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Impulsive excitation of a vibrational mode of Cs on Pt(111)
Chemical Physics Letters 366 (2002) 606–610 www.elsevier.com/locate/cplett
Impulsive excitation of a vibrational mode of Cs on Ptð1 1 1Þ Kazuya Watanabe, Noriaki Takagi, Yoshiyasu Matsumoto
*
Department of Photoscience, School of Advanced Sciences, The Graduate University for Advanced Studies (Sokendai), Hayama, Kanagawa 240-0193, Japan Received 8 August 2002; in final form 26 September 2002
Abstract We demonstrate the real time observation of coherent vibrational motions of Cs atoms adsorbed on Pt(1 1 1). A femtosecond laser pulse excites impulsively a Cs–Pt stretching mode and the subsequent evolution of the nuclear motion of Cs atoms is probed by measuring the oscillatory modulation of the second harmonic intensity of a probe pulse as a function of the pump–probe delay. The frequency and the dephasing time of the mode are found to be 2.3 THz and 1.4 ps, respectively. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction To understand the mechanism of surface photochemistry, it is very important to know how photoinduced electronic excitation couples to adsorbate nuclear motions that ultimately lead to the chemical transformations. In spite of the significant progress in understanding ultrafast processes at metal surfaces [1], direct time-domain observations of primary photochemical events are still scarce [2]. It has been widely demonstrated that a light pulse with a duration sufficiently shorter than a period of a vibrational mode can excite the mode with high degree of temporal and spatial coherence in condensed media [3,4]. Thus, pump–probe measurements with an ultrafast laser enable us to
*
Corresponding author. Fax: +81-468-58-1544. E-mail address: [email protected] (Y. Matsumoto).
monitor the coherent lattice dynamics in the time domain. Recently, Chang et al. [5,6], and our group [7] have shown that this approach is also applicable for surface modes on GaAs clean surfaces. In these works, femtosecond time-resolved second harmonic generation (TRSHG) has been employed, in which the dynamics of surface coherent phonon manifests in the intensity modulation of the second harmonic (SH) generation of probe pulses. Consequently, applying the TRSHG technique to well-defined adsorbate-substrate systems would provide direct information on the nuclear dynamics subsequent to the electronic excitation at the surfaces. In this Letter, we report a real-time observation of the adsorbate-substrate vibration at a Cscovered Pt(1 1 1) surface. We perform the TRSHG measurements with Ti:sapphire laser pulses with 150 fs duration at 1.55 eV photon energy. The laser pulse width is short enough to excite impulsively the Cs–Pt stretching vibrational mode. The
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 6 2 8 - 7
K. Watanabe et al. / Chemical Physics Letters 366 (2002) 606–610
dynamics of the nuclear motion is monitored via the intensity modulation of the SH of delayed probe pulses. This is the first demonstration, to our knowledge, of the time-domain observation of the coherent nuclear motion of adsorbate atoms on a well-defined metal surface.
2. Experimental The TRSHG experimental setup with an ultrahigh vacuum (UHV) chamber and the TRSHG measurement scheme used in this study are basically similar to the one described in the previous paper [7]. Because of poorer conversion efficiency to SH compared with GaAs surfaces, a Ti:sapphire regenerative amplifier system (Spectra Physics, Spitfire, 800 nm, 1 kHz, 0.8 mJ/pulse, 150 fs) was used as a light source instead of the Ti:sapphire oscillator in the previous work. The p-polarized pump and probe beams out of the regenerative amplifier were focused on a sample in the UHV chamber at an incidence angle of 70°. The p-polarized component of the SH of the probe pulse was detected by a photomultiplier. An optical chopper was inserted in the optical path of the pump beam, and the intensity modulation of the probe SH induced by the pump pulse was detected by a lock-in amplifier as a function of the pump–probe delay, t. All the measurements were accomplished at 110 K. A Pt(1 1 1) single crystal was held in the UHV chamber and could be cooled to 110 K by liquid N2 and resistively heated to 1200 K. The surface was cleaned by repeated cycles of sputtering, annealing and subsequent oxygen treatment until no trace of oxygen and carbon contamination was detected by Anger electron spectroscopy (AES). Cs atoms were deposited on the clean Pt surface from a welldegassed alkali metal source (SAES Getters) at the sample temperature less than 300 K. A single layer Cs adatoms were prepared by annealing the sample to 500 K right after a multilayer deposition of Cs. The Cs coverage is estimated to be 0.25 ML (1 ML ¼ 1 1015 atoms=cm2 ) from a work function change estimated by a cut off energy of the secondary electrons in AES measurements [8]. It has been reported that Cs forms a p(2 2) structure at this coverage on Pt(1 1 1) [8].
