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The dependence of the intensity of Raman bands of pyridine at a silver electrode on the wavelength of excitation
Volume 15, number
CHEMICAL
1
PHYSICS
LETTERS
1 April 1978
THE DEPENDENCE OF THE INTENSITY OF RAMAN BANDS OF PYRIDINE AT A SILVER ELECTRODE ON THE WAVELENGTH OF EXCITATION J.A. CREIGHTON,
M.G. ALBRECHT
Chemiiai Laboratory,
University of Kent, Gznterbury.
UK
and R.E. HESTER and J.A.D. MATTHEW Chemistry and Physics Departments, Received 28 November
University of York, Heslington.
York, UK
1977
On increasing the wavelength of excitation over the range 350-700 nm, Raman bands of pyridine adsorbed at a roughened silver electrode are found to increase in intensity, relative to bands of the bulk medium (aqueous perchlorate or liquid pyridine) in contact with the electrode. The increase is observed in the bands at 1000-1050 cm-r and 1600 cm-’ due to ring stretching, and similar increases are observed in other bands of the surface species, notably those due to CH stretching (3076 cm-l), ba ring deformation (669 cm-‘), and Ag-N stretching (239 cm-‘), which have not been reported previously.
The anomalously high Raman intensity of ring stretching bands of pyridine (Av = 1000-1050 cm-l) adsorbed at an electrochemically roughened silver surface have recently been reported [I]. These bands, which are slightly shifted from the frequencies characteristic of free aqueous pyridine 121, were observed to appear with high intensity at the end of an electrochemical roughening cycle, in which AgC1 was first formed and then reduced by cycling the potential of the electrode in an aqueous KCl/pyridine solution. It was estimated that the molar intensity of these bands was roughly lo5 times greater than the corresponding intensities for pure liquid pyridine, assuming a monolayer coverage of the silver surface by adsorbed pyridine, as indicated by the results of Barradas and Conway [3] and an increase of not more than tenfold in the electrode surface area as a result of the electrochemical roughening_ In order to obtain clues to the mechanism of this enhancement, which may be a new type of resonance Raman phenomenon, we have now investigated the dependence of the intensities of bands of surface pyricline on the frequency of the exciting radiation. The electrochemical cell for the Raman studies and the roughening procedure in aqueous KC1 solution
(lO_lM) containing pyridine ( 10B2 M) were as described previously [l]. With the platinum loop secondary electrode in this electrochemical cell, the intensity of the adsorbed pyridine spectra was found to vary considerably as the area of illumination was moved across the surface of the working electrode, due to the greater degree of roughening around the edge of this electrode. Pinhole diaphragms were therefore used to fix the area of illumination. In addition, for some of the measurements a platinum sheet secondary electrode was used. This had to be withdrawn before making the Raman measurements, but it enabled a working electrode with uniform Raman intensity across the surface to be prepared. Raman measurements were made with a Coderg PHI spectrometer with argon (4579-5 14.5 nm) and helium-neon (632.8 run) lasers (Canterbury), and with a Jobin Yvon HG2 spectrometer with argon and high power krypton (350.6-676.4 nm) laser excitation (York). The intensities of Raman bands of surface pyridine were measured relative to bands of the liquid medium in contact with the electrode, these being either the 935 cm-l v1 al band of aqueous NaC104 solution, the 3450 cm-r band of water, or the 991 cm-1 band of liquid pyridine. These reference 55
Volume 55, number 1
CHEMICAL
PHYSICS
bands were strongly polarized, whereas it was found that the Raman bands of the surface pyridine all had depolarization ratios close to l-0, and it was therefore important that the spectrometers were fitted with polarization scramblers so that they did not show a wavelength-dependent difference in sensitivity to the scattered light of different polarizations. Fig. la shows the 530.9 nm excited Raman spectrum of pyridine at a silver electrode at open-circuit potential after voltage cycling three times between -75 and +I 50 mV at 3 mV s-l in aqueous KCl/pyridine. It may readily be shown, for example by electrode withdrawal, that the bands in fig. la receive a negligible contribution from the 10m2 M pyridine in solution in the electrolyte, and the spectrum is thus effectively that only of adsorbed pyridine. Analysis of the bulk solution in the region of the electrode surface showed this to be measurably depleted in pyridine by the voltage cycling prccedure. Fig. la covers a much wider frequency range than was reported in earlier Raman studies of this electrode [1,2]_ Thus in addition to the intense bands at 1025 cm-l and at 1036 and 1008 cm-l, which were assigned, respectively, to symmetric ricgstretching of pyridine bonded through nitrogen direcrly to the surface, and to pyridine hydrogen-bonded to
Fig_ 1. Raman spectra from (a) Ag-adsorbed tact with aqueous lo-’ BL pyridine and 10-t successivevoltage cycles (between -75 and s-l) and (b) 0.15 M aqueous [Ag(C5HsN)z] excitation.
