that the normal Raman spectrum is dominated by a1 modes (Y8a. Yga, Y7a, V I , ~ 1 2and , Y6a), whereas the SERS spectrum by four b2 modes. Among the al modes, only ~7~ and Yl8a are observed in the SERS. Although the assignment of weak bands in the low-frequency region is not very certain, the a2 and bl modes seem to be enhanced as well but only very slightly compared with the b2 modes. We also note that the relative infrared band intensities are
,
1629 m 1590 s 1573 m 1488 s 1472 w 1440 vs
1284 s 1266 w 1206 m 1173 m 1142 1118 w 1089 vs 1011 w 960 vw
1204 w 1176 m
1280 m 1180 w 1077 m 1006 w 950 vw 921 w
1080 w 1008 w
830 vw
820 vw 798 vw
907 vw 820 w 799 m 699 vw 634 m
822 s 752 m 695 vw 634 w
~CH+YCC,3 (bz) vCC+GCH, 14 (bz) YCH, 7a' (a,)
1190 w 1142 vs
1119 w 1090 m 1010 w
6NH S C , 8a (ad YCC, 8b (bz) vCC+GCH, 19a (al) vCC+GCH, 19b (bz)
1391 s 1306 w
1310 w
assignment" VNH *H, 2 (ad vCH, 13 (a,) vSH
719 w -d
6CH, 9a (al) 6CH, 9b (bz) 6CH, 15 (bz) *S, 7a (ad yCC+yCCC, 18a (al) K H , 17a (az) K H , 5b (bi)
6SH K H , 10a (az) K H , 11 (bi) vCH+vCS+vCC, 1 (ai) K H , 10b (bz) K H f K S f K C , 4 (bl) yCCC, 12 (ad
544 w 521 w 463 m 396 w 321 w 256 w 196 m 154 w
518 m 406 vw
yCCC, 16b (bl) yCCC, 6a (ad rCC, 16a (az) 6CH+6CS, 18b (bz) 6CN+6CS, 9b (bz)
213 w
vAg-S
-d
K N + K S + C C , lob (bl)
* Approximate description of the modes (v, stretch; 6 and y , bend; x, wagging; z, torsion). For ring vibrations, the corresponding vibrational modes of benzene and the symmetry species under CZ, symmetry are indicated. Frequencies (in cm-I) followed by relative intensities (vs, very strong; s, strong; m, medium; w, weak, vw, very weak). 'Bands observed only in the UVRR spectrum at 309.1-nm excitation. The region below the line was not measured.
different in the SEIRA and solid state spectra (Figure 2). This is interpreted by using the surface selection rule that only the vibrations that give dipole changes normal to the surface are infrared active.24 Since the CH out-of-plane mode at 822 cm-l (~11,bl) is observed very weakly and bZ modes (Y19b and ~ 1 5 ) are missing in the SEIRA spectrum, it is concluded that the molecule is oriented with the z axis nearly normal to the surface ("standing-up" orientation). The orientation is the same as those of thiophenol on silve$z and 2,5-dihydroxythiophenolon gold,33 which were determined by SERS and cyclic voltammetry, respectively. Analysis of SERS by the Electromagnetic Mechanism. The change in relative Raman band intensities caused by the adsorption on metal surfaces has been discussed with both EM34*35and CT17 mechanisms. Since it has been well established that the EM mechanism largely contributes to the SERS on silver island films,36 we first tried to interpret the SERS spectrum of PATP (Figure la) with the EM models proposed by C r e i g h t ~ nand ~ ~by Moskovits and S U ~ The . ~two ~ models are essentially identical and have been applied to orientation determinations of C2, molecules (pyridine,34p h t h a l a ~ i n e , ~ ~ . ~ ~ benzoic acid,38 and thiophenolz1.22)adsorbed on silver. According to the models, az, bl, and b2 symmetry species are
J. Phys. Chem., Vol. 98, No. 48, 1994 12705
SERS of p-Aminothiophenol Adsorbed on Silver
I
-
h, = 632.8 nm
= 488.0 nm
El *
1.2
I
I
I
I
I
I
I
-0.8
-0.6
-0.4
-0.2
0
0.2
1 -
0
$ 0.8
-
I
.$ 0.6 c C 0.4 0
i
0.2 -
a
0 -1
Potential/ V vs. SCE
-0.40
Figure 4. Potential dependence of the 1440-cm-' band (VIBb, b2) for 632.8- (a), 514.5- (b), and 488.0-nm excitations. The intensities are
-0.45
normalized with the maximum values for each excitation. 1
1400
1200
1000
1400
.