607
3. Results Fig. 1a and b show the TRSHG traces taken from the clean Pt(1 1 1) and the Cs-covered Pt(1 1 1) surfaces, respectively. The TRSHG trace from the clean surface shows an instantaneous sharp rise right after the excitation. This is followed by a rapid decaying component (t < 1 ps) and by a slowly decaying background that is persistent to the longest delay (t ¼ 6 ps) of the measurement. On the other hand, the SH intensity is enhanced by about 70 times by the adsorption of Cs compared with the clean surface, and the TRSHG trace shows a significantly different feature. As in Fig. 1b, a strong peak appears at t 0, and after a rapid decay of the instantaneous response a clear oscillatory signal was observed. In addition, some background component is superimposed that does not decay within the measured time window. The oscillatory part is well fitted by a single underdamped oscillator with a frequency of 2:30 0:01 THz and a decay time of 1:39 0:05 ps. Fig. 2 shows a Fourier transform (FT) power spectrum of the oscillatory part in Fig. 1b. The spectrum is obtained by Fourier transformation of
Fig. 1. TRSHG traces taken from (a) clean and (b) Cs-covered Pt(1 1 1) surfaces. Laser fluence of the pump pulse absorbed by the substrates was 6 mJ=cm2 . The vertical scales are normalized at each signal peak.
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K. Watanabe et al. / Chemical Physics Letters 366 (2002) 606–610
Fig. 2. The FT power spectrum of the oscillatory component in Fig. 1b. The peak is located at 2.3 THz.
Fig. 3. Initial amplitude of the oscillatory component in the TRSHG traces as a function of the laser fluence of the pump pulse absorbed by the substrate (solid squares). The solid line is depicted for a guide to eyes.
the time-domain data from t ¼ 450 fs to 6 ps after subtracting the background component. In the FT power spectrum, a sharp peak at 2.3 THz (77 cm1 ) is apparent. Fig. 3 shows the initial amplitude of the oscillatory signal as a function of the absorbed fluence of the pump pulse. The absorbed fluence was calculated from Pt bulk optical constants [9]. The amplitude is defined as the difference between the signal intensity at the first minimum (t 400 fs) and at the second peak maximum (t 600 fs). Within the range of the pump power employed, the amplitude of the initial oscillatory feature is proportional to the pump fluence. 4. Discussion The observed SH response of the clean Pt(1 1 1) surface in Fig. 1a may be understood as a mani-
festation of electron and phonon dynamics induced by the pump pulse [10]. The faster decaying component in t < 1 ps may be due to the rapid thermalization of hot electrons, and the slower decaying component after t ¼ 2 ps can be ascribed to the thermalization of the lattice motions excited by the hot electrons [11]. Since we focus on the oscillatory signal of Cs/Pt(1 1 1) in this Letter, we do not discuss the response of the clean surface in further detail. The TRSHG trace of the Cs-covered Pt(1 1 1) surface is dominated by the oscillatory response. The frequency of the oscillatory signal (2.3 THz, 9.5 meV) is close to those of the Cs-substrate stretching vibrational mode of Cs/Cu(1 0 0) (6.8 meV) [12] and Cs/Ru(0 0 0 1) (6.7–9.0 meV) [13]. Thus, it is straightforward to attribute the oscillatory feature to the Cs–Pt stretching motion. The electronic states at the surface are modified by the displacement of Cs atoms along the surface normal. Thus, the coherent stretching motion of Cs atoms modulates the electronic states at the surface, and results in the variation of the nonlinear response of the surface. The second order nonlinear susceptibility, vð2Þ , of the system can be expanded around the equilibrium position, Q0 , in terms of the displacement along the normal coordinate of the Cs–Pt stretching mode, dQ, as in Eq. (1) ovð2Þ ð2Þ ð2Þ dQ þ : ð1Þ v ¼ v Q0 þ oQ Q0
