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Vibrational investigation of chemisorbed C60 by infrared-visible sum frequency generation spectroscopy
SurfaceScience377-379(1997)1071-1075
Vibrational investigation of chemisorbed C& by infrared-visible sum frequency generation spectroscopy Y. Caudano a31,A. Peremans a,2, P.A. Thiry a9*, P. Dumas b, A. Tadjeddine b a Laboratoire de Spectroscopic Molhlaire de Surface, Institutefor Studies in Interface Sciences, Faculth Universitaires Notre-Dame de la Paix, 61, rue de Bruxelles, B-5000 Namur, Belgium b Laboratoirepow I’Utilisation du Rayonnement Electromagn~tique, CNRS B&. 2090, F-91405 Orsay Cedex, France
Received 1 August 1996; accepted for publication 9 September 1996
Non-linear optical experiments were performed on Cm and K-doped C,, monolayers grown on Ag( 111). We have recorded the visible SFG signal resulting from the frequency mixing of one tunable infrared and one visible laser beam. The SFG process is forbidden in the bulk for centrosymmetric systems and thus provides the fingerprint of interfacial vibrations. The monolayer spectrum of pure C,,/Ag( 111) shows one resonance at 1445 cm-‘. We assign this peak to the A,(2) “pentagonal pinch” mode, that is Raman active in the bulk at 1468 cm-l. The spectrum of a fully K-doped C6,, monolayer exhibits two peaks at 1349 and 1431 cm-‘. The former is still attributed to the A,(2) mode while the latter is assigned to the T,,(4) infrared active mode of C,. Keywords:
Carbon; Fullerenes; Non-linear optical methods; Silver; Single crystal surfaces; Vibrations of adsorbed molecules
1. Introduction
With the development of surface science and of its applications, there has been an increased demand for surface specific spectroscopies. Under certain conditions, electron and ion spectroscopies can be extremely surface sensitive, but they suffer the disadvantage of requiring a demanding UHV environment. This drastic limitation does not exist
* Corresponding author. Fax + 32 8172 4707; e-mail: [email protected] 1 Holder of a grant from the Belgian Fund for Research in Industry and Agriculture (FRIA) ‘Senior Research Associate of the Belgian National Fund for Scientific Research (FNRS).
for infrared and Raman spectroscopies that can operate at ambient pressure and even in the liquid phase, but are not intrinsically surface sensitive, at least in the framework of the linear response. However, in favourable cases, the non-linear response of a medium to an electromagnetic excitation can be surface (or interface) sensitive. The purpose of this paper is to demonstrate that the non-linear response can be efficiently used to study interfacial systems. Therefore, we selected “infrared-visible sum frequency generation” (SFG) as the technique and C,, as the system to study. The principle of the SFG technique has been demonstrated experimentally in 1987 [ 1,2]. An IR-visible SFG setup makes use of two laser sources: A visible beam at fixed frequency, usually
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PIZ SOO39-6028(96)01548-S
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Y. Caudano et al. /Surface Science 377-379
green, and .an infrared beam that has to be widely tunable in the range of the vibrational fingerprint of the sample under study. Developing such a tunable infrared source is still challenging: free electron lasers were often used although benchtop lasers could be recently qualified for IR-visible SFG. In the SFG experiment, the two laser beams are mixed at the surface of the sample and one detects photons at the sum of the two frequencies (i.e. blue = green + infrared). Within the dipolar approximation neglecting magnetic and quadrupole electric interactions, all even-order non-linear optical processes, to which SFG belongs, are strictly forbidden in media possessing a center of inversion (as it is the case for C,,,). Indeed the number of SFG photons produced is proportional to the square of xc’) which is the second-order polarizability tensor of the material and it is straightforward to show that for centrosymmetric media, all individual components of xc’) must vanish. At this point, one could wonder why for our investigation, we chose a centrosymmetric molecule like C&,, because it should be intrinsically SFG, inactive. The answer is that any observed SFG activity should indeed indicate that the molecule symmetry is broken and provide information about its bonding at the interface and/or about the effect of doping. The SFG signal is resonantly