- Email: [email protected]
vacuum interfaces
Q
861—865. Solid State Communications, Vol.35,inpp. Pergamon Press Ltd. 1980. Printed Great Britain.
COVERAGE DEPENDENCE OF RAMAN SCATTERING FROM PYRIDINE ADSORBED TO SILVER/VACUU11 INTERFACES I. PoCkrand and A. Otto Physikalisches Institut III, Universitlit DUsseldorf, D—4000 Düsseldorf I, Fed. Rep. of Germany (Received 11 June 1980 by M. Cardona) Roman spectra from pyridine adsorbed to silver films evaporated on cooled substrates un~erUHV ~onditions have been investigated in the exposure range 3x1O L to 10 L. Surface enhanced Reman signals have been detected even for the smallest exposure and reached a maximum in intensity at about monolayer coverage. Extended exposure to pyri— dine exceeding monolayer coverage results in a variation of the Raman spectrum eventually leading to additional Roman peaks due to ordinary scattering from thick condensed pyridine layers. Several further observations make an explanation of surface enhanced Raman scattering on the basis of electromagnetic resonances unlikely.
Since the detection of surface enhanced Raman scattering (SERB) from pyridine adsorbed to silver electrodes /1—3/, the effect has been confirmed for various molecules electro— chemically deposited on different metals (for a recent review see /4/) as well as for dif— ferent systems like tunnel junctions /5,6/ and metal/vacuum interfaces /7/. The pyridine/sil— ver electrode system is, however, still the most intensely studied system: the dependence of SERB intensities, iinewidths /4,8/, and peak positions on the applied potential /4,8, 9,10/, the wavelength dependence of the en—
this temperature. The substrates have been cx— 2L — posed to various doses of pyridine (3x10 IO5L) at temperatures of about 130K and in— mediately afterwards Ranian spectra have been taken from the cooled samples. Fig. I shows Raman spectra in the region of the strong pyridine breathing vibrations from different substrates exposed to different amounts of pyridine as indicated. The single crystal (fig. Ia) and the po1~crystallineslug (fig. Ib), both exposed to 10 L pyridine, exhi— bited quite siini 1lar spectra dominjted by strong peaks at 996cm and 1037/1040cm (for the high background intensity of the polycrystal— line slug see /16/). A few additional weak peaks are also seen. As outlined in detail in /16/ these features are interpreted as being due to ordinary Raman scattering from rather thick condensed pyridine layers on the cooled silver substrates. Fig. lc,d display actual Roman scans from the evaporated silver film exposed to IL (fig3 Ic same intensity scale as fig. la,b!) and 10 L pyridin~respectively. Very intense peaks at 1006cm and 1036cm accompanied by several less pronounced peaks have been measured from at most a monolayer pyridine coverage (due to IL exposvre, fig. Ic). The strongest peak (1006cm ) could easS,ly be detected after ex1 posure to only 3x10’L. Compaying 996cm peak intensity to the 1006cm peaktheintensity the latter one must ke enhanced by at least a
hancement /2,4,9,11,12/, the time development of the signal /13/, and parameters influencing the usually performed oxidation—reduction pre— treatment /2,3,4,8,14/ have all been investi— gated. In view of this situation and the still controversal interpretation of SERB /15/ it seems desirable to study Raman scattering from pyridine adsorbed to silver in a nonelectro— chemical environment, namely at silver/vacuum interfaces, and to compare corresponding re— suits to those obtained from the Ag/electro— lyte interface. Here we report on unambiguous observation of surface enhanced Reman scattering from pyridine adsorbed to silver under 13EV conditions and the dependence of the Raman spectrum on pyridine coverage. The experiments have been performed 10torr) in an using an approximately 9Odeg scattering conf i— 13EV chamber (base pressure ~ 5xIO guration as described in detail elsewhere /16/. For all measurements 250mW of 514.5mm radia— tion of an Ar ion laser has been focused on the sample and the scattered light has been analyzed by using a_louble spectrometer set to a resolution of 3cm . Substrates have been a single crystal (110) silver surface, cleaned by Ar ion bombardment and subsequent annealing /16/, a polycrystalline silver slug, me— chanically polished and afterwards cleaned like the (110) surface /16/, and thick silver films evaporated onto cooled copper substrates (T’u’130K) under 13EV conditions and kept at