The SH intensity is proportional to the square of jvð2Þ j. When we only take into account up to the first order term with respect to dQ, the SH modulation due to the stretching motion is proportional to dQ. Fischer et al. [14] have studied alkali-induced unoccupied states on various metal surfaces by two-photon photoemission spectroscopy. The energy position of the unoccupied state shifts as a function of the coverage of alkali atoms. At the coverage that gives rise to the work function minimum, the unoccupied state was found at 0.6–1 eV above the Fermi level in all the alkali metal/ substrate systems studied. Since the Cs coverage was adjusted to the work function minimum in this work, we expect an unoccupied state similar to the ones in the other
K. Watanabe et al. / Chemical Physics Letters 366 (2002) 606–610
alkali metal/substrate systems. Thus, the employed photon energy of 1.55 eV is likely to be in resonance with the transition between the Cs induced occupied and unoccupied states, giving rise to strong enhancement of the SH intensity as compared to that at the clean Pt(1 1 1) surface [15,16]. Consequently, the displacement of Cs atoms along the Cs–Pt stretching coordinate shifts the energy levels of the alkali-induced surface states, resulting in the modulation in vð2Þ . If the pulse width of a pump pulse is shorter than the period of a vibrational mode, the vibrational mode can be excited coherently by impulsive stimulated Raman scattering and an oscillating nuclear wavepacket along the vibrational coordinate is formed. In particular, the excitation is efficiently enhanced if the photon energy of the pump pulse is resonant to the transition frequency to an electronic excited state. In molecular systems [17–19] and localized electrons in solids [20,21], the wavepacket dynamics both in the excited and the ground electronic states are expected to contribute to the temporal feature of femtosecond pump– probe signals in the resonant excitation conditions. However, since the lifetime of the electron in the anitbonding state of the alkali adsorbate on metal surfaces is considered to be less than a few tens of femtoseconds [2], the wavepacket dynamics in the excited state would not contribute to the oscillatory feature in Fig. 1b. Here we discuss the pump fluence dependence of the oscillation amplitude in Fig. 3. In the impulsive stimulated Raman process [3,22,23], the equation of motion for a normal coordinate Q can be written as o2 Q oQ þ x20 Q ¼ F ðtÞ; þ 2c ot2 ot
ð2Þ
where c and x0 are vibrational damping constant and frequency, respectively, and F ðtÞ is the force driving the oscillator. When the laser pulse width is sufficiently shorter than the vibrational period, the actual temporal shape of the F ðtÞ depends on the detuning of the excitation photon energy from an electronic transition energy P [22]. In transparent 2 media, F ðtÞ is proportional to kl Rkl Ek El / jEðtÞj , and thus becomes impulsive, where Ek is the electric field, and Rkl is the Raman tensor. In this case, the
609
coherent amplitude is proportional to the intensity 2 of the excitation laser pulse, IðtÞ ¼ jEðtÞj . In Rabt sorbing media, on the other hand, F ðtÞ / 1 R t 2 jEðt0 Þj dt0 ¼ 1 Iðt0 Þ dt0 , which is displacive in character [23]. Thus, in both cases the coherent amplitude is expected to be proportional to the fluence of the excitation laser pulse, when one measures the coherent amplitude variation as a function of the fluence without changing the pulse shape of the excitation laser; this is consistent with the pump fluence dependence shown in Fig. 3. The decay of the coherent nuclear motion is contributed by energy relaxation and pure dephasing. The vibrational relaxation of the C–O stretching mode of CO on metal surfaces is well understood in terms of energy relaxation by electron–hole pair damping associated with dynamical charge transfer [24,25]. In contrast to the intraadsorbate high-frequency mode, relatively little is known for the low-frequency adsorbate–substrate modes. Tully et al. [26] have shown by ab initio calculations that a coupling to substrate phonons can dominate the relaxation dynamics of lowfrequency modes such as a frustrated translational mode of CO adsorbed on copper. In the case of the Cs–Pt stretching mode, the frequency (77 cm1 ) falls in the continuum of Pt substrate phonon modes. Therefore, vibrational relaxation due to energy exchange with the substrate phonon modes would be efficient in addition to electron–hole pair excitation.