enhanced when the infrared tunable laser frequency matches the frequency of a sample vibration that is both infrared and Raman active. Let us emphasize that, because of the well-known mutual exclusion rule, this condition again rules out any SFG activity in centrosymmetric molecules. The vibrational structure of pure ChO is well known [3]. Due to its high symmetry, C& has only 4 infrared active modes labeled T,, and located at 526, 576, 1183 and 143Ocm-‘. The 10 optically allowed Raman modes consist of 8 H, modes and 2 totally symmetric Ag modes. The first A& 1) mode is the radial “breathing” mode of the molecule at 495 cm-’ while the A,(2) mode at 1468 cm-l is the so-called “pentagonal pinch”, for which the radius of the molecule remains constant and the carbon atoms move tangentially on the surface of the sphere in such a way that all 12
(1997) 1071-1075
pentagons shrink and expand in phase. All these optical modes are affected by doping with alkali atoms: most of the frequencies are red shifted and some infrared vibrational intensities, especially the fourth T1, mode are dramatically enhanced by a charged phonon effect [4]. In a recent theoretical paper, Giannozzi and Andreoni [5] showed that structural relaxation is primarily responsible for the frequency shifts and that the change in infrared relative intensities is directly related to charge transfer.
2. Experimental The substrate Ag( 111) single crystal was cleaned by repeated cycles of Argon ion sputtering and annealing to 570 K. C& soot was sublimated from a home-made Ta oven, on the Ag substrate held at 570 K in order to stabilize one chemisorbed C&,, monolayer and to avoid multilayer growth. K-doping was made from a SAES getter source on the substrate at 390 K and was followed by Fourier transform infrared spectroscopy (FTIR). Saturation of the Cc0 monolayer (ML) (i.e. formation of a completely doped phase, analogous to the K,C& phase in bulk fullerides) was assumed when the FTIR spectrum showed no further change. The latter phase corresponds to a complete filling of the t,, electronic band (LUMO) of the molecule which can accept up to 6 electrons. Unfortunately, the experimental conditions did not allow precise nor reproducible control of the intermediate stoichiometry of the KXt,,6$& chemisorbed phases. IR spectra were recorded at grazing incidence with unpolarized light with a FTIR spectrometer operating at a resolution of 4 cm-l and equipped with a liquid nitrogen cooled HCT detector. The infrared tunable laser used for the SFG spectroscopy uses a picosecond optical parametric oscillator (OPO) (B.M. Industries, Lisses-Evry, France) developed around an AgGaS, single crystal and synchronously pumped by a YAG laser. It is tunable from 2.5 pm (4000 cm-‘) to 10 ,um (1000 cm-l) and delivers pulses of 11 ps duration, with a bandwidth of 1.8 cm-l. The output infrared power was 7 mW at 7 ,um. The infrared and visible
Y. Caudano et al. /Surface Science 377-379
(1997) 1071-1075
beams were p-polarized with incidence angles of 65” and 55”, respectively, on the target. The SFG photons were detected by a photomultiplier after spatial and spectral filtering. A complete description of the system can be found in Ref. [6].
3. Results aud discussion Fig. 1 displays the FTIR spectra recorded between 1300 and 1500 cm-’ on a monolayer of C6,, grown on Ag(ll1) (lower trace), and after successive stages of K-doping. These spectra are not of very high quality, but the information they contain is sufficient to follow qualitatively the doping process. They are similar to the spectra by Rudolf et al. using synchrotron radiation [7]. The pristine C& layer is characterized by one single peak located at 1440 cm-l, that gradually red 1300
1350
1400
Wavenumber
1450
1500
(cm-‘)
Fig. 2. IR-visible SFG spectra recorded on Cs,/Ag( 111). The lower trace corresponds to pure C,,. The upper trace corresponds to the completely K-doped phase. Intermediate traces correspond to intermediate K-doping.
shifts and decreases in intensity upon K-doping until it completely disappears at some intermediate K-coverage. SFG spectra corresponding to Fig. 1, i.e. recorded under similar conditions, are presented in Fig. 2. 3.1. Undoped C,,
1350
1400
Wavenumber
1450
(cm-‘)
Fig. 1. Series of FTIR spectra recorded on C,,/Ag( 111). The lower trace corresponds to one undoped monolayer of Cse. The upper trace corresponds to the saturation state reached upon doping with K. The other traces correspond to intermediate K-doped C,, phases (see text).