factor of several 10 (fig. Ia,c). After extended exposure of the ~vaporated silver film to pyridine (fig. Id, 10 L) the intensity of the_Ipain features in the Raman spectrum (1006cm !~d 1036cm”1) decreased and a new peak at 994cm close to that observed in fig. clearly Note also that now the Ia,b 1036cm line developed. is the strongest peak in the spectrum. Fiç. 2 shows the ~ak intensity of the 1006cm and 994/996cm line as a function of exposure. The surface enhanced signal from theclearly evaporated silver film Roman (dots
.5
861
862
RoMAN SCATTERING FROM PYRIDINE ADSORBED TO SILVER/VACUUM INTERFACES
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WAVELENGTH (nm) Figure I spectra from pyridine adsorbed to di~fe— rent silver surfaces, a: (110) surfac~, 10 L exposure; b: polycrystalline slug, 10 L exposure; C: evapora~ed film, IL exposure; d: evaporated film, 10 L exposure. All spectra were taken from samples at about 130K, but for a,b samples have been prepared at room temperature (or higher, see text), whereas the film was e— vaporated on a cooled (130K) substrate (c,d). Note the different intensity scale for d. Roman
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EXPOSURE 101 1o (LI
Figure 2 Peak intensity of some pyridine 1Raman lines as a function of exposufe: 1006cm (dots and solid line) and 994cm (solid triangles) for ridine adsorbed to the evaporated film, 996cm for pyridine adsorbed to the (110) surface (rhombs) and to the polycrystalline slug (open triangles and dashed line).
i
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Vol. 35, No. 11
RANAB SCATTERING FROM PYRIDINE ADSORBED TO SILVER/VACUUM INTERFACES
and solid line) is increasing with exposure in the low exposure range, saturating at about 2L, decreasing after further pyridine deposition, and finally, after exposure to several 10 L, approachin~intensities those of the 996cm line for thesimilar single to crystal (rhoiths) and the polycrystalline slug (open triangles and dashed line), The solid triangles in fig. 2 de,cribe the coverage dependence of the 994cm peak intensity for the evaporated film. Actually we observed a weakly pronounced shoulder at the low energy side of the 1006cm1 peak also for small pyridine exposure (see fig. Ic) which developed into a distinct peak at 30L. This line is significantly more intense than the corresponding peak from pyridine adsorbed to the (110) surface or the polycrystalline slug for exposures ~3OL, but shows comparable in— tensities for higher exposures. Table I suimnarizes the Roman peaks from the evaporated film exposed to IL pyridine (fifth column). Besides the dominating 1006cm’ peak several other surface enhanced lines were observed (see also fig. Ic) whose relative peak intensities varied with exposure (cove— rage) most easily,,,yeen from the intensity ra— tio of the 1036cm to the 1006cm line (in— creasing with exposure). Moreover, a slight, but significant, yhift of_these lines to lover energiey (IOO7cm_ 1,I037cm2 for O.IL and 1005cm , 1035cm for 10 L) and an increase of the peak halfwidth with increasing exposure have been measured. A convincing explanation of these observations will have to await more detailed experiments and analysis of the data (currently in progress), but we would like to point at corresponding effects in SERB spectra from pyridine adsorbed to silver/electrolyte interfaces when varying the potential /4,8,9, 10,13/.