5. Summary We performed TRSHG measurements on the Cs-covered Pt(1 1 1) surface under an ultrahigh vacuum condition. In the TRSHG trace, the oscillatory signal was prominent. The FT spectrum of the oscillatory trace showed a single peak at 2.3 THz that is assigned to the Cs–Pt stretching mode. The resonant impulsive Raman scattering process could be responsible for the creation of the vibrating nuclear wavepacket motion in phase along the Cs–Pt stretching coordinate on the ground state. The dephasing time of the coherent vibration was estimated to be 1.4 ps. Further experimental investigation including temperature
610
K. Watanabe et al. / Chemical Physics Letters 366 (2002) 606–610
dependence and model calculations incorporating transient lattice heating are under way for a full understanding the relaxation dynamics.
Acknowledgements This work was supported by the Grants-in-Aid for Scientific Research by Japan Society for the Promotion of Science (14340176, 14703009, 13874068).
References [1] H.L. Dai, W. Ho (Eds.), Laser Spectroscopy and Photochemistry on Metal Surfaces, World Scientific, Singapore, 1995. [2] H. Petek, H. Nagano, M.J. Weida, S. Ogawa, J. Phys. Chem. B 105 (2001) 6767. [3] Y. Yan, K.A. Nelson, J. Chem. Phys. 87 (1987) 6240. [4] T. Dekorsy, G.C. Cho, H. Kurz, in: M. Cardona, G. G€ untherodt (Eds.), Light Scattering in Solids VIII, Springer, Berlin, 2000, p. 169. [5] Y.M. Chang, L. Xu, H.W.K. Tom, Phys. Rev. Lett. 78 (1997) 4649. [6] Y.M. Chang, L. Xu, H.W.K. Tom, Chem. Phys. 251 (2000) 283. [7] K. Watanabe, D.T. Dimitrov, N. Takagi, Y. Matsumoto, Phys. Rev. B 65 (2002) 235328. [8] J. Cousty, C.A. Papageorgopoulos, R. Riwan, Surf. Sci. 223 (1989) 479.