The pristine C& monolayer shows a clear SFG resonance at 1445 cm-l, i.e. very close to the position of the measured FTIR peak. On the other hand, on the fully K-doped layer, two resonances are observed, one at 1349 cm-’ and a second one at 143 1 cm-l. It should be mentioned that between successive dopings, the sample had to be transferred from the analysis position to the evaporation position so that it was not possible to compare the SFG intensities between the successive spectra. We assign both the FTIR and SFG peaks at 1445 cm-‘, to the “pentagonal pinching” A,(2)
1074
X Caudano et al. / Surface Science 377-379
mode, which is Raman active and measured at 1468 cn-l in bulk C,,. However, if a Raman mode is SFG active, it must have an infrared activity. We believe that this infrared activity results from the chemisorption process inducing a charge transfer from the Ag substrate toward the C&, molecules. Such charge transfer should induce a global softening of all modes, and in particular of the A,(2) and T,,(4) modes [ 51. The measured frequency of 1445 cm-l is lower than that of pristine A,(2) (1468 cm-l), but higher than the pristine T,,(4) (1430 cm-‘) so that it cannot be assigned the T,,(4) mode whose oscillator strength is most probably too weak to be detected by FTIR. 3.2. K-doped C,, During the alkali-doping process, both FTIR and SFG measurements exhibit a softening of the A,(2) mode until some intermediate doping stage where it is no longer detected by FTIR. After this intermediate stage, a new peak is observed by both FTIR and SFG at 1349 cm-’ and grows in intensity up to reaching the K-saturated phase. This second peak at 1349 cm-l is interpreted as corresponding to the fourth infrared active mode (T,,(4)) of pristine CsO at 1430 cm-l, which is red shifted due to the structural and electronic modifications induced by the charge transfer. We note that the A,(2) mode is always observed by SFG even though it is no longer detected by FTIR. Taking into account that the SFG activity can be roughly estimated from the product of the IR and Raman activities, we believe that, although the IR activity of this Raman mode has decreased below the detection limit of FTIR, it remains strong enough to provide SFG coupling. The amount of red shift of the A,(2) mode in the K-saturated phase allows to make an estimate for the charge transfer. Indeed, from Refs. [8,9], it can be assumed that 1 electron transferred induces a shift of 6.5 cm-’ in frequency. The difference in frequencies between the pristine CsO (1468 cm-‘) and the K-saturated phase (1431 cm-l) being equal to 37 cn-‘, thus corresponds to 5.7 electrons. Consequently, the K-saturated phase should be very similar to the fully K-doped insulating phase K,&,,.
(1997) 1071-1075
The astonishingly strong infrared activity of the A,(2) mode can be explained by invoking an electron-phonon coupling mechanism [ lo]. The charge transfer from the metallic Ag substrate to the C& molecules results in a partial occupancy by the Ag electrons of the 6-fold degenerate t,, molecular orbitals corresponding to the LUMO’s of the free C&, molecules. This occupancy is modulated in time by the A,(2) vibration inducing consequently a charge oscillation between the substrate and the admolecule. This oscillating charge transfer is responsible for the enhancement of the infrared activity of the pentagonal pinch mode in the chemisorbed molecule. However, increased doping contributes to fill completely these t,, states and by so doing, to decrease the effect of the modulation until it vanishes when the electronic levels are fully occupied.