lumns table I) were also seen in our_~ERS spectra. Moreover, besides the 994cm shoul— der/pealf, at least one additional line at 1057cm has been observed (the.,}ines in 1 and 1079cm , are too brackets, weak to be 972cm fixed safely). For completeness and comparison we also show the Roman lines for liquid pyridine, pyridine in aqueous solution, and thick condensed pyridine layers on cooled silver samples in table 1. Several conclusions may be drawn from our results: (i) The Reman spectrum from pyridine adsorbed to the evaporated silver film is certain— ly enhanced, the 1006cm1 line at least by a factor of several IO~as stated a— bove. Taking into account that only a fraction of the pyridine molecules may be adsorbed to ‘active’ sites (see below) this number may increase to the value re— ported for the Ag/electrolyte system /2,3/. In passing we note that our mea— surements present additional evidence for the doubt already expressed in /16/ that some recently published spectra from py~ ridine adsorbed to polycrystalline sil— ver slugs in an 13EV chamber were surface enhanced /17/ (compare spectra in /17/ to fig. Ic). (ii) The fact, that we did not observe SERB spectra from the single crystal (110) and the polycrystalline silver surface prepared at room temperature (or higher temperature) and subse~uent11cooled down demonstrates the importance of a certain surface topography (roughness). It strongly opposes models for SERS based on a chemical reaction of adsorbed pyri— dine (e.g. radical formation in the pre— sence of chloride ions in the Ag/electro— lyte system /18/) or coadsorption of ad-
Table I: Observed ordi~naryand surface enhanced pyridine Raman lines from different systems (cm ). Synunetry
Liquid /2/
Aqueous solution /2/
Thick solid film5L on Ag /16/ 10
Adsorbed terface, IL Ag/vac. in— this work
Adsorbed Ag/electrolyte —0.2V interface —0.6V /2/ /2/
B 2
942w
942w
946w
941w
958w (-‘-972) A1
991vs
1005vs
986w 1023w 996vs
6vs 994w/1006vs 1025w
IOO8s 1026w
IOO
1036s 1057w 1069w (—1079)
1036s
1035s
1068w
1066w
1029w A 1
IO3Ovs
A1
1069w
1037vs 1037/IO4Ovs 1059w 1070w 1067w
All surface enhanced Reman peaks reported for the silver/electrolyte system (we took as representative values from /2/, last two co—
863
ditional ions out of the electrolyte (see also /19/) From the presented observations it can—
864
RAXAN SCATTERING FROM PYRIDINE ADSORBED TO SILVER/VACUUM INTERFACES not be decided definitely which kind of roughness — on a lateral scale of 1OO~ to about 100O~(e.g. /20/) or micro— roughness e.g. adatoms offering special, active adsorption sites /19,21/ — is the main ingredient for SERS. However, two additional experimental observations fa— your the latter: a) Wood and Klein /22/ observed SERS from CO adsorbed to silver films deposited on copper substrates at 120K. After warming up to room temperature and several hours ‘annealing’ at this temperature the films were again cooled down to 120K and ex— posed to CO. No Ranan signal has been ob— served after this procedure. These ob— servations have been confirmed by own experiments. It is hardly imaginable that room temperature annealing influ— ences roughness on the lateral scale of a few boX. On the other hand, isolated adatoms created during deposition of the silver film and fairly stable at low ten— perature, migrate at room temperature and eventually are trapped on sinks (e.g. ledge or kink sites /23/) thus reducing the number of active adsorption sites for SERS. Consequently SERS intensities are not reestablished after the anneal— ing cycle, b) We did not find any significant correla— tion between SERS intensities and the, elastically scattered Rayleigh intensity (caused by roughness on a scale ~500~). Silver films evaporated onto predeposi— ted CaF2 layers (400nm) showed, besides increased Rayleigh scattering, a slight increase of the Reman signal (by rough— ly a factor of two), which is interpre— ted as being due to an increase of the by laser scattering field at ofthe themetal incident surface beam caused into surface plasmons as outlined in /16/. On the other hand, we observed a strong 1IV crease of the Rayleigh intensity after a room temperature annealing cycle as de— scribed in (ii)a (up to a factor of a— bout 10 probably due to a release of
stress in the film during the treatment), but no Reman signal could be detected. These results will be discussed in de— tail elsewhere /24/. (iii) The variation of the peak position, pe?k intensity and halfwidth of the 1006cm
Vol. 35, No. 11
line (and similar influence on the other lines) with exposure is only an effect of pyridine coverage. Preliminary mea— surements show that all effects are cornpletely reversible by slowly desorbing pyridine when warming up the sample. This excludes time dependent effects due to e.g. the prolonged irradiation of the sample during the measurements. (iv) Qualitatively our observations have some similarities to those of Rowe et al from silver surfaces roughened by a special procedure /25/. Although, besides confe— rence abstracts, no detailed information on their results is available some conments can be made. Rowe_?t al concluded that b~sidesthe 1006cm line also the 995cm line due to physisorbed pyridine not in direct contact with silver was surface enhanced. This has been taken as evidence for a nonlocal enhancement me— chanisiji /26/. However, we think that the 994cm shoulder/line originates from pyridine molecules in close contact with molecules directly attached to active (adatom) sites. As these pyridine mole— cules are c 1upled by_j~esonant interaction the 1006cm — 994cm doublet reflects this coupling. This may also account for the observed intensity, position,~nd halfwidth variation of the 1006cm line. On the other hand, as outlined above and in /16/, the i~creasingpeak inten— sity2of the 994cm line for exposures ~10 L is due to ordinary Raman scatter— ing from rather thick condensed layers. This ex?lains also that in fig. Id the 1036cm line is the strongest feature. Here the surface enhanced signal (1036 ) coincide. cm 37/IO4Ocrn ) and the 1ordinary Reman signal In summary (1O we have presented surface enhanced Raman spectra from pyridine adsorbed to eva— porated silver films and discussed the influ— ence of coverage. The observed effects have been interpreted within the frame of the ad— atom hypothesis /19,21/. The small increase of the SERS signal with roughness created by CaF 2 undercoating of the silver film and the irre— versable disappearance of SERS by annealing to room temperature makes an explanation on the basis of electromagnetic resonances /26/ un— likely.
