J. Phys. Chem. 1983, 87. 5314-5319
5314
van der Ploeg have recently performed molecular dynamics simulations on a membrane bilayer model consisting of two layers of hydrocarbon chains.88 They found that the effective gauche-trans free energy difference increases with increasing packing density. This is consistent with the behavior of long-chain hydrocarbons in the solid or at high pressure in the liquid phase as discussed above.
Conclusion An overview of recent theoretical studies of medium effects on conformational equilibria has been provided with emphasis on molecular dynamics and Monte Carlo simulations. Good agreement between theory and experiment has been obtained for a variety of pure organic liquids including investigations at high pressure. Simulations of dilute solutions also appear promising, though technical challenges exist. At this point in the development of the theoretical procedures it is very important to stay in touch with experiment. Although through preferential and umbrella sampling meaningful studies can be carried out for dilute solutions, if the potential functions are unreasonable, misleading results will be obtained. A good start is to test the potential functions thoroughly on pure liquids since (88) Edholm, 0.;Berendsen, H. J. C.; van der Ploeg, P. Mol. Phys. 1983,48,379. van der Ploeg, P.; Berendsen, H. J. C. Ibid. 1983,49, 233.
accurate statistics can be obtained and many experimental data are available for comparison. Then, extension to dilute solutions should be supported by calculation of as many observable properties as possible such as heats and volumes of solution. The simulation methods clearly have great potential for enhancing the understanding of many aspects of condensed-phase systems, if they are carefully applied. Overall, solvent effects on conformational equilibria are commonplace and may have profound consequences on the chemical nature and behavior of a system. Pronounced effects are promoted by (1)significant changes in polarity for a molecule as a function of conformation, (2) the presence of strong solvent-solvent interactions and a relative flat rotational potential for the solute, (3) steric effects which may dictate the relative hydrogen-bonding ability of different conformers, and (4)competitions between intra- and intermolecular hydrogen bonding.
Acknowledgment. The author is grateful to his coworkers listed in the references for their insights and assistance. Many other colleagues have provided helpful ideas and discussions over the years, particularly Drs. D. L. Beveridge and M. Mezei. Support from the National Science Foundation and Purdue University is also gratefully acknowledged.
ARTICLES Auger Electron Spectroscopic Evidence for Laser Damage on Silver Electrodes Exhibiting Surface-Enhanced Raman Scattering Ralph P. Cooney,” Terrence P. Mernagh, Merrlck R. Mahoney, Chemistty Department, University of Newcastie, New South Wales, 2308, Australia
and John A. Splnk CSIRO Division of Materials Science, University of Melbourne, Parkville, Victoria, 3052, Australia (Received: November 17, 1982; In Final Form: April 8, 1983)
Silver electrode surfaces which exhibited the surface-enhanced Raman scattering (SERS) effect were examined for evidence of laser damage by scanning electron and scanning Auger microscopy (SEM/SAM). The laser illuminated area of the silver electrodes which are the origin of the pyridine SERS effect are localized zones (ca. 0.03 mm diameter) of pure or nearly pure carbon. The carbon is thought to originate primarily in the laser-assistedcarbonization of pyridine (free or coordinated to anodically generated silver(1)). The SERS electrode from the Ag/KCN, NazS04(Hz0)system exhibits more extensive laser damage than the pyridine system. In the cyanide system, the composition of the laser damage zone is complex, being richer in K, 0, and S but depleted in Ag and C relative to the nonilluminated surface. Cathodically cleaned surfaces exhibited no evidence for laser damage. Introduction The origin of the surface-enhanced R~~~~ scattering effect (SERS) has been the subject of extensive debate.’ Theories formulated to explain the effect have assumed localized (e = 1) or nonlocalized (0 > 1) enhancement processes which were generated by adsorption of pyridine,
cyanide, etc. on a clean silver surface.2 Silver-metal generated enhancement theories can be simplistically analyzed in terms of surface morphology. They involve flat surfaces, microscoPicallY roughened surfaces, Or (chemisorption On) atomically roughened surfaces.’ In situ Cathodic cleaning procedures have been developed for silver electrode surfaces3 and such cleaned silver surfaces
(1) R. P. Cooney, M. R. Mahoney, and A. J. McQuillan in “Advances in Infrared and Raman Spectroscopy”, Vol. 9, R. J. H. Clark and R. E. Hester, Ed., Heyden, 1982, Chapter 4, and references therein.