4. Conclusions
This paper clearly demonstrates the possibility of applications of SFG spectroscopy. SFG spectra could be recorded from a monolayer of C& deposited on Ag( 111) before and after doping with K. In the measured frequency range, two peaks have been observed and could be related to the Raman A,(2) and the infared T,,(4) modes of pristine C,,. The frequency shifts and intensity variations of these peaks can be explained by an electronphonon coupling mechanism involving the valence states of Ag and the tl, orbitals of the CeO molecules.
Acknowledgements
We thank P. Rudolf, Ph. Lambin, J.-M. Gilles and J. Darville for stimulating discussions. These experiments could not have been successful without the efficient technical support of M. Tanguy and P. Monte1 to whom we are very grateful.
Y. Caudano et al. / Surface Science 377 -379 (1997)
References [l] J.H. Hunt, Ph. Guyot-Sionnest and Y.R. Shen, Chem. Phys. Lett. 133 (1987) 189. [2] Y.R. Shen, Nature 337 (1989) 519. [3] B. Chase, N. Herron and E. Holler, J. Phys. Chem. 96 (1992) 4262. [4] M.J. Rice and H.Y. Choi, Phys. Rev. B 45 (1992) 10 173. [5] P. Giannozzi and W. Andreoni, Phys. Rev. Lett. 76 (1996) 4915. [6] A. Peremans, A. Tadjeddine, W.-D. Zheng and A. Le Rille, Ann. Phys. Fr. 20 (1995) 9527.
[71P. Rudolf,
1071-1075
1075
PhD. Thesis, University of Namur, Belgium, 1995. S.J. Duclos, R.C. Haddon, S. Glarum, A.F. Hebard and K.B. Lyons, Science 254 (1991) 1625 D.W. PI R.C. Haddon, A.F. Hebard, M.J. Rosseinsky, Murphy, S.J. Duclos, K.B. Lyons, B. Miller, J.M. Rosamilia, R.M. Fleming, A.R. Kortan, S.H. Glarum, A.V. Makhija, A.J. Muller, R.H. Eick, SM. Zahurak, R. Tycko, G. Dabagh and F.A. Thiel, Nature 350 (1991) 320. t101A. Peremans, Y. Caudano, P.A. Thiry and A. Tadjeddine, to be published.
Vibrational investigation of chemisorbed C& by infrared-visible sum frequency generation spectroscopy Y. Caudano a31,A. Peremans a,2, P.A. Thiry a9*, P. Dumas b, A. Tadjeddine b a Laboratoire de Spectroscopic Molhlaire de Surface, Institutefor Studies in Interface Sciences, Faculth Universitaires Notre-Dame de la Paix, 61, rue de Bruxelles, B-5000 Namur, Belgium b Laboratoirepow I’Utilisation du Rayonnement Electromagn~tique, CNRS B&. 2090, F-91405 Orsay Cedex, France
Received 1 August 1996; accepted for publication 9 September 1996
Non-linear optical experiments were performed on Cm and K-doped C,, monolayers grown on Ag( 111). We have recorded the visible SFG signal resulting from the frequency mixing of one tunable infrared and one visible laser beam. The SFG process is forbidden in the bulk for centrosymmetric systems and thus provides the fingerprint of interfacial vibrations. The monolayer spectrum of pure C,,/Ag( 111) shows one resonance at 1445 cm-‘. We assign this peak to the A,(2) “pentagonal pinch” mode, that is Raman active in the bulk at 1468 cm-l. The spectrum of a fully K-doped C6,, monolayer exhibits two peaks at 1349 and 1431 cm-‘. The former is still attributed to the A,(2) mode while the latter is assigned to the T,,(4) infrared active mode of C,. Keywords:
Carbon; Fullerenes; Non-linear optical methods; Silver; Single crystal surfaces; Vibrations of adsorbed molecules
1. Introduction
With the development of surface science and of its applications, there has been an increased demand for surface specific spectroscopies. Under certain conditions, electron and ion spectroscopies can be extremely surface sensitive, but they suffer the disadvantage of requiring a demanding UHV environment. This drastic limitation does not exist
* Corresponding author. Fax + 32 8172 4707; e-mail: [email protected] 1 Holder of a grant from the Belgian Fund for Research in Industry and Agriculture (FRIA) ‘Senior Research Associate of the Belgian National Fund for Scientific Research (FNRS).
