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Normal raman scattering from pyridine adsorbed on the low-index faces of silver
Volume 91. number 6
NOR\lAL
CHEhIlCAL
RAMAN SCA-ITERING
PHYSICS
FROM PYRIDINE
LETTERS
11 February
1983
ADSORBED
OK THE LOW-INDEX FACES OF SILVER
\\;c find no cnh~nccmcnt
oi the Raman
adsorbed at 120 K on the (1001, (110) and unshifted from thoseof liquid pyridine, intensity rz~tios are similar to the liquid. the signal intensity is linear in covemge from submonolayer to multilayer, and the dcpolariz;ltion ratio is Io\v. Since these obwrvations are in marked contrast to those associated with surface-enhanced Rsmun scattcrins (SERS). WC’conclude that our spectra result from normal Raman scattering. Our results support the bypwhrsis that spcci;d adsorption sites are responsible for 3 substantial fraction of the total enhancement of the Raman cross section for :hc pyridinc-silver system.
( 111) f;lres of silver in ulrnhigb ticuum.
scwerin_r
The
cross section
frequcncius
I _ Introduction
Siucc the discovery of surface-enhanced Raman scattering (SERS) there has been a tremendous effort. both esperirnental and rlleoretical, devoted to the e1ucidatk~n of Ihe enhancement mechanism_ Several rcvicw articles [ 1_2] and a recent book [3] summarize the esperimental results and various rheorerical points of view. It is generally agreed that the enhancement mechrtnisms may bc divided into two broad classes, whose 11131involve enhanced local (laser) electric fields al ihc surfaces of small metallic particles or gratings, and those that increase rhe magnitude of the molecular polarizability dcrivarive, through the charge transfer that accompanies cI,lemisorption. It has not been pt)ssibie to derermine unequivocally the relative magnitudes of these Iwo contributions to the torsl SERS intensity. even for the most frequently studied system pyridine on silver_ For reasons of sensitivity, the over\rhchning majority of studies carried out to date have used geometric structures (roughened surfaces, ~olloids, gratings) that provide some degree of purely electromagnetic enhancement. Control of atomic scale order. and thus the nature and distribution of potential adsorption sites. on such surfaces is difficult at best and the problem camlot be decoupled esperi576
for pyridine
WC observe
xc
essenthlly
mentally using conventional approaches. Since we have been successful in obtaining highquality Raman spectra from molecules adsorbed on well-characterized surfaces at low coverage without enhat~ccment [4], we felt that we could address the question of molecular enhancement mechanisms in the absence of any purely electromagnetic contribution, by working with fiat surfaces. Electrodynarnic effects at flat surfaces are well understood and lead to only minor enhancement [5]. We report here the first observation of normal (unenhanced) Raman scattering from a monolayer of pyridine adsorbed on each of the low-index faces of silver.
2. Experimental The experimenti were performed using an ultrahigh vacuunl (UHV) chamber equipped with a quadrupole mass spectrometer and low-energy electron diffraction (LEED) optics that were also used for retarding field Auger electron spectroscopy. Base pressure during experiments was in the low 1O- lo Torr range. Silver samples were cut from two different singlecrystal rods, supplied by Monocrystals, Inc. The rods 0 009-2614/S3/0000-0000/S
03.00 0 1983 North-Holland
Volume 94, number 6
11 Februw
CHEMICAL PHYSICS LETTERS
1983
were aligned to within lo of the desired direction by
of =SOO cm-l.