(2) T. E. Furtak and J. Reyes, Surf.Sci., 93, 351 (1980). (3) M. W. Howard, R. P. Cooney, and A. J. McQuillan, J. Raman Spectrosc., 9, 273 (1980).
0022-3654/83/2087-5314$0 1.50/0
0 1983 American Chemical Society
AES Study of Laser Damage on Ag SERS Electrodes
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983
5315
TABLE I: Silver Surfaces Examined b y Auger/SEM Techniques predictionsb
pretreatmenta
findings (Auger/SEM)
Ag/O.l M KCI, 0.05 M Pyridine ( O R C : ” -0.6 V - t 0.12 V (SCE) a t 5 mV s-’) (a) o n e O R C t laserC cathodic cleaninga n o carbon s p o t ( n o SERS) n o carbon spot (b) o n e ORC + laser ( n o cleaning) carbon spot (SERS) pure carbon s p o t ( c ) o n e ORC + laser ( n o cleaning) carbon s p o t (SERS) (less C t h a n in ( b ) ) less carbonized s p o t (0.03 m m diameter) ( d ) ten ORC + laser ( n o cleaning) heavy carbon s p o t (SERS) pure carbon spot (0.03 m m diameter) with some cratering +
-
Ag/O.l M Na,SO,, 0.01 M KCN, p H 11(ORC:I3 - 0.95 V pulse step t o + 0 . 5 V (SCE) f o r 5 s) ( a ) cathodic cleaning ( n o laser, no O R C ) n o damage ( n o SERS) n o damage ( b ) cathodic cleaning + - 0 . 9 5 V + laser n o damage ( n o SERS) n o damage (c) as f o r ( b ) t h e n O R C + laser oxygen-rich damage zone ( SERS)13 substantial damage zone (rich in S, 0, K ; depleted in C, Ag) a +, a t t h e same time as; + followed by. Laser focus, 0.1-0.01 m m diameter. See mechanism in ref 1and 9. mW of focussed 514-nm Ar+. - 1 . 4 V for ca. 1 h (see ref 3, 4, and 6 for details).
(flat, anodically roughened, or massively roughened) exhibit no SERS effecta4 SEM investigation of the anodically generated roughness features show they are unchanged by cathodic leaning.^ Adatom-chemisorption theorieslJ fail to explain why the SERS spectra of pyridine (under electrochemical or ultrahigh vacuum conditions), benzene, and alkanes are clearly indicative of physically held and not chemisorbed specie~.‘*~n~ Carbon has been detected8 on SERS electrode surfaces and has been implicated in the pyridine SERS effect (see Figure 21 of ref 1). The evidence for surface c a r b ~ n l and *~#~ for the formulation of the scattering phase as an intercalation phase [~arbompyridine]~,~ is given elsewhere. Carbon has been detected by Auger/Raman techniques on SERS silver surfaces under ultrahigh vacuum conditions1° and for tunnel-junction interfaces.1° The Raman carbon spectrum is apparently more intense for argon-ion cleaned silver surfaces in contact with pyridine vapor.11J2 The catalytic carbonization of pyridine on actual silver surface sites or particles would therefore appear to be the major source of surface carbon in ultrahigh vacuum SERS systems. The Ag/CN- SERS system has been investigated in detail by the present authors because under working electrode conditions no carbon features were evident13and because radiochemical coverage data14J5for nonilluminated large electrode surfaces indicated 0 2 or 3. However, uniform laser illumination of wire electrodes at a photon flux much less than that used in SERS experiments revealed substantial laser-assisted oxidation effects.16 The
-
(4) T. P. Mernagh and R. P. Cooney, J. Raman Spectrosc., 14, 138 (1983). (5) A. Otto, review paper presented at the 6th Solid/Vacuum Interface Conference, Delft, The Netherlands, May, 1980. (6) M. W. Howard and R. P. Cooney, Chem. Phys. Lett., 87, 299 (1982). (7) R. P. Cooney, M. R. Mahoney, and M. W. Howard, Chem. Phys. Lett., 76, 448 (1980). (8) M. R. Mahoney, M. W. Howard, and R. P. Cooney, Chem. Phys. Lett., 71, 59 (1980). (9) R. P. Cooney, M. W. Howard, M. R. Mahoney, and T. P. Mernagh, Chem. Phys. Lett., 79, 459 (1981). (10) J. C. Tsang, J. E. Demuth, P. N. Sanda, and J. R. Kirtley, Chem. Phys. Lett., 76, 54 (1980). (11) H. Seki and M. R. Philpott, J. Chem. Phys., 73, 5376 (1980). (12) R. R. Smardzewski, R. J. Colton, and J. S. Murday, Chem. Phys. Lett., 68, 53 (1979). (13) M. R. Mahoney and R. P. Cooney, J.Raman Spectrosc., 11,141 (1981). , , (14) G. Blondeau, J. Zerbino. and N. Jaffrezic-Renault. J. ElwtroannL