[9] E.D. Palik (Ed.), Handbook of Optical Constants of Solids, Academic Press, San Diego, 1985, p. 341. [10] J.A. Misewich, T.F. Heintz, P. Weigand, A. Kalamarides, in: H.L. Dai, W. Ho (Eds.), Laser Spectroscopy and Photochemistry on Metal Surfaces, World Scientific, Singapore, 1995, p. 764. [11] J. Hohlfeld, D. Grosenick, U. Conrad, E. Matthias, Appl. Phys. A 60 (1995) 137. [12] G. Witte, J.P. Toennies, Phys. Rev. B 62 (2000) R7771. [13] P. He, K. Jacobi, Phys. Rev. B 53 (1996) 3658. [14] N. Fischer, S. Schuppler, Th. Fauster, W. Steinmann, Surf. Sci. 314 (1994) 89. . Lindgren, L. Wallden, Phys. Rev. B 45 (1992) [15] S.A 6345. [16] A. Liebsch, G. Hincelin, T. L opez-Rıos, Phys. Rev. B 41 (1990) 10463. [17] W.T. Pollard, S.L. Dexheimer, Q. Wang, L.A. Peteanu, C.V. Shank, R.A. Mathies, J. Phys. Chem. 96 (1992) 6147. [18] Y.J. Yan, S. Mukamel, Phys. Rev. A 41 (1990) 6485. [19] J. Chesnoy, A. Mokhtari, Phys. Rev. A 38 (1988) 3566. [20] Y. Kayanuma, S. Tanaka, Phys. Rev. B 62 (2000) 12838. [21] S.L. Dexheimer, A.D. Van Pelt, J.A. Brozik, B.I. Swanson, J. Phys. Chem. A 104 (2000) 4308. [22] M.D. Levenson, S.S. Kano, Introduction to Nonlinear Laser Spectroscopy, Academic Press, San Diego, 1987. [23] T.E. Stevens, J. Kuhl, R. Merlin, Phys. Rev. B 65 (2002) 144304. [24] H. Ueba, Prog. Surf. Sci. 55 (1997) 115. [25] D.C. Langreth, M. Persson, in: H.L. Dai, W. Ho (Eds.), Laser Spectroscopy and Photochemistry on Metal Surfaces, World Scientific, Singapore, 1995, p. 498. [26] J.C. Tully, M. Gomez, M. Head-Gordon, J. Vac. Sci. Technol. A 11 (1993) 1914.
Impulsive excitation of a vibrational mode of Cs on Ptð1 1 1Þ Kazuya Watanabe, Noriaki Takagi, Yoshiyasu Matsumoto
*
Department of Photoscience, School of Advanced Sciences, The Graduate University for Advanced Studies (Sokendai), Hayama, Kanagawa 240-0193, Japan Received 8 August 2002; in final form 26 September 2002
Abstract We demonstrate the real time observation of coherent vibrational motions of Cs atoms adsorbed on Pt(1 1 1). A femtosecond laser pulse excites impulsively a Cs–Pt stretching mode and the subsequent evolution of the nuclear motion of Cs atoms is probed by measuring the oscillatory modulation of the second harmonic intensity of a probe pulse as a function of the pump–probe delay. The frequency and the dephasing time of the mode are found to be 2.3 THz and 1.4 ps, respectively. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction To understand the mechanism of surface photochemistry, it is very important to know how photoinduced electronic excitation couples to adsorbate nuclear motions that ultimately lead to the chemical transformations. In spite of the significant progress in understanding ultrafast processes at metal surfaces [1], direct time-domain observations of primary photochemical events are still scarce [2]. It has been widely demonstrated that a light pulse with a duration sufficiently shorter than a period of a vibrational mode can excite the mode with high degree of temporal and spatial coherence in condensed media [3,4]. Thus, pump–probe measurements with an ultrafast laser enable us to
*
Corresponding author. Fax: +81-468-58-1544. E-mail address: [email protected] (Y. Matsumoto).
monitor the coherent lattice dynamics in the time domain. Recently, Chang et al. [5,6], and our group [7] have shown that this approach is also applicable for surface modes on GaAs clean surfaces. In these works, femtosecond time-resolved second harmonic generation (TRSHG) has been employed, in which the dynamics of surface coherent phonon manifests in the intensity modulation of the second harmonic (SH) generation of probe pulses. Consequently, applying the TRSHG technique to well-defined adsorbate-substrate systems would provide direct information on the nuclear dynamics subsequent to the electronic excitation at the surfaces. In this Letter, we report a real-time observation of the adsorbate-substrate vibration at a Cscovered Pt(1 1 1) surface. We perform the TRSHG measurements with Ti:sapphire laser pulses with 150 fs duration at 1.55 eV photon energy. The laser pulse width is short enough to excite impulsively the Cs–Pt stretching vibrational mode. The
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 6 2 8 - 7
K. Watanabe et al. / Chemical Physics Letters 366 (2002) 606–610
dynamics of the nuclear motion is monitored via the intensity modulation of the SH of delayed probe pulses. This is the first demonstration, to our knowledge, of the time-domain observation of the coherent nuclear motion of adsorbate atoms on a well-defined metal surface.