for infrared and Raman spectroscopies that can operate at ambient pressure and even in the liquid phase, but are not intrinsically surface sensitive, at least in the framework of the linear response. However, in favourable cases, the non-linear response of a medium to an electromagnetic excitation can be surface (or interface) sensitive. The purpose of this paper is to demonstrate that the non-linear response can be efficiently used to study interfacial systems. Therefore, we selected “infrared-visible sum frequency generation” (SFG) as the technique and C,, as the system to study. The principle of the SFG technique has been demonstrated experimentally in 1987 [ 1,2]. An IR-visible SFG setup makes use of two laser sources: A visible beam at fixed frequency, usually
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PIZ SOO39-6028(96)01548-S
1072
Y. Caudano et al. /Surface Science 377-379
green, and .an infrared beam that has to be widely tunable in the range of the vibrational fingerprint of the sample under study. Developing such a tunable infrared source is still challenging: free electron lasers were often used although benchtop lasers could be recently qualified for IR-visible SFG. In the SFG experiment, the two laser beams are mixed at the surface of the sample and one detects photons at the sum of the two frequencies (i.e. blue = green + infrared). Within the dipolar approximation neglecting magnetic and quadrupole electric interactions, all even-order non-linear optical processes, to which SFG belongs, are strictly forbidden in media possessing a center of inversion (as it is the case for C,,,). Indeed the number of SFG photons produced is proportional to the square of xc’) which is the second-order polarizability tensor of the material and it is straightforward to show that for centrosymmetric media, all individual components of xc’) must vanish. At this point, one could wonder why for our investigation, we chose a centrosymmetric molecule like C&,, because it should be intrinsically SFG, inactive. The answer is that any observed SFG activity should indeed indicate that the molecule symmetry is broken and provide information about its bonding at the interface and/or about the effect of doping. The SFG signal is resonantly enhanced when the infrared tunable laser frequency matches the frequency of a sample vibration that is both infrared and Raman active. Let us emphasize that, because of the well-known mutual exclusion rule, this condition again rules out any SFG activity in centrosymmetric molecules. The vibrational structure of pure ChO is well known [3]. Due to its high symmetry, C& has only 4 infrared active modes labeled T,, and located at 526, 576, 1183 and 143Ocm-‘. The 10 optically allowed Raman modes consist of 8 H, modes and 2 totally symmetric Ag modes. The first A& 1) mode is the radial “breathing” mode of the molecule at 495 cm-’ while the A,(2) mode at 1468 cm-l is the so-called “pentagonal pinch”, for which the radius of the molecule remains constant and the carbon atoms move tangentially on the surface of the sphere in such a way that all 12
(1997) 1071-1075
pentagons shrink and expand in phase. All these optical modes are affected by doping with alkali atoms: most of the frequencies are red shifted and some infrared vibrational intensities, especially the fourth T1, mode are dramatically enhanced by a charged phonon effect [4]. In a recent theoretical paper, Giannozzi and Andreoni [5] showed that structural relaxation is primarily responsible for the frequency shifts and that the change in infrared relative intensities is directly related to charge transfer.