Laue backscattering, and sample disks were cut with a spark cutter. The crystals were polished through a series of emery papers and 6 pm and 1 I.tm diamond paste. A final chemical polish in a solution of potassium dichromate resulted in a mirror-like, scratch-free surface. The crystals were cleaned in UHV by repeated argon ion sputtering (3 keV, a-10 PA cmm2 for 10 min) and annealing (3OOOC) cycles_ Surfaces prepared in this manner showed no features under the scanning electron microscope at 500 A resolution, except for a few isolated etch pits of I--1Oprn diameter. Surface impurities were less than 1% as determined by Auger spectroscopy, and the LEED patterns consisted of very sharp spots with a low background Intensity_ Consequently, we consider these surfaces to be smooth, flat, and essentially free of both submicroscopic and atomic scaie roughness. Pyridine was dosed onto the cold surface (120150 K) of the crystal by backfiiing the chamber (typically 5 X loss Torr for 100 s)_ Because of rather long pumpdown times for pyridine, the actual dose might have been 50% larger than the indicated dose. Raman scattering was excited by the 5145 _&line of an argon ion laser, focused to a 100 pm diameter spot on the crystal at an angle of incidence of 70° with respect to the surface normal_ A half-wave Fresnel rhomb was used to select the input polarization. The laser power was typically 1 W, although powers as low as a few hundred mW and as high as 2.5 W were occasionally used. The signal intensity was always linear in power and we never observed any ill effects due to laser heating. This behavior is easily understood- Even for p polarization, silver reflects more than 95% of the incident radiation_ Furthermore, since it is such an excellent thermal conductor, neither the bulk nor the surface temperature rises more than a few degrees under laser illumination. The scattered radiation was collected using f/O.95optics at 55” to the surface normal in a plane orthogonal to the plane of incidence_ A Schott OG 530 colored glass filter was used to reject light scattered at the laser frequency_ An Instruments SA. HR-320 spectrograph fitted with an 1800 lines/mm holographic grating blazed at 550 nm dispersed the scattered radiation across the faceplate of a cooled, intensified vidicon detector (EG & G PARC 1254). With this arrangement the dispersion is =l cm- 1 per channel, giving a total spectral coverage
linewidth is 10 cm-l_ For the polarization measurements, a polaroid film and polarization scrambler were inserted into the optical path.
With the slits used, the instrument
3. ResuIts and discussion Fig. 1 shows the 1000 cm- 1 region of the Raman spectrum of pyridine adsorbed on the low-index silver faces following a dose of 5 L (uncorrected for ion gauge sensitivity) at 120 K. This dose produces approximately monolayer coverage, as determined by the UPS measurements of Sanda et al. [6] for the (111) face. Additional evidence for rhe coverage is provided by our Auger measurements. A plot of the intensity of the carbon Auger signal at 272 V versus exposure shows a change in slope near 5 L on all three crystal faces. Such behavior has been taken to indicate monolayer completion [7,8]. The vibrational frequencies are essentially identical to those of liquid pyridine and the scattered intensities are comparable on all three faces. The intensities are extremely low, considering the laser power used and the high detec-
1100
1050
1000
950
900
AVkm-‘1 Fig- 1_ Normal Raman spectra of the 1000 cm-l re$on of pyridine adsorbed on the low-index faces of lilver at 120 I;. following a 5 L dose. Integration time was 500 s total. with 1 W of 5145 J%radiation_
577
Volume 94. nurnbcr 6
CHEMICAL PHYSICS LETTERS
11 February 1983
lion efficiency of our system, immediately sugesting that SERS is absent_ Using the observed count rate, the laser power. our calibrated detection efficiency and assuming the coverage to be 6 X 10’” molecults cm- 2 [9], we calculate a scattering cross seciion of5 X 1O-2g ctn2 moiecule-1 ST- 1 for the ad-
sorbed pyridine which is close to the liquid value of 1.5 X IO- ‘g cm? molecule-1 sr- l [IO] (integrated over the lineshape and corrected to 5145 A excitation). The Raman intensity is linear in exposure from I ---20 L in the present experiments, unlike SERS where short-range (first layer) enhancements are oftrn observed [6.7.1 I]_ Fig. I! shows several different spectral regions for pyridine adsorbed on A&l IO). Similar results were obtained for the other two facts. The band positions and relative intensiries are quite similar to those of liquid pyridine. SERS spectra differ from these in two important ways. C-H stretching modes are genemlly weak (if observed at all) in SERS [ 1l] and the 992 c111- 1 ring breathing mode of pyridine is invariably shif‘ted to -51005 cm- 1 [3]_ This latter point is imporrant.
particularly
when comparing our results to
previous work on “smooth” silver surfaces. Schultz et al. 112 j. have argued that an enhancement of 10” exists on a smooth (unanodized) silver electrode, which may Ix increased to Z=1O6 by electrochemical roughcning. Cj&~wa er al. [ 71) have presented evidence lh;lI an cnhancemenl of zt5 X IO2 occurs when pyridmc is adsorbed WI the Ag( 100) face in UHV at low Ic-mpcr:~Iurc. 111 both cases the enhancement is ac
L--II-
Scm-’
-II-5cm-’
P-POLARIZED
8 . II00
,
,
,
1 .