Chem.., 112. 127 11980). (15) J. G. Bergman, J. P. Heritage, A. Pinczuk, J. M. Worlock, and J. H. McFee, Chem. Phys. Lett., 68, 412 (1s180). ---r
~-
\ - - - - ,
100
laser-induced oxidation of the surface, clearly indicated by these results, was identified in preliminary SEM analysis of conventional evaporated silver SERS surfaces.16 The p H dependence of the cyanide SERS signal (which terminates at pH -7) has been interpreted in terms of silver oxide involvement in the intensely scattering phase at the natural alkalinity (pH -11) of the SERS system.13 The related gold/cyanide system, which is also a natural corrosion system, is known to involve oxide/ hydroxide passivating films.17 The present Auger investigation of the silver SERS electrode surfaces has been undertaken to establish if laser damage is evident and, if it is, to confirm that the damage is in the form of localized (0.1-0.01 mm) carbon zones for pyridine/silver surfaces and oxide-rich zones for cyanide/silver surfaces. Previous Auger studies,18 which excluded pyridine and did not consider the possibility of laser damage, revealed the presence of AgCl on the non-illuminated anodized surface.
Experimental Section The electrochemical and Raman spectroscopic equipment and the materials used were described in detail p r e v i o ~ s l y .A~ Raman ~ ~ ~ spectroelectrochemical cell following the design of Pettinger et al.19 was employed in generating SERS spectra, and 100 mW of 514-nm Ar+ was used in these experiments. The scanning electron/scanning Auger (SEM/SAM) facility was designed and constructed in the CSIRO Materials Science Laboratoryz0 and included a fine-focus electron gun and a Vacuum Generators HCVA Model 850 hemicylindrical electron analyzer incorporating an internal fluorescent screen and an optically coupled external photomultiplier tube. The ultrahigh vacuum system was ion pumped to a working background pressure of Pa. Auger analyses in both static and scanning modes were performed with a 3-kV, 1-pA incident electron beam which had a fwhm of -10 pm. The analyzer modulation was varied from 1 to 4 eV rms.
-
(16) M. R. Mahoney and R. P. Cooney, J. Phys. Chem., 87, 4589 (1983). (17) D. W. Kirk, F. R. Faulkes, and W. F. Graydon, J . Electrochem. SOC.,127, 1962 (1980). (18) J. F. Evans, M. G. Albrecht, D. M. Ullevig, and R. M. Hexter, J. ElectroanaL Chem., 106, 209 (1980). (19) B. Pettinger, U. Wenning, and D. M. Kolb, Ber. Bunsenges. Phys. Chem., 82, 1326 (1978). (20) J. A. Spink and P. I. Davey, “Scanning Auger Studies in Catalysis and Materials Science”, presented at the Royal Australian Chemical Institute 7th National Convention, Solid State Division Symposium, Canberra, August, 1982.
5316
Cooney et ai.
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983 (a1 LASER OXIDATION
, SURFACE
*
t
t
t
C
J PY *[Ag(PY),lX e 8 IO
ADSORBED LAYE S
-
0.03mm LASER DAMAGE ZONE U
b
R CARBONIZATION
SOLUTION
'
LASER
'!,
r
SOLUTION
/ /PYRIDINE
(bl LASER OXIDATION
E LASER
SOLUTION
II
a
rA
/
2
(a)
SOLUTION KCN rn
5
3
> U
a U
Flgure 1. Possible mechanisms of laser damage on anodized SERS electrode surfaces: (a) Ag/pyridine, KCI (H,O), 8 data from ref 14; (b) Ag/KCN, Na,SO, (H,O), 8 data from ref 14 and confirmed In the present study. For a 1-mmdlameter electrode surface, [noqllluminated area/illuminated area] lo3.
