Scanning tunneling microscopy is utilized to investigate the structural changes of AgO chains on clean and carbidic-carbon containing Ag(110) surfaces under UV photoirradiation and CO exposure. Although AgO chains are arranged with the (2x1)structure on both of the surfaces, AgO chains are bundled to make the (2x1) bands
on the C-containing surface, whereas they make much larger domains on the entire surface of clean Ag(110). The photo-induced elimination of O in AgO chains ccurs only on the C-containing surface. Kinetics of oxygen elimination by CO exposure are very different between the two surfaces. Oxygen coverage decreases steadily on the C-containing surface with CO exposure, whereas the reaction is accelerated in the lower O coverage range where AgO chains with (nx1)(n≥4) configurations show significant structural fluctuation. Comparison
between the two surfaces and simulations based on the Ising model indicate that the acceleration of the reaction originates from the dynamical formation of active O adatoms by fluctuation of AgO chains.
The photo-induced processes of methane adsorbed on Pt(111) and Pd(111) surfaces have been studied by post-irradiation temperature-programmed desorption and angle-resolved time- of-flight measurements. Methane adsorbs weakly on those metals. Although gaseous methane does not show any appreciable absorption cross sections of 6.4 eV, methane weakly adsorbed on those metals is photodissociated to produce methyl and hydrogen by the irradiation of 6.4-eV photons. The incident angle dependence of cross sections of the photochemistry obtained with linearly polarized light indicates that direct electronic excitation of methane adsorbate plays an important role in the photochemistry of methane. We interpreted that the photochemistry is induced via the electronic transition from the ground state localized at methane to the excited state of the methane- substrate atom complex where the first excited Rydberg-like state of methane significantly mixed with substrate empty states. Photofragments of methane, H and CH3, further react with preadsorbed methyl and hydrogen species, respectively. In particular, methane is desorbed via associative recombination between a `hot' hydrogen and a methyl adsorbate. The average translational energy of the desorbed methane is 0.26 eV and 0.53 eV for Pd(111) and Pt(111), respectively. This difference can be explained by the difference in the surface electronic structure between Pd(111) and Pt(111).
Photodissociation dynamics of N2O adsorbed on Si(100) has been compared with that on Pt(111). N2O is adsorbed molecularly on both of the surfaces at lower than 95 K. Upon the irradiation of excimer laser pulses at 193 and 248 nm, adsorbed N2O is dissociated to oxygen and N2. While oxygen remains on the surfaces, N2 desorbs from the surfaces. Translational energy distributions of N2 are measured by time-of-flight (TOF) spectroscopy. The TOF distributions of N2 desorbing from Si(100) as well as Pt(111) show nonthermal multiple velocity components. There are some differences in the N2 desorption characteristics between the two cases. In particular, the translational energy distribution of N2 fragments from Si(100) depends on the desorption angle. Furthermore, the N2 desorption from Si(100) is peaked at approximately 30 degree(s) from the surface normal. These peculiar features observed in N2O photodissociation on Si(100) are discussed in relation to the adsorption structure of N2O.
Photochemistry of N2O adsorbed on a Pt(111) surface at 193 nm has been studied by temperature-programmed desorption (TPD) and time-of-flight (TOF) distribution analysis. Upon the irradiation of excimer laser pulses at 193 nm, adsorbed N2O molecules are either desorbed or dissociated to produce oxygen adatoms and nitrogen molecules in the gas phase. In addition, a small amount of adsorbed NO is found by TPD after the irradiation. The production yields of oxygen adatoms depend on the polarization and angle-of-incidence of laser light. This suggests that electron transfer to adsorbed N2O initiated by the surface excitation is a predominant primary step for the production of oxygen adatoms. Both TOF distributions of N2O and N2 reveal nonthermal multiple velocity components. The mechanisms of the photochemical processes and the origins of the multiple velocity components are discussed.
Conference Committee Involvement (1)
Physical Chemistry of Interfaces and Nanomaterials III
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