56
pyridine in hi KC1 after i-150 mV at NO3.530.9
conthree. 3 mV nm
LETTERS
1 April 1978
adsorbed water molecules [2], there are other prominent bands at 3076,1610,1218,669 and 239 cm-l. All but the lowest of these-are similar m frequencies and relative intensities to bands of free pyridine, and they are assigned, respectively, to aI (CH stretching), aL (ringstretching), aI (CH deformation), b,(ring deformation) by analogy with the assignment for the free molecule [4]_ The lowest frequency band of the surface species at 239 cm-l is of particular interest since it lies in the middle of the range of frequencies found for metal-ligand stretching in a variety of Nbonded metal-pyridine complexes [5]. It is certainly well below the frequency of any fundamentals of free pyridine, and is thus almost certainly due to Agpyridine bond stretching. Fig. I b shows for comparison the spectrum of 0.15 M aqueous solution of [Ag(CSHIN)2] NO,. The u (Ag-N) band for the IAg(CSHsN)21 t cation, which was measured at 245 cm-l in the infrared spectrum of the crystalline per&orate, was too weak to be observed in the Raman spectrum (fig. lb), and is thus considerably weaker relative to the internal pyridine vibrations than it is for the adsorbed pyridine. For the intensity measurements at different excitation wavelengths the electrode was first roughened (triangular sweep between -50 and +i80 mV, 3 mV s-l) in an aqueous solution lo-* M in KC1 and 10-Z M in pyridine, and then transferred to a Raman cell containing either liquid pyridine or a 1 .OM aqueous NaC!04 solution containing 10S2 M pyridine. Fig. 2 shows a series of spectra in the region 950- 1050 cm-* for the electrode in liquid pyridine with different exciting lines, in which the 99 1 cm-l band of free pyridine serves as an intensity standard for the 1008 cm-l band of the surface pyridine. The 1025 cm-l band of the surface species was found to disappear on immersing the electrode in pyridine, while the 1033 cm-l band of liquid pyridine and the 1036 cm-l band of the surface species overlad in fig. 2, though they may be resolved by use of smaller slits. Apart from clearly demonstrating the large enhancement whereby bands of surface pyridine are clearly seen in the presence of the concentrated bulk liquid, fig. 2 shows that there is a strong wavelength-dependence in the intensity of the 1008 and 1036 cm-l Raman bands of the surface-adsorbed molecules which is not shown by free pyridine. A qualitatively similar wavelength-dependence relative-to-&r internal standard
Volume 55, number I
CHEMICAL
PHYSICS
1 April 1978
LE-I-IERS
i-
i
&ci
I
17ooo
I
19ooo
’
2looo
Wwemmber/cm’
Fig. 2. Raman bands from Ag-adsorbed pyridine in contact with pure liquid pyridine.
Arrows indicate free pyridine bands.