1
.
1
1
1200
1
1
1
3
Wavenumber I cml Wavenumber Icm-1 Figure 3. Potential and excitation energy dependence of the SERS from PATP adsorbed on a polycrystalline silver electrode surface in 0.1 M NaC104. enhanced in the order bl = b2 > a2 for the standing-up orientation. If the molecule were lying flat, the enhancement order should be a2 = bl > b2. The a1 modes are difficult to use for orientation analysis since they are composed of a linear combination of three diagonal elements of Raman tensor. However, the experimental results are apparently different from the expectations. The selective and tremendous enhancement only of the four b2 modes could not be explained by the EM models for any other considerable orientations. SEW in Electrochemical Environments. Another possible explanation of the selective enhancement of the b2 modes is that based on the CT me~hanism.'~Since the CT enhancement process is similar to the resonance Raman process, there exists a possibility that only limited numbers of modes are selectively enhanced, as is the case of usual resonance Raman scattering. The contribution of CT excitation to SERS can be probed in an approach described by Otto and Furtak et al.,l49l5 and Lombardi et al.17 This approach is based on tuning of the CT excitation into and out of the resonance by changing either Ef or hw. Since the electronic levels of the adsorbed molecule are easily changed relative to Efin electrochemical environments by changing applied potential, SERS intensity becomes large at a certain potential where the energy gap between Efand a molecular electronic level becomes equal to the excitation energy.6-8,12-18 Figure 3 shows the SERS spectra of PATP adsorbed on a polycrystalline silver electrode in 0.1 M NaC104 aqueous solution taken with 488.0- or 632.8-nm excitation at the potentials shown in the figure. The figure shows that the SERS spectrum is changed dramatically on applied potential and photon energy. For 488.0-nm excitation, the SERS spectrum at -0.2 V is identical to that in Figure la. As the potential is made more negative, the SERS bands assigned to b2 species greatly decrease in intensity and disappear completely around -0.4 V versus SCE, while the v7 (al) mode at 1077 cm-l is almost insensitive to the potential change. Similar behavior is seen for 632.8-nm excitation. However, the potential where the b2 modes disappear shifts to around -0.75 V. The spectral change was reversible and responded quickly (within 1 s) with the potential changes between 0.2 and -1.0 V, indicating that the molecule is not desorbed from the electrode surface in the potential range examined. No electrochemical reactions of the adsorbed PATP were observed by cyclic voltammetry. Since
the differential capacitance of the silver electrode modified with P A P is constant in the potential range between 0.2 and -0.6 V,39orientation change is not considerable. The peak intensity of the 1440-cm-' band is shown in Figure 4 as a function of applied potential for three excitation wavelengths of 632.8 (hw = 1.96 eV), 514.5 (2.41 eV), and 488.0 nm (2.54 eV). The intensities are normalized to the maximum value observed in the investigated potential range for each excitation. This band (and also other b2 modes observed in this spectral range) resonantly increases in intensity around -0.5 V for 623.8-nm excitation. The intensity-potential curve shifts to more positive potentials as the excitation energy increases. The potential where the intensity reaches a maximum linearly shifts toward positive values at a slope of about 1 eV/ V. This result is well interpreted in terms of the CT mechanism.l2-l8 The direction of the CT is from the metal to an affinity level of the adsorbed molecule. The affinity level is located at about 2.5 eV above Efat 0 V versus SCE. Since the v7 (al) mode does not show clear potential dependence, this mode is believed to be enhanced only by the EM mechanism. According to the theoretical CT model of SERS by Lombardi et al.,17 the intensity-potential profile should be symmetric if the affinity level involved in CT is symmetric. However, the observed profile for 623.8-nm excitation is not symmetric and the intensity falls rapidly at the negative potential side of the peak. Since differential capacitance of the silver electrode modified with PATP is abruptly increased at more negative potentials than -0.6 V,39 the rapid fall of the SERS intensity will be correlated to this capacitance change although details of this are not clear. UVRR Spectra. Since the CT mechanism of SERS is believed to be a kind of resonance Raman we can expect that a Raman spectrum similar to the SERS spectrum shown in Figure l a may be observed without the metal if the direct electronic transition from the ground state to the affinity level involved in the CT in the mewadsorbate system is excited. In other word, the affinity level in SERS will be probed by resonance Raman measurements. Figure 5 shows the UV spectrum of a 0.1 mM methanol solution of PATP. Three absorption bands are observed around 200, 250, and 300 nm. The spectrum is closely related to the parent benzene molecule, and these bands can be assigned to Bpb, La, and Lb in Platt's notation.40.41 The latter two transitions are symmetry-forbidden for benzene, but substituents on the benzene ring lower the symmetry and introduce some allowed character into the symmetry-forbidden transitions. In addition, the transition energies are shifted to longer wavelengths. La and Lb transitions transform as x (al) and y (bz),
Osawa et al.