1050
Au
.
,
,
1 , 1000
,
,
.
, 950
,
( .
( 900
km-‘)
Fig. 3. Raman-scattered intensity as a function of input laser polarization for a monolayer (5 L dose) of pyridine adsorbed on Ag(ll1) at 120 K;.
due to chemisorption at sites which are not available on the low index faces. Polarization studies further support the conclusion that there is no enhancement on these surfaces. A simple calculation using Fresnel’s equations is sufficient to predict the dependence of the signal upon input
laser polarization for a flat surface [ 13]_ Such a calculation shows that for 70° incidence, the mean square normal component of the electric field produced with p polarization is about a factor of ten greater than the tangential component produced with s polarized radiation. Fig_ 3 shows the spectra observed with p- and s-polarized light at 70” incidence. The large increase in signal observed with p polarization is entirely consistent with this analysis. The scattered radiation is
-II-5
cm-’
--IISLIT
Izig.2. L)il’i’erwIspcctr4 regions for a monolayer (5 L dose) pyridinc adsorbed on &(110) at 120 K.
II00
1050
1000
950
900
Av 1cm-‘) Fi_ 4. Polarization of the Raman scattering from a monolayer (5 L dose) of pyridine adsorbed on Ag(lll) at 120 K.
Volume94;number 6
CHEMICALPHYSICSLETTERS
also highly polarized, in contrast to SERS 1123. Fig. 4 shows the spectra obtained with the analyzer polarizer oriented parallel and perpendicular to the surface normal. At 70” incidence, with p-polarized excitation, the laser electric field at the surface is oriented very close to the surface normal. This means that for these totally symmetric modes, in the adsorption geometry proposed by Demuth et al. [9], the induced dipole will be similarly oriented_ Therefore, we expect a strong component polarized along the surface normal, as observed. Our experiments were carried out at 120 K. In order to ensure that both phases of pyridine (x-bonded and n-bonded) could be prepared [9], we repeated these experiments at 150 K. No evidence of enhanced scattering was observed at the higher temperature.
11 February 1983
iting small quantities of silver on a very cold (a10 K) single-crystal silver substrate in an effort to stabilize isolated adatoms [15]. Careful determination of the enhancement factors, their correlation with atomic scale morphology and measurements of Raman esciration profiles, should lead to a better understanding of the molecular origins of surface-enhanced Raman scattering. Acknowledgement The support of the Robert A. Welch Foundation (F-751) is gratefully acknowledged_ DRM was the recipient of a Charles M. Share Fellowship.