(the 935 cm-t band of CIO; or the 3450 cm-l band of water) was demonstrated for the 1036 and 1008 cm-’ bands of adsorbed pyridine, and also for the bands at 3076,1026 and 239 cm-l, for the Ag elec-
trode immersed in aqueous sodium perchlorate solution_ Because of the closeness of the bands in the 1000-1050 cm-l region it was possible to compare their relative intensity changes particularly critically, and it was observed that the 1026 cm-l band changed in a distinctly different way from the 1036 and 1008 cm-l bands, as shown in fig. 3. The most remarkable feature of fig. 3 is, however, the substantial fircrease in the band intensities with decrease in excitation frequency. This trend is markedly different from that of Raman bands of free pyridine or of the [Ag(C5H5N)2] + complex, both of which approximately follow a z$ intensity law with excitation in the visible region. The scatter in the data plotted in fig. 3 is, in part, due to time-dependent effects, as illustrated by the results shown in fig.4. These effects are of unknown origin but indicate changes in surface conditions with time. There is also the difficulty of using intensity standards which, unlike the species of interest, are not truly a part of the electrode surface, although they are in contact with it. Fig. 4 does, however, indicate the validity of the solution-replacement technique used, and the generally good agreement between data ob-
Fig. 3. Excitation-frequency dependence of Raman band intensities from Ag-adsorbed pyridine. o, 1026 cm-‘band (Y); D, 1008 cm-t (C); A, 1008 cm-r (Y);c, 1036 cm-l (Y). Y = York result, measured relative to aqueous Cl02 and Hz0 bands; C = Canterbun result, measured relative to Iiqtiid pyridine standard_
tained with quite different equipment in our two laboratories gives us confidence in the overall result. The assignment of the 3076 cm-l band of Ag-adsorbed pyridine to C-H stretching, rather than to a second overtone of the 1026 cm-l mode, was confirm-
Fig. 4. Raman bands from Ag-adsorbed pyridine in contact with aqueous pyridine and NaC104 solution. 900-l 100 cm-’ region (a) after four successive voltage cycles (between -75 and +150 mV at 3 mV s-‘) in aqueous lo-* hl pyridine, 10-l hi KCI, (b) immediately after replacement of bulk solution by aqueous 10 -* M pyridine, 1.0 hl NaC104, (c) 17 h after (b), (d) 41 h after (bi_ Arrows indicate 935 cm-’ CIO; band. 568.2 nm excitation.
57
Volume 55, number 1
CHEMICAL PHYSICS LETTERS
ed by deuterium substitution. The C-D stretching band of Ag-adsorbed pyridine-d5 was observed at 2306 cm-l_ The intense band at 239 cm-l was unaf-
fected by deuteration, as expected for an Ag-N stretching mode. This band was selectively eliminated, however, together with the 1026 cm-l band [2], by
increasing the Ag-electrode potential to -0.60 V (S.C.E.)_ This, together with the resutts in fig. 3, may be taken as further evidence for a two-species model [2] of pyridine adsorption at asilver electrode surfaceThe anomalous increase in Raman Wensity with
decreasing excitation frequency reported here may result from a resonant Raman process involving excitations dominantly at the red end of the visible spectrum, and it is interesting to speculate on what the mechanism might be_ The lack of correlation between the Raman experiments on pyridine adsorbed at a silver surface and on pyridine bonded to a sing!e silver atom in [Ag(C5H5N)2]* suggests that absorption associated with metaLpyridine charge transfer cannot be responsible for the observed enhancement in Raman intensity at long wavelength_ Another possibility which has been suggested [I ] is that the enhancement proceeds via surface plasmon excitation in the metal substrate, an effect which is crucially dependent on the cooperative response of many electrons in the metal rather than the promotion of a single electron in a charge transfer process. It is well known that light incident on rough metal surfaces is capable of exciting bulk and surface plasmons [6,7], which may decay radiatively by a Rayleigh-like process, giving scattered light at the same wavelength as the incident light. It is possible that Raman transitions involving vibrational excitation of molecules adsorbed on the surface might couple with such processes, and give rise to the observed effects. However, for Ag the surface plasmon energy corresponds to radiation at the birce end of the visible spectrum (ca. 390 MI, depending on the contact medium). Although the presence of a liquid of dielectric constant E may lower the maximum frequency 0, of ‘optical” surface plasmons in a simple free-electron metal through the relation [8] 2
us0
=
US/(1 + E),
where we is the bulk plasmon frequency at zero wave vector, the value wW G 360 nm has been reported [9] 58
1 April 1978
for silver (which is knotin to depart substantially frbm the free-electron approximation) in contact with water. However, under special experiment&l conditions it is possible to preferentially excite long wavelength surface plasmons with o < oso [ IO,1 11. Foi example, Hartmann and Raether [lo] deposit an Ag film on the back of a quartz prism, and excite surface plasmons of different wavelengths at the Ag-air interface by varying the angle of incidence of light on the fdm. Near the critical angle for the quartz-air interface, low frequency surface plasmons are strongly excited by radiation at the red end of the visible spectrum. However, it is difficult to see how such processes will be important in the experimental set-up used in the present work, and it is far from obvious how they can form the basis of a resonant Raman mechanism under these conditions_ On present evidence it seems possible that the high Raman intensities and the anomalous increase in intensity with decreasing excitation frequency reported here may be associated with resonant surface pIasmon excitation, but further work involving the dependence of these effects on geometry and polarization of the incident radiation and on the state of the surface are needed to confhtn the inteqretation. References
111M.G. Albrecht and J.A. Creighton,J. Am. Chem. Sot. 99 (197 7) 5215.
