Time-resolved SFG study of the vibrational excitation of adsorbed CO on Ni(111) and NiO(111) surfaces under the irradiation of UV and visible photons
Introduction
There are many reports on the state-resolved characterization of the molecules that desorb from metal surfaces by the irradiation of laser pulses, and the photo-excitation of vibrational and rotational degrees of freedom in the desorbed molecules is a major issue in the interpretation of electronic states established between the surface and the adsorbate [1], [2], [3]. Recently developed sum-frequency generation (SFG) spectroscopy and time-resolved infrared reflection absorption spectroscopy (IRAS) opened the way to characterize the transient states of adsorbates on solid surfaces such as metals and semiconductors [4], [5], [6], [7], [8]. The relaxation of vibrational energy of adsorbates, thermal heating of substrate, and the excitation of low-frequency modes such as frustrated translation via the hot-electron mediated processes have now been reported [1], [2], [3], [4], [5], [6], [7], [8]. No report has been published, however, on observations of the excitation of the internal vibration mode of non-desorbing molecules.
Reported here is the finding that photogenerated hot electrons probably led to the excitation of the stretching vibrational mode of molecules that stayed adsorbed. Pump-probe experiments using infrared-visible SFG spectroscopy for the probe were carried out to study the CO adsorbed on Ni(111) and NiO(111)/Ni(111) surfaces under the irradiation of picosecond UV (266 nm) and visible (532 nm) laser pulses. The irradiation of UV pulses of the CO/Ni(111) and CO/NiO(111)/Ni(111) systems and the irradiation of visible pulses of the CO/NiO(111)/Ni(111) system resulted in the appearance of a transient hot band transition (v=2←1) of a CO stretching band, whereas the irradiation of visible pulses of the CO/Ni(111) system resulted in a feature that was ascribed to a thermal effect.
Section snippets
Experimental
The experiments were performed in an ultra-high-vacuum (UHV) chamber. Tunable picosecond infrared (IR) pulses for SFG were obtained by difference frequency generation between near-IR and 1064 nm pulses in a AgGaS2 crystal [9]. Visible pulses with a wavelength of 532 nm were obtained by second harmonic generation (SHG) and divided into two beams; one for SFG and the other for the UV- and visible-pumping. The IR and visible pulses for SFG irradiated the sample surface from the same direction with
Results and discussion
The open circles in Fig. 1a and b denote the SFG spectra observed for CO adsorbed on to the Ni(111) and NiO(111), respectively, with and without irradiation using 266 nm pulses. The spectra were observed by using both p-polarized visible and IR pulses. The resonance peaks at 2076 and 2144 cm−1 in the upper traces of Fig. 1a and b, respectively, observed without UV irradiation, were assigned to the C–O stretching modes of linearly bonded CO and the CO adsorbed on fully oxidized sites [11]. The
Summary
The time-resolved SFG spectroscopy has been applied to the CO/Ni(111) and CO/NiO(111)/Ni(111) systems under the irradiation of picosecond UV and visible laser pulses. The UV irradiation resulted in a highly efficient excitation of molecular vibration of the adsorbed CO, and the transient responses of both the fundamental and hot band signals suggested that the excitation arose from an electronically driven process; the involvement of hot electrons in the excitation of the CO internal stretching
References (31)
- et al.
Surf. Sci. Rep.
(1991) - et al.
Surf. Sci. Rep.
(1995) - et al.
Surf. Sci.
(1997) - et al.
Chem. Phys. Lett.
(1995) - et al.
J. Mol. Spectrosc.
(1966) - et al.
Surf. Sci.
(1979) - et al.
Surf. Sci.
(1988) - et al.
Surf. Sci.
(1990) - et al.
Surf. Sci.
(1997)
J. Phys. Chem.
Phys. Rev. Lett.
J. Chem. Phys.
J. Chem. Phys.
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Ultrafast surface vibrational dynamics
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2008, Developments in Surface Contamination and Cleaning - Fundamentals and Applied AspectsElucidating the Fundamental Interactions of Very Small Particles. Ultrafast Science.
2007, Developments in Surface Contamination and Cleaning: Fundamentals and Applied AspectsSum Frequency Generation and Polarization-Modulation Infrared Reflection Absorption Spectroscopy of Functioning Model Catalysts from Ultrahigh Vacuum to Ambient Pressure
2007, Advances in CatalysisCitation Excerpt :For example, after photoexcitation of the surface with an intense near-IR femtosecond-laser pulse (“pump”), a time-delayed weak SFG (IR+VIS) “probe” is employed to monitor the changes in the vibrational properties of the adsorbate–substrate complex by taking snapshots of the transient vibrational spectrum (48). Systems investigated include CO desorption (147,148,157,417,421,422) and formic acid decomposition on NiO(1 1 1) (419,420). The SFG spectra discussed in the preceding sections were obtained in ppp-polarization combination (i.e., by detecting a p (parallel)-polarized SFG signal produced by a p-polarized visible and a p-polarized IR beam).
Structural change of CO adsorbed on Pt(111) by laser heating: Time-resolved sum-frequency generation study
2003, Chemical Physics LettersCitation Excerpt :This interpretation is supported by the temporal feature of SFG intensity, which is similar to the temporal feature of surface temperature described in the last section. Although the line shape is similar to that observed in the conventional heating method (Fig. 1), the spectral change induced by the pump pulses at the delay of 20 ps is smaller than that expected from the increase in surface temperature (estimated to be 250 K at 20 ps [13–15]). The reason for this discrepancy is that the equilibrium coverage of CO in the conventional method differs from that in the transient method, because the temperature rise induced by the pump pulse ends in a few nanoseconds and the equilibrium coverage under the irradiation of pump pulses must be larger than in the case of conventional heating.