Photochemical reaction at metal cluster surfaces as revealed by ultrafast vibrational spectroscopy

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We have investigated how intermediate species with excess energy contribute to various surface chemical reactions. We have mainly focused on the following topics :1. Reaction of carbon dioxide during the oxidation process of silicon surface :Carbon dioxide is known as a very stable molecule in gas phase. However, we have found that it is readily converted to carbonate species when it is adsorbed on silicon surface at low temperature and a surface oxidation procedure is conducted.2. Photodesorption of rare gas atoms on silicon surfacesRare gas atoms (Kr, Xe) weakly adsorb on a Si(100) surface. We found for the first time that these adsorbates are desorbed by the irradiation of photos whose wavelength range from near infrared to uv. It is revealed that the desorption occurs via energy transfer from surface hot phonon which is excited by charge recombination of photocarriers at surface states. We also found that the desorption mechanism changes to that involves charge transfer state between the substrate and the adsorbate by surface modification with oxygen or deuterium.3. Photochemistry of oxygen adsorbed on Ag(110)Photochemistry of oxygen atoms on Ag(110) has been investigated. The chemisorbed oxygen atoms are removed by uv light irradiation. It is found that slight amount of carbon atom in the surface region is necessary for the reaction. Wavelength dependence of photo-yield indicates that the phtogenerated hot-electron plays a crucial role in this reaction.4. Development of the light source for time resolved vibrational spectroscopyTHz electro-magnetic field radiation from femtosecond laser irradiated InAs surface has been investigated. Under an ultra-high vacuum condition, an effect of the surface cleanness and a temperature dependence of the THz radiation were examined. It was found that the THz radiation intensity was enhanced up to two-times by surface cleaning and up to 4 times by temperature lowering to 50 K compared to that at room temperature in air.