PJ_ Hendm and A-J. McQuilIan, Chem. PI M. Fleischm~~~n,
Phys. Letters 26 (1974) 163; A.J. McQuiUan, P.J. Hendra and M. meischmann, J. Electroanal. Chem. 65 (197.5) 933. 131 R.G. Barradas and B-E. Conway, J. Electroanal. Chem. 6 (19631314. 141 J.H.S. Green, IV_Kynaston and H-M_Paisley, Spectro_ Chim. Acta 19 (1963) 549; J-K. Wilmfiurst and H.J. Bernstein, Can. J. Chem. 35 t19-57) ! 183. 151 R.J.H. Clalk and C.S. Williams, Inorg. Chem- 4 (1965) 350; 3-R. Durig and D-W_Wertz, Appl. Spectry. 22 (1968) 627. PI D. BeagIehoIe an* U. Efunderi, Phys. Rev. 82 (1970) 309,321. Kretschma:m and H. Raether, 2. Naturforsch. 23a [71 E. __-_-. ____ (IY68) 2135. [8] R-H. Ritchie, Surface Sci 34 (1973) 1. [9] J.K. Sass, R.K. Sen, E. Meyer and H. Gerischer, Surface Sci. 44 (1974) 515.. [lOI D. Hartmann and H- Raether, Surface Sci. 59 (1976) 17. [ll] R. Orlowski and H. Raether, Surface Sci. 54 (1976) 303.
CHEMICAL
1
PHYSICS
LETTERS
1 April 1978
THE DEPENDENCE OF THE INTENSITY OF RAMAN BANDS OF PYRIDINE AT A SILVER ELECTRODE ON THE WAVELENGTH OF EXCITATION J.A. CREIGHTON,
M.G. ALBRECHT
Chemiiai Laboratory,
University of Kent, Gznterbury.
UK
and R.E. HESTER and J.A.D. MATTHEW Chemistry and Physics Departments, Received 28 November
University of York, Heslington.
York, UK
1977
On increasing the wavelength of excitation over the range 350-700 nm, Raman bands of pyridine adsorbed at a roughened silver electrode are found to increase in intensity, relative to bands of the bulk medium (aqueous perchlorate or liquid pyridine) in contact with the electrode. The increase is observed in the bands at 1000-1050 cm-r and 1600 cm-’ due to ring stretching, and similar increases are observed in other bands of the surface species, notably those due to CH stretching (3076 cm-l), ba ring deformation (669 cm-‘), and Ag-N stretching (239 cm-‘), which have not been reported previously.