References /1/ M. Fleischmann, P. J. Hendra and A. J. McQuillan, Chem. Phys. Letters 26 (1974) 163 /2/ D.L. Jeanmaire and R. P. VanDuyne, J. E— lectroanal. Chem. 84 (1977) 1 /3/ M. G. Albrecht and J. A. Creighton, J. Am. Chem. Soc. 99 (1977) 5215 /4/ R. P. Van Duyne in: Chemical and Bioche— mical Applications of Lasers, Vol.4, Ed. C. 8. Moore (Academic Press, New York, 1978) pp 101—185 /5/ J. C. Tsang and J. Kirtley, Sol. State Cousnun. 30 (1979) 617
/6/ J. C. Tsang, J. R. Kirtley and J. A. Bradley, Phys. Rev. Letters 43 (1979) 772 /7/ T. H. Wood and H. V. Klein, J. Vac. Sci. Technol. 16 (1979) 459 /8/ B. Pettinger and U. Wenning, Chein. Phys. Letters 56 (1978) 253 /9/ J. A. Creighton, N. G. Albrecht, R. E. Hester and J. A. D. Matthew, Chem. Phys. Letters 55 (1978) 55 /10/ R. P. VanDuyne, J. Physique (Paris) 38 (1977) C5—239 /11/ B. Pettinger, U. Weaning and D. M. Kolb, Ber. Bunsenges. Physik. Chem. 82 (1978)
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RAMAB SCATTERING FROM PYRIDINE ADSORBED TO SILVER/VACUUM INTERFACES
1326 /12/ A. Girlando, 3. C. Gordon II, D. Heitmann, N. R. Philpott, H. Seki and J. D. Swalen, Surf. Sci, (in press) /13/ R. Doruhaus, N, B. Long, K. E. Benner and K. K. Chang, Surf. Sci. 93 (1980) 24 /14/ N. Yleischmann, P. 3. Hendra, A. 3. McQuillan, R. L. Paul and E. S. Reid, .J. Reman Spectrosc. 4 (1976) 269 /15/ T. E. Furtak and 3. Reyes, Surf. Sd. 93 (1980) 351 /16/ I. Pockrand and A. Otto, Proc. VIth tnt. Conf. on the Sol.—Vacuum Interface (Delft May 1980), to appear in Applic. Surf. Sd. /17/ K. R. Smardzeweki, K. 3. Colton and 3. S. Murday, them. Phys. Letters 68 (1979) 53 /18/ A. Regis and 3. Corset, them. Phys. Let— ters 70 (1980) 305
/19/ 3. Billmann, C. Kovacs and A. Otto, Surf, Sci. 92 (1980) 153 /20/ N. Moskovits, 3. them. Phys. 69 (1978) 4159 /21/ A. Otto, 3. Timper, 3. Billmann, G, (0— vacs and I. Pockrand, Surf. Sci. 92 (1980) L55 /22/ T. H. Wood and H. V. Klein, to be published /23/ J. P. Chauvineau, Surf. Sd. 93 (1980) 471 /24/ I. Pockrand and A. Otto, to be published /25/ 3. E. Rowe, D. A. Zwemer, C. A. Murray and C. V. Shank, Bull. Am. Phys. Soc. 25 (1980) 424 /26/ S. L. McCall, P. N. Platzmann and P. A. Wolff, to be published
865
861—865. Solid State Communications, Vol.35,inpp. Pergamon Press Ltd. 1980. Printed Great Britain.