2. Experimental The substrate Ag( 111) single crystal was cleaned by repeated cycles of Argon ion sputtering and annealing to 570 K. C& soot was sublimated from a home-made Ta oven, on the Ag substrate held at 570 K in order to stabilize one chemisorbed C&,, monolayer and to avoid multilayer growth. K-doping was made from a SAES getter source on the substrate at 390 K and was followed by Fourier transform infrared spectroscopy (FTIR). Saturation of the Cc0 monolayer (ML) (i.e. formation of a completely doped phase, analogous to the K,C& phase in bulk fullerides) was assumed when the FTIR spectrum showed no further change. The latter phase corresponds to a complete filling of the t,, electronic band (LUMO) of the molecule which can accept up to 6 electrons. Unfortunately, the experimental conditions did not allow precise nor reproducible control of the intermediate stoichiometry of the KXt,,6$& chemisorbed phases. IR spectra were recorded at grazing incidence with unpolarized light with a FTIR spectrometer operating at a resolution of 4 cm-l and equipped with a liquid nitrogen cooled HCT detector. The infrared tunable laser used for the SFG spectroscopy uses a picosecond optical parametric oscillator (OPO) (B.M. Industries, Lisses-Evry, France) developed around an AgGaS, single crystal and synchronously pumped by a YAG laser. It is tunable from 2.5 pm (4000 cm-‘) to 10 ,um (1000 cm-l) and delivers pulses of 11 ps duration, with a bandwidth of 1.8 cm-l. The output infrared power was 7 mW at 7 ,um. The infrared and visible
Y. Caudano et al. /Surface Science 377-379
(1997) 1071-1075
beams were p-polarized with incidence angles of 65” and 55”, respectively, on the target. The SFG photons were detected by a photomultiplier after spatial and spectral filtering. A complete description of the system can be found in Ref. [6].
3. Results aud discussion Fig. 1 displays the FTIR spectra recorded between 1300 and 1500 cm-’ on a monolayer of C6,, grown on Ag(ll1) (lower trace), and after successive stages of K-doping. These spectra are not of very high quality, but the information they contain is sufficient to follow qualitatively the doping process. They are similar to the spectra by Rudolf et al. using synchrotron radiation [7]. The pristine C& layer is characterized by one single peak located at 1440 cm-l, that gradually red 1300
1350
1400
Wavenumber
1450
1500
(cm-‘)
Fig. 2. IR-visible SFG spectra recorded on Cs,/Ag( 111). The lower trace corresponds to pure C,,. The upper trace corresponds to the completely K-doped phase. Intermediate traces correspond to intermediate K-doping.
shifts and decreases in intensity upon K-doping until it completely disappears at some intermediate K-coverage. SFG spectra corresponding to Fig. 1, i.e. recorded under similar conditions, are presented in Fig. 2. 3.1. Undoped C,,
1350
1400
Wavenumber
1450
(cm-‘)
Fig. 1. Series of FTIR spectra recorded on C,,/Ag( 111). The lower trace corresponds to one undoped monolayer of Cse. The upper trace corresponds to the saturation state reached upon doping with K. The other traces correspond to intermediate K-doped C,, phases (see text).
The pristine C& monolayer shows a clear SFG resonance at 1445 cm-l, i.e. very close to the position of the measured FTIR peak. On the other hand, on the fully K-doped layer, two resonances are observed, one at 1349 cm-’ and a second one at 143 1 cm-l. It should be mentioned that between successive dopings, the sample had to be transferred from the analysis position to the evaporation position so that it was not possible to compare the SFG intensities between the successive spectra. We assign both the FTIR and SFG peaks at 1445 cm-‘, to the “pentagonal pinching” A,(2)
1074
X Caudano et al. / Surface Science 377-379
mode, which is Raman active and measured at 1468 cn-l in bulk C,,. However, if a Raman mode is SFG active, it must have an infrared activity. We believe that this infrared activity results from the chemisorption process inducing a charge transfer from the Ag substrate toward the C&, molecules. Such charge transfer should induce a global softening of all modes, and in particular of the A,(2) and T,,(4) modes [ 51. The measured frequency of 1445 cm-l is lower than that of pristine A,(2) (1468 cm-l), but higher than the pristine T,,(4) (1430 cm-‘) so that it cannot be assigned the T,,(4) mode whose oscillator strength is most probably too weak to be detected by FTIR. 3.2. K-doped C,, During the alkali-doping process, both FTIR and SFG measurements exhibit a softening of the A,(2) mode until some intermediate doping stage where it is no longer detected by FTIR. After this intermediate stage, a new peak is observed by both FTIR and SFG at 1349 cm-’ and grows in intensity up to reaching the K-saturated phase. This second peak at 1349 cm-l is interpreted as corresponding to the fourth infrared active mode (T,,(4)) of pristine CsO at 1430 cm-l, which is red shifted due to the structural and electronic modifications induced by the charge transfer. We note that the A,(2) mode is always observed by SFG even though it is no longer detected by FTIR. Taking into account that the SFG activity can be roughly estimated from the product of the IR and Raman activities, we believe that, although the IR activity of this Raman mode has decreased below the detection limit of FTIR, it remains strong enough to provide SFG coupling. The amount of red shift of the A,(2) mode in the K-saturated phase allows to make an estimate for the charge transfer. Indeed, from Refs. [8,9], it can be assumed that 1 electron transferred induces a shift of 6.5 cm-’ in frequency. The difference in frequencies between the pristine CsO (1468 cm-‘) and the K-saturated phase (1431 cm-l) being equal to 37 cn-‘, thus corresponds to 5.7 electrons. Consequently, the K-saturated phase should be very similar to the fully K-doped insulating phase K,&,,.