-
Silver electrode surfaces in the form of the 1-mm circular end section of a Johnson-Matthey Specpure silver wire were employed to geometrically limit the possible area within which laser damage might occur. The various surfaces examined by Auger analysis and SEM are described in Table I in terms of their pretreatment, predictions of laser damage, and a statement of the key findings.
Results and Discussion &/Pyridine Surfaces. The predictions (based on earlier Raman/electrochemid studies) for the surfaces examined by SEM/SAM are summarized in Table I. The mechanism of pyridine intercalation in carbonlfgis the specific basis of these predictions for the pyridine system. Other carbonizable adsorbates (Le., organic molecules) would be expected to react in a similar fashion. Electrode surfaces which have been cathodically cleaned3t4and are free from carbon and anodically formed silver(1) carbonizable species (i.e., are fully reduced) do not exhibit the pyridine SERS effect and so would not be expected (Table I) to exhibit laser damage zones. Surfaces which have been subjected to a single oxidation-reduction cycle (ORC) contain carbonizable multilayer~l,'~ of [Ag( p ~ ) ~ ] Cand l this material together with multilayered pyridine on the surface1J4would be predicted to carbonize selectively within the laser spot (see Figure 1). Solution pyridine may migrate to the illuminated zone either to carbonize or to adsorb (Figure la). A carbon film 0.03 mm in diameter, with a laser penetration depth24of ca. 30 nm, assuming an average carbon-carbon distance of 0.3 nm, could form from only ca. mol of pyridine. If the laser is focussed on the surface during the ORC, it would be expected to assist the oxidation of the surface (reflected in a small although significant photoanodic displacement of the voltammogram). The mechanism of photon-tosurface energy transfer may well involve the plasmonphoton resonance processes invoked in enhancement theories.lP2 If the laser is focussed on the surface after the ORC, only those multilayers of pyridine surviving the reduction half-cycle of the ORC (i.e., the 8 = 10-100 described by Blondeau et al.14) would be expected (Table I)
t m U
a
v
w
9 P z
K
100
300
500
I
100
I
I
300
I
I
500
II
I
100
I
I
300
I
I
500
ELECTRON ENERGY (eV)
Figure 2. A. Low magnification scanning electron mlcrograph of silver/pyridine specimen showing positions from whlch Auger spectra were recorded: (a) background; (b) laser exposure after a single ORC; (c) laser exposure during a slngle ORC. Angle between incident electron beam and speciment normal was -30'. B. Auger spectra from silver/pyridine SERS electrode surfaces (see Table 11): (a) no laser; (b) laser exposure after a single ORC; (c) laser exposure during a single ORC.
to carbonize. The carbonization would therefore be predicted to be less than for laser irradiation during the ORC. Repetitive ORC cycling (ten in this study) with simultaneous laser illumination would be predicted (Table I) to lead to a heavily carbonized laser spot. Significant cratering is also expected because of the accumulation of laser photooxidative stages with repetitive ORC. These repetitively anodized surfaces are grey in color and would be expected to resemble the surfaces prepared in early SERS studies.21 They differ in macroscopic appearance and microscopic (SEM) surface s t r ~ c t u r e lfrom , ~ the conventional SERS surface described earlier. The results of the Auger/SEM investigation (see Figure 2 and Table 11)agree well with these predictions. No laser damage spot was observed for the cathodically cleaned surface. However, for a surface subjected to an ORC under focussed laser light a spot of pure carbon (of dimensions expected for the focussed laser) was found (see Figure 1, Figure 2 and Table 11). For a surface subjected to focussed laser light after the ORC, a less extensively carbonized spot of similar dimensions was observed (Figure 1and Table 11). For a surface subjected to ten ORC with simultaneous focussed-laser illumination a very well-defined crater (0.03 (21) M. Fleischmann, P. J. Hendra, and A. J. McQuillan, Chem. Phys.
Lett., 26, 163 (1974).
