Elsevier

Surface Science

Volumes 427–428, 1 June 1999, Pages 331-336
Surface Science

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

https://doi.org/10.1016/S0039-6028(99)00298-8Get rights and content

Abstract

The effects of irradiation by picosecond ultraviolet (266 nm) and visible (532 nm) pulses of CO-covered Ni(111) and NiO(111)/Ni(111) surfaces were investigated by infrared-visible sum-frequency generation (SFG) spectroscopy. As a result of the irradiation of UV pulses, the SFG signal by the v=1←0 bands of the CO stretching mode weakened and transient peaks tentatively assigned to the v=2←1 hot band transition appeared with a downshift by 18 and 14 cm−1 from v=1←0 bands on the metal and oxide-layered surfaces, respectively. The time profile of the weakening and that of the transient signal were coincident with the same rise and fall times of less than ∼10 ps. The irradiation of visible pulses also induced a weakening of the v=1←0 bands and transient peaks, but the decay time on the metal surface was longer than 100 ps. The temporal and spectral features imply that the persistent v=1←0 band and the transient band originated from the same CO molecule. Short decay times observed on irradiating UV pulses on to the Ni(111) and NiO(111) surfaces and that of the visible pulses on to the NiO(111) surface suggest that the photo-generated hot electrons induced the observed spectral changes. The long decay time observed on irradiating visible pulses on to the Ni(111) surface suggests the occurrence of a thermally driven process. Participation of the negative ion state was considered to be one of the possible mechanisms for the rapidly decaying features.

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

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