Fujiyoshi Satoru, Ishibashi Taka-Aki, Onishi Hiroshi
Department of Chemistry, Faculty of Science, Kobe University, Nada, Kobe, 657-8501 Japan.
J Phys Chem B. 2005 May 12;109(18):8557-61. doi: 10.1021/jp051203i.
Low-frequency surface vibrations were observed on a rutile TiO(2)(110) surface covered with trimethyl acetate (TMA) by using fourth-order Raman spectroscopy. The TMA-covered surface interfaced to air was irradiated with 18-fs light at a wavelength of 630 nm. A pump pulse excited vibrational coherence of Raman-active modes and a probe pulse interacts with the coherently excited surface to generate second harmonic light (315 nm), the intensity of which oscillated as a function of the pump-probe delay. Four bands were recognized at 180, 357, 444, and 826 cm(-1) in the Fourier transformation spectrum of the oscillation and assigned to bulk phonons modified by the presence of the surface boundary condition. The Raman transition for the pump was nonresonant to the band gap excitation of TiO(2), as evidenced by the oscillation phase relative to the pump irradiation and by the oscillation amplitude as a function of the pump power. The observable range of this surface-selective spectroscopy is extended to wide-band gap materials on which one-photon resonance enhancement of the Raman-pump efficiency cannot be expected.
通过使用四阶拉曼光谱，在覆盖有乙酸三甲酯（TMA）的金红石型TiO₂(110)表面观察到了低频表面振动。将与空气接触的TMA覆盖表面用波长为630 nm的18飞秒光进行照射。泵浦脉冲激发拉曼活性模式的振动相干性，探测脉冲与相干激发的表面相互作用以产生二次谐波光（315 nm），其强度随泵浦 - 探测延迟而振荡。在振荡的傅里叶变换光谱中，在180、357、444和826 cm⁻¹处识别出四个谱带，并将其归因于受表面边界条件影响而改变的体声子。泵浦的拉曼跃迁与TiO₂的带隙激发非共振，这由相对于泵浦照射的振荡相位以及作为泵浦功率函数的振荡幅度所证明。这种表面选择性光谱的可观测范围扩展到了不能期望拉曼泵浦效率有单光子共振增强的宽带隙材料。