COVERAGE DEPENDENCE OF RAMAN SCATTERING FROM PYRIDINE ADSORBED TO SILVER/VACUU11 INTERFACES I. PoCkrand and A. Otto Physikalisches Institut III, Universitlit DUsseldorf, D—4000 Düsseldorf I, Fed. Rep. of Germany (Received 11 June 1980 by M. Cardona) Roman spectra from pyridine adsorbed to silver films evaporated on cooled substrates un~erUHV ~onditions have been investigated in the exposure range 3x1O L to 10 L. Surface enhanced Reman signals have been detected even for the smallest exposure and reached a maximum in intensity at about monolayer coverage. Extended exposure to pyri— dine exceeding monolayer coverage results in a variation of the Raman spectrum eventually leading to additional Roman peaks due to ordinary scattering from thick condensed pyridine layers. Several further observations make an explanation of surface enhanced Raman scattering on the basis of electromagnetic resonances unlikely.
Since the detection of surface enhanced Raman scattering (SERB) from pyridine adsorbed to silver electrodes /1—3/, the effect has been confirmed for various molecules electro— chemically deposited on different metals (for a recent review see /4/) as well as for dif— ferent systems like tunnel junctions /5,6/ and metal/vacuum interfaces /7/. The pyridine/sil— ver electrode system is, however, still the most intensely studied system: the dependence of SERB intensities, iinewidths /4,8/, and peak positions on the applied potential /4,8, 9,10/, the wavelength dependence of the en—
this temperature. The substrates have been cx— 2L — posed to various doses of pyridine (3x10 IO5L) at temperatures of about 130K and in— mediately afterwards Ranian spectra have been taken from the cooled samples. Fig. I shows Raman spectra in the region of the strong pyridine breathing vibrations from different substrates exposed to different amounts of pyridine as indicated. The single crystal (fig. Ia) and the po1~crystallineslug (fig. Ib), both exposed to 10 L pyridine, exhi— bited quite siini 1lar spectra dominjted by strong peaks at 996cm and 1037/1040cm (for the high background intensity of the polycrystal— line slug see /16/). A few additional weak peaks are also seen. As outlined in detail in /16/ these features are interpreted as being due to ordinary Raman scattering from rather thick condensed pyridine layers on the cooled silver substrates. Fig. lc,d display actual Roman scans from the evaporated silver film exposed to IL (fig3 Ic same intensity scale as fig. la,b!) and 10 L pyridin~respectively. Very intense peaks at 1006cm and 1036cm accompanied by several less pronounced peaks have been measured from at most a monolayer pyridine coverage (due to IL exposvre, fig. Ic). The strongest peak (1006cm ) could easS,ly be detected after ex1 posure to only 3x10’L. Compaying 996cm peak intensity to the 1006cm peaktheintensity the latter one must ke enhanced by at least a
hancement /2,4,9,11,12/, the time development of the signal /13/, and parameters influencing the usually performed oxidation—reduction pre— treatment /2,3,4,8,14/ have all been investi— gated. In view of this situation and the still controversal interpretation of SERB /15/ it seems desirable to study Raman scattering from pyridine adsorbed to silver in a nonelectro— chemical environment, namely at silver/vacuum interfaces, and to compare corresponding re— suits to those obtained from the Ag/electro— lyte interface. Here we report on unambiguous observation of surface enhanced Reman scattering from pyridine adsorbed to silver under 13EV conditions and the dependence of the Raman spectrum on pyridine coverage. The experiments have been performed 10torr) in an using an approximately 9Odeg scattering conf i— 13EV chamber (base pressure ~ 5xIO guration as described in detail elsewhere /16/. For all measurements 250mW of 514.5mm radia— tion of an Ar ion laser has been focused on the sample and the scattered light has been analyzed by using a_louble spectrometer set to a resolution of 3cm . Substrates have been a single crystal (110) silver surface, cleaned by Ar ion bombardment and subsequent annealing /16/, a polycrystalline silver slug, me— chanically polished and afterwards cleaned like the (110) surface /16/, and thick silver films evaporated onto cooled copper substrates (T’u’130K) under 13EV conditions and kept at
factor of several 10 (fig. Ia,c). After extended exposure of the ~vaporated silver film to pyridine (fig. Id, 10 L) the intensity of the_Ipain features in the Raman spectrum (1006cm !~d 1036cm”1) decreased and a new peak at 994cm close to that observed in fig. clearly Note also that now the Ia,b 1036cm line developed. is the strongest peak in the spectrum. Fiç. 2 shows the ~ak intensity of the 1006cm and 994/996cm line as a function of exposure. The surface enhanced signal from theclearly evaporated silver film Roman (dots
.5
861
862
RoMAN SCATTERING FROM PYRIDINE ADSORBED TO SILVER/VACUUM INTERFACES
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Vol. 35, No. 11
I
cts/sec
30
.~
1006cm
~O cml
J
1O1’L
1O~L
1L
103L
lO35cr 1 Tc
994 544
51.2
540
544
542
51.0
542
544
540
541.