References 4. Conclusions We have observed normal Raman scattering from pyridine adsorbed on the low-index faces of silver. Evidence that the scattering is not enhanced includes the magnitude of the scattering cross section, the frequencies and relative intensities of the bands observed, the linearity of the scattered intensity with exposure and the high degree of polarization in both excitation and scattering. These results provide a firm base upon which detailed studies of molecular enhancement mechanisms can begin without the complication of purely electromagnetic enhancement. It is clear that no sites exist on these “perfect” surfaces that give rise to enhanced scattering_ Enhancement of the molecular polarizabllity derivative may therefore be associated with chemisorption at defect sites which might be steps, kinks, adatoms or vacancies [ 14]_ The Raman spectra themselves include a marker for such chemisorption. Virtually all enhanced surface Raman spectra show the 992 cm-l ring breathing mode shifted to xl005 cm- 1 in contrast to our results where the mode appears at 992 cm-l_ We intend to investigate systematically the nature of these special sites by looking at stepped and kinked surfaces and by depos-
[I] A. Otto. Appl. Surface Sci. 6 (1980) 309. [2] T.E. Furtak and 1. Reyes. Surface Sci. 93 (19SO) 351. [3] R. Chang and TX. Furtak, eds., Surface enhanced Raman spectroscopy (Plenum Press. New York, 1951). (41 A. Campion, J.K. Brown and V.M. Grizzle. Surhcc Sci. 115 (1982) L153. [5] S. Efrimaand H. hletiu, J. Chem. Phys. 70 (1979) 1601. [61 P.H. Sanda, J.M. Warlaumont. J-E. Demuth. J.C. Tsang, K. Christmann and J.A. Bradley. Phys. Rev. Letters45 (1980) 1519. [‘I] M. Ugadawa. C.-C_ Chou, J.C. Hemminger and S. Ushioda, Phys. Rev. B23 (1981) 6843. IS] I.E. Rowe, C-V. Shank. D.A. Zwemer and CA. Murray’. Phys. Rev. Letters44 (1980) 1770. 191 J.E. Demuth, K. Christmann and P-N. Sanda. Chcm. Phys. Letters 76 (1980) 201. [IO] J-G. Skinner and W_C. Nilsen. J. Opt. Sot. Am. 58 (1968) 113. [ 111 hl. hioskovitz and D-P-D. Lella. in: Surface enhanced Raman spectroscopy, eds. R. Chang and T.E. Furtak (Plenum Press. New York, 1981)_ [ 121 S.G. Schultz, M. JanikGzacher and R.P. van Duyne. Surface Sci. 104 (1981) 419. [ 131 J-D-E. McIntyre, in: Advances in electrochemistry and electrochemical engneering. Vol. 9. cd. R.H. hluller (Wiley. New York, 1974) p. 227. 1141 A. Otto. I. Pockrand, J. Billman and C_ Pettenkofer. in: Surface enhanced Raman spectroscopy, eds. R. Chag and T-E. Furtak (Plenum Press, New York, 1981) p. 147. 1151 J.P. Chauvineau. Surface Sci 93 (1980) 471.
579
NOR\lAL
CHEhIlCAL
RAMAN SCA-ITERING
PHYSICS
FROM PYRIDINE
LETTERS
11 February
1983
ADSORBED
OK THE LOW-INDEX FACES OF SILVER
\\;c find no cnh~nccmcnt
oi the Raman
adsorbed at 120 K on the (1001, (110) and unshifted from thoseof liquid pyridine, intensity rz~tios are similar to the liquid. the signal intensity is linear in covemge from submonolayer to multilayer, and the dcpolariz;ltion ratio is Io\v. Since these obwrvations are in marked contrast to those associated with surface-enhanced Rsmun scattcrins (SERS). WC’conclude that our spectra result from normal Raman scattering. Our results support the bypwhrsis that spcci;d adsorption sites are responsible for 3 substantial fraction of the total enhancement of the Raman cross section for :hc pyridinc-silver system.
( 111) f;lres of silver in ulrnhigb ticuum.
scwerin_r
The
cross section
frequcncius
I _ Introduction
Siucc the discovery of surface-enhanced Raman scattering (SERS) there has been a tremendous effort. both esperirnental and rlleoretical, devoted to the e1ucidatk~n of Ihe enhancement mechanism_ Several rcvicw articles [ 1_2] and a recent book [3] summarize the esperimental results and various rheorerical points of view. It is generally agreed that the enhancement mechrtnisms may bc divided into two broad classes, whose 11131involve enhanced local (laser) electric fields al ihc surfaces of small metallic particles or gratings, and those that increase rhe magnitude of the molecular polarizability dcrivarive, through the charge transfer that accompanies cI,lemisorption. It has not been pt)ssibie to derermine unequivocally the relative magnitudes of these Iwo contributions to the torsl SERS intensity. even for the most frequently studied system pyridine on silver_ For reasons of sensitivity, the over\rhchning majority of studies carried out to date have used geometric structures (roughened surfaces, ~olloids, gratings) that provide some degree of purely electromagnetic enhancement. Control of atomic scale order. and thus the nature and distribution of potential adsorption sites. on such surfaces is difficult at best and the problem camlot be decoupled esperi576
for pyridine
WC observe
xc
essenthlly
mentally using conventional approaches. Since we have been successful in obtaining highquality Raman spectra from molecules adsorbed on well-characterized surfaces at low coverage without enhat~ccment [4], we felt that we could address the question of molecular enhancement mechanisms in the absence of any purely electromagnetic contribution, by working with fiat surfaces. Electrodynarnic effects at flat surfaces are well understood and lead to only minor enhancement [5]. We report here the first observation of normal (unenhanced) Raman scattering from a monolayer of pyridine adsorbed on each of the low-index faces of silver.