The anomalously high Raman intensity of ring stretching bands of pyridine (Av = 1000-1050 cm-l) adsorbed at an electrochemically roughened silver surface have recently been reported [I]. These bands, which are slightly shifted from the frequencies characteristic of free aqueous pyridine 121, were observed to appear with high intensity at the end of an electrochemical roughening cycle, in which AgC1 was first formed and then reduced by cycling the potential of the electrode in an aqueous KCl/pyridine solution. It was estimated that the molar intensity of these bands was roughly lo5 times greater than the corresponding intensities for pure liquid pyridine, assuming a monolayer coverage of the silver surface by adsorbed pyridine, as indicated by the results of Barradas and Conway [3] and an increase of not more than tenfold in the electrode surface area as a result of the electrochemical roughening_ In order to obtain clues to the mechanism of this enhancement, which may be a new type of resonance Raman phenomenon, we have now investigated the dependence of the intensities of bands of surface pyricline on the frequency of the exciting radiation. The electrochemical cell for the Raman studies and the roughening procedure in aqueous KC1 solution
(lO_lM) containing pyridine ( 10B2 M) were as described previously [l]. With the platinum loop secondary electrode in this electrochemical cell, the intensity of the adsorbed pyridine spectra was found to vary considerably as the area of illumination was moved across the surface of the working electrode, due to the greater degree of roughening around the edge of this electrode. Pinhole diaphragms were therefore used to fix the area of illumination. In addition, for some of the measurements a platinum sheet secondary electrode was used. This had to be withdrawn before making the Raman measurements, but it enabled a working electrode with uniform Raman intensity across the surface to be prepared. Raman measurements were made with a Coderg PHI spectrometer with argon (4579-5 14.5 nm) and helium-neon (632.8 run) lasers (Canterbury), and with a Jobin Yvon HG2 spectrometer with argon and high power krypton (350.6-676.4 nm) laser excitation (York). The intensities of Raman bands of surface pyridine were measured relative to bands of the liquid medium in contact with the electrode, these being either the 935 cm-l v1 al band of aqueous NaC104 solution, the 3450 cm-r band of water, or the 991 cm-1 band of liquid pyridine. These reference 55
Volume 55, number 1
CHEMICAL
PHYSICS
bands were strongly polarized, whereas it was found that the Raman bands of the surface pyridine all had depolarization ratios close to l-0, and it was therefore important that the spectrometers were fitted with polarization scramblers so that they did not show a wavelength-dependent difference in sensitivity to the scattered light of different polarizations. Fig. la shows the 530.9 nm excited Raman spectrum of pyridine at a silver electrode at open-circuit potential after voltage cycling three times between -75 and +I 50 mV at 3 mV s-l in aqueous KCl/pyridine. It may readily be shown, for example by electrode withdrawal, that the bands in fig. la receive a negligible contribution from the 10m2 M pyridine in solution in the electrolyte, and the spectrum is thus effectively that only of adsorbed pyridine. Analysis of the bulk solution in the region of the electrode surface showed this to be measurably depleted in pyridine by the voltage cycling prccedure. Fig. la covers a much wider frequency range than was reported in earlier Raman studies of this electrode [1,2]_ Thus in addition to the intense bands at 1025 cm-l and at 1036 and 1008 cm-l, which were assigned, respectively, to symmetric ricgstretching of pyridine bonded through nitrogen direcrly to the surface, and to pyridine hydrogen-bonded to
Fig_ 1. Raman spectra from (a) Ag-adsorbed tact with aqueous lo-’ BL pyridine and 10-t successivevoltage cycles (between -75 and s-l) and (b) 0.15 M aqueous [Ag(C5HsN)z] excitation.
56
pyridine in hi KC1 after i-150 mV at NO3.530.9
conthree. 3 mV nm
LETTERS
1 April 1978