2. Experimental The TRSHG experimental setup with an ultrahigh vacuum (UHV) chamber and the TRSHG measurement scheme used in this study are basically similar to the one described in the previous paper [7]. Because of poorer conversion efficiency to SH compared with GaAs surfaces, a Ti:sapphire regenerative amplifier system (Spectra Physics, Spitfire, 800 nm, 1 kHz, 0.8 mJ/pulse, 150 fs) was used as a light source instead of the Ti:sapphire oscillator in the previous work. The p-polarized pump and probe beams out of the regenerative amplifier were focused on a sample in the UHV chamber at an incidence angle of 70°. The p-polarized component of the SH of the probe pulse was detected by a photomultiplier. An optical chopper was inserted in the optical path of the pump beam, and the intensity modulation of the probe SH induced by the pump pulse was detected by a lock-in amplifier as a function of the pump–probe delay, t. All the measurements were accomplished at 110 K. A Pt(1 1 1) single crystal was held in the UHV chamber and could be cooled to 110 K by liquid N2 and resistively heated to 1200 K. The surface was cleaned by repeated cycles of sputtering, annealing and subsequent oxygen treatment until no trace of oxygen and carbon contamination was detected by Anger electron spectroscopy (AES). Cs atoms were deposited on the clean Pt surface from a welldegassed alkali metal source (SAES Getters) at the sample temperature less than 300 K. A single layer Cs adatoms were prepared by annealing the sample to 500 K right after a multilayer deposition of Cs. The Cs coverage is estimated to be 0.25 ML (1 ML ¼ 1 1015 atoms=cm2 ) from a work function change estimated by a cut off energy of the secondary electrons in AES measurements [8]. It has been reported that Cs forms a p(2 2) structure at this coverage on Pt(1 1 1) [8].
607
3. Results Fig. 1a and b show the TRSHG traces taken from the clean Pt(1 1 1) and the Cs-covered Pt(1 1 1) surfaces, respectively. The TRSHG trace from the clean surface shows an instantaneous sharp rise right after the excitation. This is followed by a rapid decaying component (t < 1 ps) and by a slowly decaying background that is persistent to the longest delay (t ¼ 6 ps) of the measurement. On the other hand, the SH intensity is enhanced by about 70 times by the adsorption of Cs compared with the clean surface, and the TRSHG trace shows a significantly different feature. As in Fig. 1b, a strong peak appears at t 0, and after a rapid decay of the instantaneous response a clear oscillatory signal was observed. In addition, some background component is superimposed that does not decay within the measured time window. The oscillatory part is well fitted by a single underdamped oscillator with a frequency of 2:30 0:01 THz and a decay time of 1:39 0:05 ps. Fig. 2 shows a Fourier transform (FT) power spectrum of the oscillatory part in Fig. 1b. The spectrum is obtained by Fourier transformation of
Fig. 1. TRSHG traces taken from (a) clean and (b) Cs-covered Pt(1 1 1) surfaces. Laser fluence of the pump pulse absorbed by the substrates was 6 mJ=cm2 . The vertical scales are normalized at each signal peak.
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K. Watanabe et al. / Chemical Physics Letters 366 (2002) 606–610
Fig. 2. The FT power spectrum of the oscillatory component in Fig. 1b. The peak is located at 2.3 THz.
Fig. 3. Initial amplitude of the oscillatory component in the TRSHG traces as a function of the laser fluence of the pump pulse absorbed by the substrate (solid squares). The solid line is depicted for a guide to eyes.