542
WAVELENGTH (nm) Figure I spectra from pyridine adsorbed to di~fe— rent silver surfaces, a: (110) surfac~, 10 L exposure; b: polycrystalline slug, 10 L exposure; C: evapora~ed film, IL exposure; d: evaporated film, 10 L exposure. All spectra were taken from samples at about 130K, but for a,b samples have been prepared at room temperature (or higher, see text), whereas the film was e— vaporated on a cooled (130K) substrate (c,d). Note the different intensity scale for d. Roman
10 4
_______________________________________________________________________________ • •IIl~9 I ~III 9
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EXPOSURE 101 1o (LI
Figure 2 Peak intensity of some pyridine 1Raman lines as a function of exposufe: 1006cm (dots and solid line) and 994cm (solid triangles) for ridine adsorbed to the evaporated film, 996cm for pyridine adsorbed to the (110) surface (rhombs) and to the polycrystalline slug (open triangles and dashed line).
i
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Vol. 35, No. 11
RANAB SCATTERING FROM PYRIDINE ADSORBED TO SILVER/VACUUM INTERFACES
and solid line) is increasing with exposure in the low exposure range, saturating at about 2L, decreasing after further pyridine deposition, and finally, after exposure to several 10 L, approachin~intensities those of the 996cm line for thesimilar single to crystal (rhoiths) and the polycrystalline slug (open triangles and dashed line), The solid triangles in fig. 2 de,cribe the coverage dependence of the 994cm peak intensity for the evaporated film. Actually we observed a weakly pronounced shoulder at the low energy side of the 1006cm1 peak also for small pyridine exposure (see fig. Ic) which developed into a distinct peak at 30L. This line is significantly more intense than the corresponding peak from pyridine adsorbed to the (110) surface or the polycrystalline slug for exposures ~3OL, but shows comparable in— tensities for higher exposures. Table I suimnarizes the Roman peaks from the evaporated film exposed to IL pyridine (fifth column). Besides the dominating 1006cm’ peak several other surface enhanced lines were observed (see also fig. Ic) whose relative peak intensities varied with exposure (cove— rage) most easily,,,yeen from the intensity ra— tio of the 1036cm to the 1006cm line (in— creasing with exposure). Moreover, a slight, but significant, yhift of_these lines to lover energiey (IOO7cm_ 1,I037cm2 for O.IL and 1005cm , 1035cm for 10 L) and an increase of the peak halfwidth with increasing exposure have been measured. A convincing explanation of these observations will have to await more detailed experiments and analysis of the data (currently in progress), but we would like to point at corresponding effects in SERB spectra from pyridine adsorbed to silver/electrolyte interfaces when varying the potential /4,8,9, 10,13/.