AES Study of Laser Damage on Ag SERS Electrodes
The Journal of Physical Chemlstty, Vol. 87, No. 26, 1983 5317
SECONDARY ELECTRON IMAGE AND AUGER IMAGES OF Ag,K,O & C FROM A SILVER /CYANIDE SERS SPECIMEN. SEM
K
0
C
Flgure 3. Low magnification secondary electron Image and scanning Auger images of a silver/cyanlde SERS specimen exhibiting laser-Induced damage.
mm) was observed with an associated zone of pure carbon (Table 11). The carbon/silver ratio (Table 11) was observed to increase from a nonilluminated surface ratio of -5 to laser-damage zone ratios of m for laser irradiation carried out both after and during a single ORC. The spectra show, however, that whereas other elements are present in the former instance (Figure 2B(b)), only pure carbon is observed in the latter (Figure 2B(c)) where the conditions represent the typical SERS experimental conditions. In general (Table I), it can be seen that laser damage coincides with the SERS effect. The probable mechanism of laser-assisted surface oxidation and complexation with subsequent laser carbonization of pyridine is illustrated in Figure 1for laser illumination during the ORC. Some cratering occurs under these conditions which would be expected to be less evident for laser illumination after the ORC. The carbonization of silver(1) pyridine complexes under the laser is a macroscopic reaction of the same type. Laser carboniza-
tion of the silver(1) pyridine complexes would passivate the damage zone. Carbon overlayer passivation is a common feature of the interfacial electrochemistryof metals.22 Further, the resultant carbon “plug” would absorb laser light and so prevent damage zones from spreading laterally across the surface. &/Cyanide Surfaces. In this case preliminary investigations pointed to extensive laser-induced oxidation of the surface.16 For example, uniform (unfocussed) laser illumination (at 12% SERS flux levels) of a 1-mm silver wire electrode resulted in photogalvanic effects. The major voltammetric waves were displaced in an anodic (oxidizing) direction by the introduction of laser flux.16 As a result of this observation, the surface of a vapor-deposited electrode, after anodization under focussed laser light, was examined by SEM and shown to exhibit significant laser damage. While Auger spectroscopy appears to be insensitive to cyanide15 it was hoped that the technique would (22) K. D.Snell and A. G. Keenan, Chem. SOC.Rev., 8, 259 (1979).
5318
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983
Gooney et al. ~
I
I
l
l
Ag/KCN
I
l
l
l
I
l
I
(a)
I
I
t
200 400 600 800 1000 ELECTRON ENERGY (eV)
Ag/KCN
I
I
I
l
1
1
I
(b)
I
I
200 400 600 800 1000 ELECTRON ENERGY (eV)
Flgure 4. Auger electron spectra of silver/cyanide specimen (shown In Figure 3) from nonilluminatedarea (a) and from area exposed to laser beam (b).
TABLE 11: AES Analysesa of Ag/Pyridine S E R S Electrode Surfaces one ORCb
Auger peak, eV silicon, 92 sulfur, 152 chlorine, 181 potassium, 252 carbon, 272 silver, 356 oxygen, 510 carbonlsilver ratiosd a
no laser
6 30 20 310 72 66 16 -5
TABLE 111: AES Analysis‘ of Ag/KCN S E R S Electrode Surface
t e n ORC Auger peak, eV
laser laser laser after during n o during O R C O R C laser ORC NDC 5 14 34 187 ND ND m
ND ND ND ND 167 ND ND m
ND 13 33 190 50 48 11 -5
silicon, 9 2 sulfur, 152 chlorine, 181 potassium, 252 carbon, 272 silver, 356 nitrogen, 379 oxygen, 510 sodium, 990
ND ND ND ND 95 ND ND
oxygen/silver ratiosC
m
T h e AES data represent peak-to-peak excursions in t h e
cW/dEvs. E curves. T h e arbitrary units are normalized t o an electron beam current of 1 fiA, and a modulation voltO R C : see Table I; laser (focussed): age of 2 e V rms. 100 mW at 514-nm Ar’. ND: n o t detectable. AES data modified to allow for relative Auger sensitivities [ cf. Davis, L. E. e t al. “ H a n d b o o k of Auger Electron Spectroscopy”; Physical Elec. Indust. : Eden Prairie, MN, 19671.
assist in the identification of the phase formed by laser damage. The present authors had concluded on the basis of pH dependence that the phase involved silver oxide but other authors interpreted their results in terms of [Ag(CN),] 2- and related surface complexes.23 The Auger data indicated that the zone of laser damage for anodized-illuminated surfaces (see Figures 3 and 4 and Table 111) is richer in potassium, oxygen, sulfur, and sodium but depleted in silver and carbon relative to the nonilluminated surface. The high sulfur levels in the damage zones are unlikely to be of direct significance to the SERS effect, as SERS is observed for perchlorate13 (in place of sulfate) aqueous media and the sulfate spectrum is not enhanced.23 The observation of higher oxygen levels in the damage zones (Table 111) would appear to support (23)J. Billman, G. Kovacs, and A. Otto, Surf. Sci., 92, 153 (1980). (24) R.P.Vidano, D. B. Fischbach, L. J. WiIlis, and T. M. Loehr, Solid State Comrnun., 39,341 (1981). (25) T. P. Mernagh, R. P. Cooney, and R. A. Johnson, Carbon, in press.