12706 J. Phys. Chem., Vol. 98, No. 48, 1994
A 1
1
ho=239.5nm
$ c
-
400
WAVELENGTH I nm
Figure 5. UV spectrum of PATP in methanol. The arrows represent the excitation wavelength used in the UVRR measurements in Figure 6.
The arrows in the figure represent the excited wavelengths used in the UVRR measurements. The UVRR spectra of methanol solutions of PATP at 239.5, 282.3-, 299.1-, 309.9-, and 319.9-nm excitations are shown in Figure 6 from the top to bottom. The Raman bands of methanol were subtracted from the spectra. The concentration of PATP is 10-50 mM. These spectra are scaled such that the most intense bands are the same. The UVRR spectra are very similar to those of p-disubstituted benzenes41except the differences in peak positions of the substituent dependent modes: ~ 7 (1280 ~ ' cm-') and qa(1 102 and 1092 cm-l). Therefore, the assignment is straightforward. The splitting of v.la was observed also in the solution Raman spectra at visible excitations and is probably due to the Fermi resonance between ~ 7 and a the v6a -t ~ 1 combination mode (613 478 = 1091 cm-' in methanol). The bands at 1602, 1499, and 1180 cm-l are undoubtedly assigned to VSa, v19a, and v9a.41 It is noticeable here that the v& mode observed with visible excitations only in SERS is clearly seen in the UVRR spectra at 1572 cm-'. In addition, v19b (1445 cm-l), v14b (1320 cm-I), and Y9b (1 142 cm-I) are also weakly observed at 309.1- and 299.1-nm excitations within the Lb band. The latter three bands are not observed at other excitations. This finding strongly suggests the contribution of LUMO (n*b2) in the SERS as the affinity level. However, the b2 modes are not so strong in the UVRR scattering as in the SERS. The difference in enhancement pattern is discussed in the next section. Vibronic Coupling in UVRR and SERS. In resonance Raman scattering, three different enhancement mechanisms are involved. According to A l b r e ~ h t $ Raman ~ $ ~ ~ tensor elements can be represented by three terms under resonance Raman conditions as follows:
+
aeo= A -tB + C
(1)
where e and u represent x, y , or z . The A term represents a Franck-Condon contribution, and B and C terms HerzbergTeller contributions (vibronic terms). Term C is usually neglected because the energy gaps between the electronic ground state and excited states are too large to be vibronically coupled. In resonance with allowed electronic transitions, the dominant Raman scattering mechanism is represented by the A term. Only totally symmetric modes are enhanced by the A term. Since the excitations used in the present UVRR study are far from the allowed Ba,b transition, however, the contribution of the A term is small. For weakly allowed transitions, Raman scattering may instead be enhanced via the B term. This term is large if the weakly allowed resonance state is coupled vibronically to a nearby strongly allowed state. Both totally and nontotally symmetric modes can be enhanced via the B term. For substituted benzenes the La transition is coupled to the a component of the Ba,btransition via the normal coordinate v&,
2
1600
1400
1200
1000
WAVENUMBER / cm-1
Figure 6. WRR spectra of PATP in methanol at the excitation wavelengths of 239.5, 282.3, 299.1, 309.1, and 3199.9 nm.