the time-domain data from t ¼ 450 fs to 6 ps after subtracting the background component. In the FT power spectrum, a sharp peak at 2.3 THz (77 cm1 ) is apparent. Fig. 3 shows the initial amplitude of the oscillatory signal as a function of the absorbed fluence of the pump pulse. The absorbed fluence was calculated from Pt bulk optical constants [9]. The amplitude is defined as the difference between the signal intensity at the first minimum (t 400 fs) and at the second peak maximum (t 600 fs). Within the range of the pump power employed, the amplitude of the initial oscillatory feature is proportional to the pump fluence. 4. Discussion The observed SH response of the clean Pt(1 1 1) surface in Fig. 1a may be understood as a mani-
festation of electron and phonon dynamics induced by the pump pulse [10]. The faster decaying component in t < 1 ps may be due to the rapid thermalization of hot electrons, and the slower decaying component after t ¼ 2 ps can be ascribed to the thermalization of the lattice motions excited by the hot electrons [11]. Since we focus on the oscillatory signal of Cs/Pt(1 1 1) in this Letter, we do not discuss the response of the clean surface in further detail. The TRSHG trace of the Cs-covered Pt(1 1 1) surface is dominated by the oscillatory response. The frequency of the oscillatory signal (2.3 THz, 9.5 meV) is close to those of the Cs-substrate stretching vibrational mode of Cs/Cu(1 0 0) (6.8 meV) [12] and Cs/Ru(0 0 0 1) (6.7–9.0 meV) [13]. Thus, it is straightforward to attribute the oscillatory feature to the Cs–Pt stretching motion. The electronic states at the surface are modified by the displacement of Cs atoms along the surface normal. Thus, the coherent stretching motion of Cs atoms modulates the electronic states at the surface, and results in the variation of the nonlinear response of the surface. The second order nonlinear susceptibility, vð2Þ , of the system can be expanded around the equilibrium position, Q0 , in terms of the displacement along the normal coordinate of the Cs–Pt stretching mode, dQ, as in Eq. (1) ovð2Þ ð2Þ ð2Þ dQ þ : ð1Þ v ¼ v Q0 þ oQ Q0
The SH intensity is proportional to the square of jvð2Þ j. When we only take into account up to the first order term with respect to dQ, the SH modulation due to the stretching motion is proportional to dQ. Fischer et al. [14] have studied alkali-induced unoccupied states on various metal surfaces by two-photon photoemission spectroscopy. The energy position of the unoccupied state shifts as a function of the coverage of alkali atoms. At the coverage that gives rise to the work function minimum, the unoccupied state was found at 0.6–1 eV above the Fermi level in all the alkali metal/ substrate systems studied. Since the Cs coverage was adjusted to the work function minimum in this work, we expect an unoccupied state similar to the ones in the other
K. Watanabe et al. / Chemical Physics Letters 366 (2002) 606–610
alkali metal/substrate systems. Thus, the employed photon energy of 1.55 eV is likely to be in resonance with the transition between the Cs induced occupied and unoccupied states, giving rise to strong enhancement of the SH intensity as compared to that at the clean Pt(1 1 1) surface [15,16]. Consequently, the displacement of Cs atoms along the Cs–Pt stretching coordinate shifts the energy levels of the alkali-induced surface states, resulting in the modulation in vð2Þ . If the pulse width of a pump pulse is shorter than the period of a vibrational mode, the vibrational mode can be excited coherently by impulsive stimulated Raman scattering and an oscillating nuclear wavepacket along the vibrational coordinate is formed. In particular, the excitation is efficiently enhanced if the photon energy of the pump pulse is resonant to the transition frequency to an electronic excited state. In molecular systems [17–19] and localized electrons in solids [20,21], the wavepacket dynamics both in the excited and the ground electronic states are expected to contribute to the temporal feature of femtosecond pump– probe signals in the resonant excitation conditions. However, since the lifetime of the electron in the anitbonding state of the alkali adsorbate on metal surfaces is considered to be less than a few tens of femtoseconds [2], the wavepacket dynamics in the excited state would not contribute to the oscillatory feature in Fig. 1b. Here we discuss the pump fluence dependence of the oscillation amplitude in Fig. 3. In the impulsive stimulated Raman process [3,22,23], the equation of motion for a normal coordinate Q can be written as o2 Q oQ þ x20 Q ¼ F ðtÞ; þ 2c ot2 ot