n o laser
laser during O R C b
9 95 25 33 146 218 7 32 21 0.28
9 20 7 16 56 103 117 7 48 32 0.78
T h e AES data represent peak-to-peak excursions in t h e m/dE vs. E curves. T h e arbitrary units are normalized to an electron beam current of 1 M Aand a modulation voltage of 2 eV rms. O R C : see Table I; laser (focussed): 100 mW 514-nm Ar’. AES data modified t o allow for relative Auger sensitivities [ cf. Davis, L. E. e t al. “Handbook of Auger Electron Spectroscopy”; Physical Elec. Indust.: E d e n Prairie, MN, 19761. a
the assignment to a silver oxide associated phase.l3 However, the presence of high levels of potassium suggests the latter is playing a role as a countercation and that the intensely scattering damage zone (i.e., SERS) phase is of the type x(KCN).y(Ag,O)-z(AgCN) rather than x(Ag20).y(AgCN) or x(KCN).y(AgCN). The more striking feature of the Ag/CN- study is the extent of laser damage. While individual laser spots can be recognized it is apparent that extended illumination results in some extension of the damage zone acrow the surface and presumably also into the surface. This has been rationalized (see Figure 1b) in terms of the scattering phase (unlike carbonized pyridine) being transparent to laser light thus permitting the laser-assisted oxidation processes to continue. Further, as cyanide is not carbonizable, no subsequent damage-zone passivation (of the type observed for carbonized pyridine) occurs.
Conclusions The data presented in this paper support conclusions drawn from earlier experimental studies of the silver SERS effect.
J. Phys. Chem. 1983, 87,5319-5325
5319
Carbonizuble Adsorbate: AglPyridine SERS. The Auger data suggest that the pyridine SERS effect emanates from the relatively minute laser region of the electrode surface which is pure or nearly pure carbon. This is in accord with the previous formulation of the intensely scattering phase as [carbon-pyridine] and not pyridine itself. The enhancement factor (lo4-lo5)for this system is calculated from parameters for the nonilluminated26 silver surface (viz. pyridine coverage, roughness). The calculation does not consider either pyridine accumulation on laser-zone carbon or resonance (nonsurface) enhancement due to carbon i t ~ e l f . Therefore ~ ~ , ~ ~ the residual silver-surface enhancement of the [pyridine-carbon] phase is, a t present, unknown. Recent studies2Ihave revealed that under typical SERS conditions28black laser surface damage spots are visible to the unaided eye for a range of organic adsorbates
(pyridine, glycine, ethylenediamine, 4-aminopyridine, etc.). In each case, the observation of the black damage spot is accompanied by the appearance of intense carbon spectral features. Noncarbonizable Adsorbate: AglCyanide SERS. Extensive laser damage is clearly evident in this case; however, the identity of the intensely-scattering phase is not clearly defined by the Auger data. The data provide some support for both surface phases previously proposed (silver(1)oxide cyanide and silver(1) cyanide complex). For this reason, a mixed formulation is suggested. However, cyanide coverage data based on the nonilluminated surface are unlikely to relate to coverage within the laser (SERS) zone on the surface. Therefore the magnitude of the enhancement factor is again uncertain. The common factor in both of these systems is the active role of the focussed laser in modifying the local composition of anodized silver surfaces.
(26) Nonilluminated surface: a surface not exposed to laser light or those zones of an illuminated surface outside the (relatively) minute laser zone. (27) R. A. Kydd and R. P. Cooney, J . Chem. Soc., Faraday Trans. I , in press. (28) Aqueous KC1 electrolyte; ORC: -0.6 V F= +0.2 V at 5 mV s& and 5100 mW at 514-nm Ar+.
Acknowledgment. The authors are grateful to A.R.G.S. for providing Raman spectroscopic and electrochemical equipment, and to Dr. J. A. D. Matthew for helpful advice. Registry No. Ag, 7440-22-4; KCN, 151-50-8; Na2S04,775782-6; pyridine, 110-86-1.