and, to a lesser extent, via vga.41 The v8b mode can also couple the La transition with the b component of the Ba,b transition. On the other hand, the Lb transition is coupled to the Ba,b transition via v&,, v&, v19a, and v9, modes.41 Therefore, these vibrational modes are greatly enhanced by the intensity borrowing from the Ba,btransition. The same enhancement pattern is observed for P A P (Figure 6), and this is well understood in terms of dominant vibronic ( B term) scattering in resonance with the quasi-forbidden La and Lb states, as shown in Figure 7a. Vlgb, V14b, and v9b also can be enhanced, to a lesser extent, through the vibronic coupling between Lb and Ba,b transitions. Lombardi et al.17 proposed a theoretical CT model for SERS based on the theory of A l b r e ~ h t . ~ The ~ , Raman ~~ polarizability tensors are represented by three terms also in SERS. Different from the usual resonance Raman scattering, however, the C term CaMOt be neglected here because the energy gap between the ground state of the molecule and Efis small if Ef is located between the ground and excited states. Term A represents the Franck-Condon contribution, and only totally symmetric modes are enhanced by this mechanism, as is the case of resonance Raman scattering. Terms B and C arise from the HerzbergTeller term and represent the enhancement via molecule-to-metal CT and metal-to-molecule CT, respectively. Both totally and nontotally symmetric modes are enhanced by these terms. Since the CT is from metal to molecule for PATP on silver, as described above, the nontotally symmetric modes b2 are believed to be enhanced via the C term. The vibronic coupling in the SERS is between Efand the ground state of the molecule, as shown in Figure 7b.17 Therefore, the different enhancement patterns in the SERS and UVRR are attributed to the difference in electronic states vibronically coupled. Term C in the model of Lombardi et al.17 is represented by
SERS of p-Aminothiophenol Adsorbed on Silver
.
I
I
A
selectively enhanced in SERS via the Herzberg-Teller (vibronic) term. Due to the difference in electronic levels which are vibronically coupled, the enhancement pattern of SERS is different from that of resonance Raman scattering.
I
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J. Phys. Chem., Vol. 98, No. 48, 1994 12707
La (ai)
Acknowledgment. We gratefully acknowledge Prof. Hideo Takeuchi at Tohoku University for permission to use the UV Raman system and for helpful discussions. This work is partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture of Japan. References and Notes (a) Resonance Raman scattering
(b)SERS
Figure 7. Diagrams of electronic energy levels and vibronic couplings in resonance Raman scattering (a) and SERS (b) of PATP. The thick dashed lines represent vibronic couplings. In the resonance Raman scattering, the a1 and bz modes shown are selectively enhanced for Lb and La transitions via vibronic couplings with Ba,b transition. In the SERS,the CT transition from metal states IM> near the Fermi level Ef to the affinity molecular state IK), MMK,borrows intensity from the allowed transition MIKby means of vibronic coupling between IM) and the ground state IZ) through the matrix element hIM. The affinity level is identified as LUMO (Lb state).
where i andfare the initial and final quanta of the normal mode Q. MMKrepresents the electronic transition from metal M to the affinity level K, which obtains its intensity through intensity borrowing from the allowed transition from the ground state I to K , MKI. hIM represents the vibronic coupling of the metal to the ground molecular level through some vibrational mode. A resonance is predicted when o x WMK. In order for the C term to be nonvanishing, (ilQv>must be nonzero. This leads to the usual selection rule that f = i f 1, Le., the fundamental mode. This agrees well with our observation. It is further required that neither hlM nor components of MMKand MKI vanish. If a totally symmetric ground state is assumed, the symmetry species and r K of these electronic states must each correspond to re, the species of at least one translation. At the same time the direct product r K x r e x M r must contain the totally symmetric representationto prevent h m from vanishing.17 represents the irreducible representation of the excited normal mode. Since the molecular z axis is r = oriented normal to the metal surface for PATP on silver,M a1 and r K = b2 if the K state is LUMO. Therefore, only b2 modes can be enhanced via the C term. This prediction agrees well with the observation in Figure la. If second and third lowest unoccupied molecular orbitals were the K state, a1 and both a1 and b2 modes should be enhanced, respectively. Therefore, the affinity level is identified as LUMO.
Conclusion We have demonstrated that the CT mechanism dominantly contributes to the SERS of PATP adsorbed on silver on the basis of intensity versus potential profiles measured in electrochemical environments. The CT is from metal to molecule. The affinity level of the adsorbed molecule is attributed to LUMO, z* b2 under C2,, symmetry, from a comparison of the SERS spectrum with UVRR spectra of the free molecule in methanol. Four of the nontotally symmetric b2 modes are
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