adsorbed water molecules [2], there are other prominent bands at 3076,1610,1218,669 and 239 cm-l. All but the lowest of these-are similar m frequencies and relative intensities to bands of free pyridine, and they are assigned, respectively, to aI (CH stretching), aL (ringstretching), aI (CH deformation), b,(ring deformation) by analogy with the assignment for the free molecule [4]_ The lowest frequency band of the surface species at 239 cm-l is of particular interest since it lies in the middle of the range of frequencies found for metal-ligand stretching in a variety of Nbonded metal-pyridine complexes [5]. It is certainly well below the frequency of any fundamentals of free pyridine, and is thus almost certainly due to Agpyridine bond stretching. Fig. I b shows for comparison the spectrum of 0.15 M aqueous solution of [Ag(CSHIN)2] NO,. The u (Ag-N) band for the IAg(CSHsN)21 t cation, which was measured at 245 cm-l in the infrared spectrum of the crystalline per&orate, was too weak to be observed in the Raman spectrum (fig. lb), and is thus considerably weaker relative to the internal pyridine vibrations than it is for the adsorbed pyridine. For the intensity measurements at different excitation wavelengths the electrode was first roughened (triangular sweep between -50 and +i80 mV, 3 mV s-l) in an aqueous solution lo-* M in KC1 and 10-Z M in pyridine, and then transferred to a Raman cell containing either liquid pyridine or a 1 .OM aqueous NaC!04 solution containing 10S2 M pyridine. Fig. 2 shows a series of spectra in the region 950- 1050 cm-* for the electrode in liquid pyridine with different exciting lines, in which the 99 1 cm-l band of free pyridine serves as an intensity standard for the 1008 cm-l band of the surface pyridine. The 1025 cm-l band of the surface species was found to disappear on immersing the electrode in pyridine, while the 1033 cm-l band of liquid pyridine and the 1036 cm-l band of the surface species overlad in fig. 2, though they may be resolved by use of smaller slits. Apart from clearly demonstrating the large enhancement whereby bands of surface pyridine are clearly seen in the presence of the concentrated bulk liquid, fig. 2 shows that there is a strong wavelength-dependence in the intensity of the 1008 and 1036 cm-l Raman bands of the surface-adsorbed molecules which is not shown by free pyridine. A qualitatively similar wavelength-dependence relative-to-&r internal standard
Volume 55, number I
CHEMICAL
PHYSICS
1 April 1978
LE-I-IERS
i-
i
&ci
I
17ooo
I
19ooo
’
2looo
Wwemmber/cm’
Fig. 2. Raman bands from Ag-adsorbed pyridine in contact with pure liquid pyridine.
Arrows indicate free pyridine bands.
(the 935 cm-t band of CIO; or the 3450 cm-l band of water) was demonstrated for the 1036 and 1008 cm-’ bands of adsorbed pyridine, and also for the bands at 3076,1026 and 239 cm-l, for the Ag elec-
trode immersed in aqueous sodium perchlorate solution_ Because of the closeness of the bands in the 1000-1050 cm-l region it was possible to compare their relative intensity changes particularly critically, and it was observed that the 1026 cm-l band changed in a distinctly different way from the 1036 and 1008 cm-l bands, as shown in fig. 3. The most remarkable feature of fig. 3 is, however, the substantial fircrease in the band intensities with decrease in excitation frequency. This trend is markedly different from that of Raman bands of free pyridine or of the [Ag(C5H5N)2] + complex, both of which approximately follow a z$ intensity law with excitation in the visible region. The scatter in the data plotted in fig. 3 is, in part, due to time-dependent effects, as illustrated by the results shown in fig.4. These effects are of unknown origin but indicate changes in surface conditions with time. There is also the difficulty of using intensity standards which, unlike the species of interest, are not truly a part of the electrode surface, although they are in contact with it. Fig. 4 does, however, indicate the validity of the solution-replacement technique used, and the generally good agreement between data ob-
Fig. 3. Excitation-frequency dependence of Raman band intensities from Ag-adsorbed pyridine. o, 1026 cm-‘band (Y); D, 1008 cm-t (C); A, 1008 cm-r (Y);c, 1036 cm-l (Y). Y = York result, measured relative to aqueous Cl02 and Hz0 bands; C = Canterbun result, measured relative to Iiqtiid pyridine standard_
tained with quite different equipment in our two laboratories gives us confidence in the overall result. The assignment of the 3076 cm-l band of Ag-adsorbed pyridine to C-H stretching, rather than to a second overtone of the 1026 cm-l mode, was confirm-
Fig. 4. Raman bands from Ag-adsorbed pyridine in contact with aqueous pyridine and NaC104 solution. 900-l 100 cm-’ region (a) after four successive voltage cycles (between -75 and +150 mV at 3 mV s-‘) in aqueous lo-* hl pyridine, 10-l hi KCI, (b) immediately after replacement of bulk solution by aqueous 10 -* M pyridine, 1.0 hl NaC104, (c) 17 h after (b), (d) 41 h after (bi_ Arrows indicate 935 cm-’ CIO; band. 568.2 nm excitation.