ð2Þ
where c and x0 are vibrational damping constant and frequency, respectively, and F ðtÞ is the force driving the oscillator. When the laser pulse width is sufficiently shorter than the vibrational period, the actual temporal shape of the F ðtÞ depends on the detuning of the excitation photon energy from an electronic transition energy P [22]. In transparent 2 media, F ðtÞ is proportional to kl Rkl Ek El / jEðtÞj , and thus becomes impulsive, where Ek is the electric field, and Rkl is the Raman tensor. In this case, the
609
coherent amplitude is proportional to the intensity 2 of the excitation laser pulse, IðtÞ ¼ jEðtÞj . In Rabt sorbing media, on the other hand, F ðtÞ / 1 R t 2 jEðt0 Þj dt0 ¼ 1 Iðt0 Þ dt0 , which is displacive in character [23]. Thus, in both cases the coherent amplitude is expected to be proportional to the fluence of the excitation laser pulse, when one measures the coherent amplitude variation as a function of the fluence without changing the pulse shape of the excitation laser; this is consistent with the pump fluence dependence shown in Fig. 3. The decay of the coherent nuclear motion is contributed by energy relaxation and pure dephasing. The vibrational relaxation of the C–O stretching mode of CO on metal surfaces is well understood in terms of energy relaxation by electron–hole pair damping associated with dynamical charge transfer [24,25]. In contrast to the intraadsorbate high-frequency mode, relatively little is known for the low-frequency adsorbate–substrate modes. Tully et al. [26] have shown by ab initio calculations that a coupling to substrate phonons can dominate the relaxation dynamics of lowfrequency modes such as a frustrated translational mode of CO adsorbed on copper. In the case of the Cs–Pt stretching mode, the frequency (77 cm1 ) falls in the continuum of Pt substrate phonon modes. Therefore, vibrational relaxation due to energy exchange with the substrate phonon modes would be efficient in addition to electron–hole pair excitation.
5. Summary We performed TRSHG measurements on the Cs-covered Pt(1 1 1) surface under an ultrahigh vacuum condition. In the TRSHG trace, the oscillatory signal was prominent. The FT spectrum of the oscillatory trace showed a single peak at 2.3 THz that is assigned to the Cs–Pt stretching mode. The resonant impulsive Raman scattering process could be responsible for the creation of the vibrating nuclear wavepacket motion in phase along the Cs–Pt stretching coordinate on the ground state. The dephasing time of the coherent vibration was estimated to be 1.4 ps. Further experimental investigation including temperature
610
K. Watanabe et al. / Chemical Physics Letters 366 (2002) 606–610
dependence and model calculations incorporating transient lattice heating are under way for a full understanding the relaxation dynamics.
Acknowledgements This work was supported by the Grants-in-Aid for Scientific Research by Japan Society for the Promotion of Science (14340176, 14703009, 13874068).
References [1] H.L. Dai, W. Ho (Eds.), Laser Spectroscopy and Photochemistry on Metal Surfaces, World Scientific, Singapore, 1995. [2] H. Petek, H. Nagano, M.J. Weida, S. Ogawa, J. Phys. Chem. B 105 (2001) 6767. [3] Y. Yan, K.A. Nelson, J. Chem. Phys. 87 (1987) 6240. [4] T. Dekorsy, G.C. Cho, H. Kurz, in: M. Cardona, G. G€ untherodt (Eds.), Light Scattering in Solids VIII, Springer, Berlin, 2000, p. 169. [5] Y.M. Chang, L. Xu, H.W.K. Tom, Phys. Rev. Lett. 78 (1997) 4649. [6] Y.M. Chang, L. Xu, H.W.K. Tom, Chem. Phys. 251 (2000) 283. [7] K. Watanabe, D.T. Dimitrov, N. Takagi, Y. Matsumoto, Phys. Rev. B 65 (2002) 235328. [8] J. Cousty, C.A. Papageorgopoulos, R. Riwan, Surf. Sci. 223 (1989) 479.
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