Combined Surface-Enhanced and Resonance-Raman Scattering from the Aspartic Acid Derivative of Methyl Orange on Colloidal Silver 0. Sllman, A. Lepp, and M. Kerker” Department of Chemistty, Clarkson College of Technology, Potsdam, New York 13676 (Received: November 23, 1982; In Final Form: March 10, 1983)
By selecting a chromophore, dabsyl (N-4-dimethylaminoazobenzene-4’-sulfonyl) aspartate (DABS-ASP), whose absorption spectrum overlaps with the surface-enhanced Raman scattering (SERS) excitation profile for colloidal silver sols, we have found it possible to study the triple combination of resonant Raman scattering (RRS) of the DABS-ASP in solution and both the combined RRS-SERS (SERRS) and the SERS of DABS-ASP adsorbed on the colloidal silver particles. The SERRS and SERS spectra were distinctly different from each other. The measured surface enhancements were in excess of lo3. This is less than the values of the order of lo5 measured earlier for citrate on these silver sols. Just as for citrate, the excitation profiles peaked at about 500 nm whereas the main absorption band was in the region of 400 nm, a disparity from our electrodynamic model for SERS.
Introduction Surface-enhanced Raman scattering (SERS) studies of species adsorbed to colloidal silver,l roughened silver electrodes,2 and silver films3 have shown that Raman signals of the adsorbate can be enhanced by as much as 105-106-fold. Thus, when combined with resonance scattering4 (RRS) from an appropriate chromophoric ad-
sorbate, total Raman enhancement factors might reach the lo1O to range- In those examples, where RRS and SERRS spectra of chromophoric adsorbates have been reported (crystal violet and methyl orange on a Ag electrode? p-nitrosodimethylaniline on Ag and Pt electrode^,^^ myoglobin and cytochrome c on a Ag electrode,k dithizone on a Ag e l e ~ t r o d e , ~rhodamine ~-~ 6G on silver-island
(1) (a) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem.
(4) (a) Behringer, J. In “Raman Spectroscopy. Theory and Practice”; Szymanski, H. A., Ed.; Plenum: New York, 1967; Vol. 1, p 168. (b) Clark, R. J. H.; Stewart, B. Struct. Bonding (Berlin) 1979, 36, 1. (c) Carey, P. R.; Salares, V. R. In “Advances in Infrared and Raman Spectroscopy”; Clark, R. J. H.; Hester, R. E., Ed.; Heyden: London, 1980; Vol. 7, p 1. (5) (a) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 8 4 , l . (b) Hagen, G.; Glavaski, B. S.; Yeager, E. Ibid. 1978,88, 269. (c) Cotton, T. M.; Schultz, S.G.; Van Duyne, R. P. J. Am. Chem. SOC.1980, 102,7962. (d) Pemberton, J. E.; Buck, R. P. J. Phys. Chem. 1981,85,248. (e) Pemberton, J. E.; Buck, R. P. Anal. Chem. 1981,53, 2263. (f) Pemberton, J. E.; Buck, R. P. J. Electroanal. Chem. 1982,132,291. (g) Weitz, D. A.; Garoff, S.; Gramila, T. J. Opt. Lett. 1982, 7, 168.
Soc., Faraday Trans. 2 1979,75,790. (b) Kerker, M.; Siiman, 0.;Bumm, L. A,; Wang, D.4. Appl. Opt. 1980, 19, 3253.
(2) (a) Van Duyne, R. P. In ’Chemical and Biological Applications of Lasers”; Moore, C. B., Ed.; Academic Press: New York, 1979; Vol. 4, p 101. (b) Furtak, T. E.; Reyes, J. Surf. Sci. 1980, 93, 251. (c) Otto, A.
Appl. Surf. Sci. 1980, 6, 309. (3) (a) DiLella, D. P.; Moskovits, M. J . Phys. Chem. 1981, 85, 2042. (b) Murray, C. A.; Allara, D. L. J . Chem. Phys. 1982,76,1290. (c) Chang,
R. K.; Furtak, T. E., Ed.; “Surface Enhanced Raman Scattering”; Plenum Press: New York, 1982. (d) Otto, A. In “Light Scattering in Solids”; Cardona, M.; Guntherodt, G., Ed.; Springer: Berlin; Vol. IV, in press.
0022-3654/83/2087-5319$01.50/00 1983 American Chemical Society