lumns table I) were also seen in our_~ERS spectra. Moreover, besides the 994cm shoul— der/pealf, at least one additional line at 1057cm has been observed (the.,}ines in 1 and 1079cm , are too brackets, weak to be 972cm fixed safely). For completeness and comparison we also show the Roman lines for liquid pyridine, pyridine in aqueous solution, and thick condensed pyridine layers on cooled silver samples in table 1. Several conclusions may be drawn from our results: (i) The Reman spectrum from pyridine adsorbed to the evaporated silver film is certain— ly enhanced, the 1006cm1 line at least by a factor of several IO~as stated a— bove. Taking into account that only a fraction of the pyridine molecules may be adsorbed to ‘active’ sites (see below) this number may increase to the value re— ported for the Ag/electrolyte system /2,3/. In passing we note that our mea— surements present additional evidence for the doubt already expressed in /16/ that some recently published spectra from py~ ridine adsorbed to polycrystalline sil— ver slugs in an 13EV chamber were surface enhanced /17/ (compare spectra in /17/ to fig. Ic). (ii) The fact, that we did not observe SERB spectra from the single crystal (110) and the polycrystalline silver surface prepared at room temperature (or higher temperature) and subse~uent11cooled down demonstrates the importance of a certain surface topography (roughness). It strongly opposes models for SERS based on a chemical reaction of adsorbed pyri— dine (e.g. radical formation in the pre— sence of chloride ions in the Ag/electro— lyte system /18/) or coadsorption of ad-
Table I: Observed ordi~naryand surface enhanced pyridine Raman lines from different systems (cm ). Synunetry
Liquid /2/
Aqueous solution /2/
Thick solid film5L on Ag /16/ 10
Adsorbed terface, IL Ag/vac. in— this work
Adsorbed Ag/electrolyte —0.2V interface —0.6V /2/ /2/
B 2
942w
942w
946w
941w
958w (-‘-972) A1
991vs
1005vs
986w 1023w 996vs
6vs 994w/1006vs 1025w
IOO8s 1026w
IOO
1036s 1057w 1069w (—1079)
1036s
1035s
1068w
1066w
1029w A 1
IO3Ovs
A1
1069w
1037vs 1037/IO4Ovs 1059w 1070w 1067w
All surface enhanced Reman peaks reported for the silver/electrolyte system (we took as representative values from /2/, last two co—
863
ditional ions out of the electrolyte (see also /19/) From the presented observations it can—
864
RAXAN SCATTERING FROM PYRIDINE ADSORBED TO SILVER/VACUUM INTERFACES not be decided definitely which kind of roughness — on a lateral scale of 1OO~ to about 100O~(e.g. /20/) or micro— roughness e.g. adatoms offering special, active adsorption sites /19,21/ — is the main ingredient for SERS. However, two additional experimental observations fa— your the latter: a) Wood and Klein /22/ observed SERS from CO adsorbed to silver films deposited on copper substrates at 120K. After warming up to room temperature and several hours ‘annealing’ at this temperature the films were again cooled down to 120K and ex— posed to CO. No Ranan signal has been ob— served after this procedure. These ob— servations have been confirmed by own experiments. It is hardly imaginable that room temperature annealing influ— ences roughness on the lateral scale of a few boX. On the other hand, isolated adatoms created during deposition of the silver film and fairly stable at low ten— perature, migrate at room temperature and eventually are trapped on sinks (e.g. ledge or kink sites /23/) thus reducing the number of active adsorption sites for SERS. Consequently SERS intensities are not reestablished after the anneal— ing cycle, b) We did not find any significant correla— tion between SERS intensities and the, elastically scattered Rayleigh intensity (caused by roughness on a scale ~500~). Silver films evaporated onto predeposi— ted CaF2 layers (400nm) showed, besides increased Rayleigh scattering, a slight increase of the Reman signal (by rough— ly a factor of two), which is interpre— ted as being due to an increase of the by laser scattering field at ofthe themetal incident surface beam caused into surface plasmons as outlined in /16/. On the other hand, we observed a strong 1IV crease of the Rayleigh intensity after a room temperature annealing cycle as de— scribed in (ii)a (up to a factor of a— bout 10 probably due to a release of
stress in the film during the treatment), but no Reman signal could be detected. These results will be discussed in de— tail elsewhere /24/. (iii) The variation of the peak position, pe?k intensity and halfwidth of the 1006cm
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line (and similar influence on the other lines) with exposure is only an effect of pyridine coverage. Preliminary mea— surements show that all effects are cornpletely reversible by slowly desorbing pyridine when warming up the sample. This excludes time dependent effects due to e.g. the prolonged irradiation of the sample during the measurements. (iv) Qualitatively our observations have some similarities to those of Rowe et al from silver surfaces roughened by a special procedure /25/. Although, besides confe— rence abstracts, no detailed information on their results is available some conments can be made. Rowe_?t al concluded that b~sidesthe 1006cm line also the 995cm line due to physisorbed pyridine not in direct contact with silver was surface enhanced. This has been taken as evidence for a nonlocal enhancement me— chanisiji /26/. However, we think that the 994cm shoulder/line originates from pyridine molecules in close contact with molecules directly attached to active (adatom) sites. As these pyridine mole— cules are c 1upled by_j~esonant interaction the 1006cm — 994cm doublet reflects this coupling. This may also account for the observed intensity, position,~nd halfwidth variation of the 1006cm line. On the other hand, as outlined above and in /16/, the i~creasingpeak inten— sity2of the 994cm line for exposures ~10 L is due to ordinary Raman scatter— ing from rather thick condensed layers. This ex?lains also that in fig. Id the 1036cm line is the strongest feature. Here the surface enhanced signal (1036 ) coincide. cm 37/IO4Ocrn ) and the 1ordinary Reman signal In summary (1O we have presented surface enhanced Raman spectra from pyridine adsorbed to eva— porated silver films and discussed the influ— ence of coverage. The observed effects have been interpreted within the frame of the ad— atom hypothesis /19,21/. The small increase of the SERS signal with roughness created by CaF 2 undercoating of the silver film and the irre— versable disappearance of SERS by annealing to room temperature makes an explanation on the basis of electromagnetic resonances /26/ un— likely.