2. Experimental The experimenti were performed using an ultrahigh vacuunl (UHV) chamber equipped with a quadrupole mass spectrometer and low-energy electron diffraction (LEED) optics that were also used for retarding field Auger electron spectroscopy. Base pressure during experiments was in the low 1O- lo Torr range. Silver samples were cut from two different singlecrystal rods, supplied by Monocrystals, Inc. The rods 0 009-2614/S3/0000-0000/S
03.00 0 1983 North-Holland
Volume 94, number 6
11 Februw
CHEMICAL PHYSICS LETTERS
1983
were aligned to within lo of the desired direction by
of =SOO cm-l.
Laue backscattering, and sample disks were cut with a spark cutter. The crystals were polished through a series of emery papers and 6 pm and 1 I.tm diamond paste. A final chemical polish in a solution of potassium dichromate resulted in a mirror-like, scratch-free surface. The crystals were cleaned in UHV by repeated argon ion sputtering (3 keV, a-10 PA cmm2 for 10 min) and annealing (3OOOC) cycles_ Surfaces prepared in this manner showed no features under the scanning electron microscope at 500 A resolution, except for a few isolated etch pits of I--1Oprn diameter. Surface impurities were less than 1% as determined by Auger spectroscopy, and the LEED patterns consisted of very sharp spots with a low background Intensity_ Consequently, we consider these surfaces to be smooth, flat, and essentially free of both submicroscopic and atomic scaie roughness. Pyridine was dosed onto the cold surface (120150 K) of the crystal by backfiiing the chamber (typically 5 X loss Torr for 100 s)_ Because of rather long pumpdown times for pyridine, the actual dose might have been 50% larger than the indicated dose. Raman scattering was excited by the 5145 _&line of an argon ion laser, focused to a 100 pm diameter spot on the crystal at an angle of incidence of 70° with respect to the surface normal_ A half-wave Fresnel rhomb was used to select the input polarization. The laser power was typically 1 W, although powers as low as a few hundred mW and as high as 2.5 W were occasionally used. The signal intensity was always linear in power and we never observed any ill effects due to laser heating. This behavior is easily understood- Even for p polarization, silver reflects more than 95% of the incident radiation_ Furthermore, since it is such an excellent thermal conductor, neither the bulk nor the surface temperature rises more than a few degrees under laser illumination. The scattered radiation was collected using f/O.95optics at 55” to the surface normal in a plane orthogonal to the plane of incidence_ A Schott OG 530 colored glass filter was used to reject light scattered at the laser frequency_ An Instruments SA. HR-320 spectrograph fitted with an 1800 lines/mm holographic grating blazed at 550 nm dispersed the scattered radiation across the faceplate of a cooled, intensified vidicon detector (EG & G PARC 1254). With this arrangement the dispersion is =l cm- 1 per channel, giving a total spectral coverage
linewidth is 10 cm-l_ For the polarization measurements, a polaroid film and polarization scrambler were inserted into the optical path.