(1997) 1071-1075
The astonishingly strong infrared activity of the A,(2) mode can be explained by invoking an electron-phonon coupling mechanism [ lo]. The charge transfer from the metallic Ag substrate to the C& molecules results in a partial occupancy by the Ag electrons of the 6-fold degenerate t,, molecular orbitals corresponding to the LUMO’s of the free C&, molecules. This occupancy is modulated in time by the A,(2) vibration inducing consequently a charge oscillation between the substrate and the admolecule. This oscillating charge transfer is responsible for the enhancement of the infrared activity of the pentagonal pinch mode in the chemisorbed molecule. However, increased doping contributes to fill completely these t,, states and by so doing, to decrease the effect of the modulation until it vanishes when the electronic levels are fully occupied.
4. Conclusions
This paper clearly demonstrates the possibility of applications of SFG spectroscopy. SFG spectra could be recorded from a monolayer of C& deposited on Ag( 111) before and after doping with K. In the measured frequency range, two peaks have been observed and could be related to the Raman A,(2) and the infared T,,(4) modes of pristine C,,. The frequency shifts and intensity variations of these peaks can be explained by an electronphonon coupling mechanism involving the valence states of Ag and the tl, orbitals of the CeO molecules.
Acknowledgements
We thank P. Rudolf, Ph. Lambin, J.-M. Gilles and J. Darville for stimulating discussions. These experiments could not have been successful without the efficient technical support of M. Tanguy and P. Monte1 to whom we are very grateful.
Y. Caudano et al. / Surface Science 377 -379 (1997)
References [l] J.H. Hunt, Ph. Guyot-Sionnest and Y.R. Shen, Chem. Phys. Lett. 133 (1987) 189. [2] Y.R. Shen, Nature 337 (1989) 519. [3] B. Chase, N. Herron and E. Holler, J. Phys. Chem. 96 (1992) 4262. [4] M.J. Rice and H.Y. Choi, Phys. Rev. B 45 (1992) 10 173. [5] P. Giannozzi and W. Andreoni, Phys. Rev. Lett. 76 (1996) 4915. [6] A. Peremans, A. Tadjeddine, W.-D. Zheng and A. Le Rille, Ann. Phys. Fr. 20 (1995) 9527.
[71P. Rudolf,
1071-1075
1075
PhD. Thesis, University of Namur, Belgium, 1995. S.J. Duclos, R.C. Haddon, S. Glarum, A.F. Hebard and K.B. Lyons, Science 254 (1991) 1625 D.W. PI R.C. Haddon, A.F. Hebard, M.J. Rosseinsky, Murphy, S.J. Duclos, K.B. Lyons, B. Miller, J.M. Rosamilia, R.M. Fleming, A.R. Kortan, S.H. Glarum, A.V. Makhija, A.J. Muller, R.H. Eick, SM. Zahurak, R. Tycko, G. Dabagh and F.A. Thiel, Nature 350 (1991) 320. t101A. Peremans, Y. Caudano, P.A. Thiry and A. Tadjeddine, to be published.