References /1/ M. Fleischmann, P. J. Hendra and A. J. McQuillan, Chem. Phys. Letters 26 (1974) 163 /2/ D.L. Jeanmaire and R. P. VanDuyne, J. E— lectroanal. Chem. 84 (1977) 1 /3/ M. G. Albrecht and J. A. Creighton, J. Am. Chem. Soc. 99 (1977) 5215 /4/ R. P. Van Duyne in: Chemical and Bioche— mical Applications of Lasers, Vol.4, Ed. C. 8. Moore (Academic Press, New York, 1978) pp 101—185 /5/ J. C. Tsang and J. Kirtley, Sol. State Cousnun. 30 (1979) 617
/6/ J. C. Tsang, J. R. Kirtley and J. A. Bradley, Phys. Rev. Letters 43 (1979) 772 /7/ T. H. Wood and H. V. Klein, J. Vac. Sci. Technol. 16 (1979) 459 /8/ B. Pettinger and U. Wenning, Chein. Phys. Letters 56 (1978) 253 /9/ J. A. Creighton, N. G. Albrecht, R. E. Hester and J. A. D. Matthew, Chem. Phys. Letters 55 (1978) 55 /10/ R. P. VanDuyne, J. Physique (Paris) 38 (1977) C5—239 /11/ B. Pettinger, U. Weaning and D. M. Kolb, Ber. Bunsenges. Physik. Chem. 82 (1978)
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1326 /12/ A. Girlando, 3. C. Gordon II, D. Heitmann, N. R. Philpott, H. Seki and J. D. Swalen, Surf. Sci, (in press) /13/ R. Doruhaus, N, B. Long, K. E. Benner and K. K. Chang, Surf. Sci. 93 (1980) 24 /14/ N. Yleischmann, P. 3. Hendra, A. 3. McQuillan, R. L. Paul and E. S. Reid, .J. Reman Spectrosc. 4 (1976) 269 /15/ T. E. Furtak and 3. Reyes, Surf. Sd. 93 (1980) 351 /16/ I. Pockrand and A. Otto, Proc. VIth tnt. Conf. on the Sol.—Vacuum Interface (Delft May 1980), to appear in Applic. Surf. Sd. /17/ K. R. Smardzeweki, K. 3. Colton and 3. S. Murday, them. Phys. Letters 68 (1979) 53 /18/ A. Regis and 3. Corset, them. Phys. Let— ters 70 (1980) 305
/19/ 3. Billmann, C. Kovacs and A. Otto, Surf, Sci. 92 (1980) 153 /20/ N. Moskovits, 3. them. Phys. 69 (1978) 4159 /21/ A. Otto, 3. Timper, 3. Billmann, G, (0— vacs and I. Pockrand, Surf. Sci. 92 (1980) L55 /22/ T. H. Wood and H. V. Klein, to be published /23/ J. P. Chauvineau, Surf. Sd. 93 (1980) 471 /24/ I. Pockrand and A. Otto, to be published /25/ 3. E. Rowe, D. A. Zwemer, C. A. Murray and C. V. Shank, Bull. Am. Phys. Soc. 25 (1980) 424 /26/ S. L. McCall, P. N. Platzmann and P. A. Wolff, to be published
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