With the slits used, the instrument
3. ResuIts and discussion Fig. 1 shows the 1000 cm- 1 region of the Raman spectrum of pyridine adsorbed on the low-index silver faces following a dose of 5 L (uncorrected for ion gauge sensitivity) at 120 K. This dose produces approximately monolayer coverage, as determined by the UPS measurements of Sanda et al. [6] for the (111) face. Additional evidence for rhe coverage is provided by our Auger measurements. A plot of the intensity of the carbon Auger signal at 272 V versus exposure shows a change in slope near 5 L on all three crystal faces. Such behavior has been taken to indicate monolayer completion [7,8]. The vibrational frequencies are essentially identical to those of liquid pyridine and the scattered intensities are comparable on all three faces. The intensities are extremely low, considering the laser power used and the high detec-
1100
1050
1000
950
900
AVkm-‘1 Fig- 1_ Normal Raman spectra of the 1000 cm-l re$on of pyridine adsorbed on the low-index faces of lilver at 120 I;. following a 5 L dose. Integration time was 500 s total. with 1 W of 5145 J%radiation_
577
Volume 94. nurnbcr 6
CHEMICAL PHYSICS LETTERS
11 February 1983
lion efficiency of our system, immediately sugesting that SERS is absent_ Using the observed count rate, the laser power. our calibrated detection efficiency and assuming the coverage to be 6 X 10’” molecults cm- 2 [9], we calculate a scattering cross seciion of5 X 1O-2g ctn2 moiecule-1 ST- 1 for the ad-
sorbed pyridine which is close to the liquid value of 1.5 X IO- ‘g cm? molecule-1 sr- l [IO] (integrated over the lineshape and corrected to 5145 A excitation). The Raman intensity is linear in exposure from I ---20 L in the present experiments, unlike SERS where short-range (first layer) enhancements are oftrn observed [6.7.1 I]_ Fig. I! shows several different spectral regions for pyridine adsorbed on A&l IO). Similar results were obtained for the other two facts. The band positions and relative intensiries are quite similar to those of liquid pyridine. SERS spectra differ from these in two important ways. C-H stretching modes are genemlly weak (if observed at all) in SERS [ 1l] and the 992 c111- 1 ring breathing mode of pyridine is invariably shif‘ted to -51005 cm- 1 [3]_ This latter point is imporrant.
particularly
when comparing our results to
previous work on “smooth” silver surfaces. Schultz et al. 112 j. have argued that an enhancement of 10” exists on a smooth (unanodized) silver electrode, which may Ix increased to Z=1O6 by electrochemical roughcning. Cj&~wa er al. [ 71) have presented evidence lh;lI an cnhancemenl of zt5 X IO2 occurs when pyridmc is adsorbed WI the Ag( 100) face in UHV at low Ic-mpcr:~Iurc. 111 both cases the enhancement is ac
L--II-
Scm-’
-II-5cm-’
P-POLARIZED
8 . II00
,
,
,
1 .
1050
Au
.
,
,
1 , 1000
,
,
.
, 950
,
( .
( 900
km-‘)
Fig. 3. Raman-scattered intensity as a function of input laser polarization for a monolayer (5 L dose) of pyridine adsorbed on Ag(ll1) at 120 K;.
due to chemisorption at sites which are not available on the low index faces. Polarization studies further support the conclusion that there is no enhancement on these surfaces. A simple calculation using Fresnel’s equations is sufficient to predict the dependence of the signal upon input
laser polarization for a flat surface [ 13]_ Such a calculation shows that for 70° incidence, the mean square normal component of the electric field produced with p polarization is about a factor of ten greater than the tangential component produced with s polarized radiation. Fig_ 3 shows the spectra observed with p- and s-polarized light at 70” incidence. The large increase in signal observed with p polarization is entirely consistent with this analysis. The scattered radiation is
-II-5
cm-’
--IISLIT
Izig.2. L)il’i’erwIspcctr4 regions for a monolayer (5 L dose) pyridinc adsorbed on &(110) at 120 K.
II00
1050
1000
950
900
Av 1cm-‘) Fi_ 4. Polarization of the Raman scattering from a monolayer (5 L dose) of pyridine adsorbed on Ag(lll) at 120 K.