57
Volume 55, number 1
CHEMICAL PHYSICS LETTERS
ed by deuterium substitution. The C-D stretching band of Ag-adsorbed pyridine-d5 was observed at 2306 cm-l_ The intense band at 239 cm-l was unaf-
fected by deuteration, as expected for an Ag-N stretching mode. This band was selectively eliminated, however, together with the 1026 cm-l band [2], by
increasing the Ag-electrode potential to -0.60 V (S.C.E.)_ This, together with the resutts in fig. 3, may be taken as further evidence for a two-species model [2] of pyridine adsorption at asilver electrode surfaceThe anomalous increase in Raman Wensity with
decreasing excitation frequency reported here may result from a resonant Raman process involving excitations dominantly at the red end of the visible spectrum, and it is interesting to speculate on what the mechanism might be_ The lack of correlation between the Raman experiments on pyridine adsorbed at a silver surface and on pyridine bonded to a sing!e silver atom in [Ag(C5H5N)2]* suggests that absorption associated with metaLpyridine charge transfer cannot be responsible for the observed enhancement in Raman intensity at long wavelength_ Another possibility which has been suggested [I ] is that the enhancement proceeds via surface plasmon excitation in the metal substrate, an effect which is crucially dependent on the cooperative response of many electrons in the metal rather than the promotion of a single electron in a charge transfer process. It is well known that light incident on rough metal surfaces is capable of exciting bulk and surface plasmons [6,7], which may decay radiatively by a Rayleigh-like process, giving scattered light at the same wavelength as the incident light. It is possible that Raman transitions involving vibrational excitation of molecules adsorbed on the surface might couple with such processes, and give rise to the observed effects. However, for Ag the surface plasmon energy corresponds to radiation at the birce end of the visible spectrum (ca. 390 MI, depending on the contact medium). Although the presence of a liquid of dielectric constant E may lower the maximum frequency 0, of ‘optical” surface plasmons in a simple free-electron metal through the relation [8] 2
us0
=
US/(1 + E),
where we is the bulk plasmon frequency at zero wave vector, the value wW G 360 nm has been reported [9] 58
1 April 1978
for silver (which is knotin to depart substantially frbm the free-electron approximation) in contact with water. However, under special experiment&l conditions it is possible to preferentially excite long wavelength surface plasmons with o < oso [ IO,1 11. Foi example, Hartmann and Raether [lo] deposit an Ag film on the back of a quartz prism, and excite surface plasmons of different wavelengths at the Ag-air interface by varying the angle of incidence of light on the fdm. Near the critical angle for the quartz-air interface, low frequency surface plasmons are strongly excited by radiation at the red end of the visible spectrum. However, it is difficult to see how such processes will be important in the experimental set-up used in the present work, and it is far from obvious how they can form the basis of a resonant Raman mechanism under these conditions_ On present evidence it seems possible that the high Raman intensities and the anomalous increase in intensity with decreasing excitation frequency reported here may be associated with resonant surface pIasmon excitation, but further work involving the dependence of these effects on geometry and polarization of the incident radiation and on the state of the surface are needed to confhtn the inteqretation. References
111M.G. Albrecht and J.A. Creighton,J. Am. Chem. Sot. 99 (197 7) 5215.
PJ_ Hendm and A-J. McQuilIan, Chem. PI M. Fleischm~~~n,
Phys. Letters 26 (1974) 163; A.J. McQuiUan, P.J. Hendra and M. meischmann, J. Electroanal. Chem. 65 (197.5) 933. 131 R.G. Barradas and B-E. Conway, J. Electroanal. Chem. 6 (19631314. 141 J.H.S. Green, IV_Kynaston and H-M_Paisley, Spectro_ Chim. Acta 19 (1963) 549; J-K. Wilmfiurst and H.J. Bernstein, Can. J. Chem. 35 t19-57) ! 183. 151 R.J.H. Clalk and C.S. Williams, Inorg. Chem- 4 (1965) 350; 3-R. Durig and D-W_Wertz, Appl. Spectry. 22 (1968) 627. PI D. BeagIehoIe an* U. Efunderi, Phys. Rev. 82 (1970) 309,321. Kretschma:m and H. Raether, 2. Naturforsch. 23a [71 E. __-_-. ____ (IY68) 2135. [8] R-H. Ritchie, Surface Sci 34 (1973) 1. [9] J.K. Sass, R.K. Sen, E. Meyer and H. Gerischer, Surface Sci. 44 (1974) 515.. [lOI D. Hartmann and H- Raether, Surface Sci. 59 (1976) 17. [ll] R. Orlowski and H. Raether, Surface Sci. 54 (1976) 303.