Volume94;number 6
CHEMICALPHYSICSLETTERS
also highly polarized, in contrast to SERS 1123. Fig. 4 shows the spectra obtained with the analyzer polarizer oriented parallel and perpendicular to the surface normal. At 70” incidence, with p-polarized excitation, the laser electric field at the surface is oriented very close to the surface normal. This means that for these totally symmetric modes, in the adsorption geometry proposed by Demuth et al. [9], the induced dipole will be similarly oriented_ Therefore, we expect a strong component polarized along the surface normal, as observed. Our experiments were carried out at 120 K. In order to ensure that both phases of pyridine (x-bonded and n-bonded) could be prepared [9], we repeated these experiments at 150 K. No evidence of enhanced scattering was observed at the higher temperature.
11 February 1983
iting small quantities of silver on a very cold (a10 K) single-crystal silver substrate in an effort to stabilize isolated adatoms [15]. Careful determination of the enhancement factors, their correlation with atomic scale morphology and measurements of Raman esciration profiles, should lead to a better understanding of the molecular origins of surface-enhanced Raman scattering. Acknowledgement The support of the Robert A. Welch Foundation (F-751) is gratefully acknowledged_ DRM was the recipient of a Charles M. Share Fellowship.
References 4. Conclusions We have observed normal Raman scattering from pyridine adsorbed on the low-index faces of silver. Evidence that the scattering is not enhanced includes the magnitude of the scattering cross section, the frequencies and relative intensities of the bands observed, the linearity of the scattered intensity with exposure and the high degree of polarization in both excitation and scattering. These results provide a firm base upon which detailed studies of molecular enhancement mechanisms can begin without the complication of purely electromagnetic enhancement. It is clear that no sites exist on these “perfect” surfaces that give rise to enhanced scattering_ Enhancement of the molecular polarizabllity derivative may therefore be associated with chemisorption at defect sites which might be steps, kinks, adatoms or vacancies [ 14]_ The Raman spectra themselves include a marker for such chemisorption. Virtually all enhanced surface Raman spectra show the 992 cm-l ring breathing mode shifted to xl005 cm- 1 in contrast to our results where the mode appears at 992 cm-l_ We intend to investigate systematically the nature of these special sites by looking at stepped and kinked surfaces and by depos-
[I] A. Otto. Appl. Surface Sci. 6 (1980) 309. [2] T.E. Furtak and 1. Reyes. Surface Sci. 93 (19SO) 351. [3] R. Chang and TX. Furtak, eds., Surface enhanced Raman spectroscopy (Plenum Press. New York, 1951). (41 A. Campion, J.K. Brown and V.M. Grizzle. Surhcc Sci. 115 (1982) L153. [5] S. Efrimaand H. hletiu, J. Chem. Phys. 70 (1979) 1601. [61 P.H. Sanda, J.M. Warlaumont. J-E. Demuth. J.C. Tsang, K. Christmann and J.A. Bradley. Phys. Rev. Letters45 (1980) 1519. [‘I] M. Ugadawa. C.-C_ Chou, J.C. Hemminger and S. Ushioda, Phys. Rev. B23 (1981) 6843. IS] I.E. Rowe, C-V. Shank. D.A. Zwemer and CA. Murray’. Phys. Rev. Letters44 (1980) 1770. 191 J.E. Demuth, K. Christmann and P-N. Sanda. Chcm. Phys. Letters 76 (1980) 201. [IO] J-G. Skinner and W_C. Nilsen. J. Opt. Sot. Am. 58 (1968) 113. [ 111 hl. hioskovitz and D-P-D. Lella. in: Surface enhanced Raman spectroscopy, eds. R. Chang and T.E. Furtak (Plenum Press. New York, 1981)_ [ 121 S.G. Schultz, M. JanikGzacher and R.P. van Duyne. Surface Sci. 104 (1981) 419. [ 131 J-D-E. McIntyre, in: Advances in electrochemistry and electrochemical engneering. Vol. 9. cd. R.H. hluller (Wiley. New York, 1974) p. 227. 1141 A. Otto. I. Pockrand, J. Billman and C_ Pettenkofer. in: Surface enhanced Raman spectroscopy, eds. R. Chag and T-E. Furtak (Plenum Press, New York, 1981) p. 147. 1151 J.P. Chauvineau. Surface Sci 93 (1980) 471.
579