Ayao Okiji

Charge exchange scattering of particles from metal surfaces

Abstract The ionization probability of a particle scattered from a metal surface is investigated with the aid of the time-dependent Newns-Anderson model laying stress on the effects of the intra-atomic Coulomb interaction. Also, the relationship is investigated between the charge transfer and the various quantities such as the velocity of the particle, its initial probability of being charged, the work function and the temperature of the surface. A good account of the experimental data is given for the ionization of sodium atoms scattered from the W(110) surface.

Electron-hole pair mechanism for excitation of intramolecular vibrations in molecule-surface scattering

Abstract The vibrational excitation of NO molecules scattered from Ag surfaces is re-examined on the basis of the model introduced by Newns. The Keldysh Green function is used in order to calculate the excitation probability for intra-molecular vibrations. It is shown that the excitation probability from the vibrational ground state to the first excited state is strongly reduced if the de-excitation process subsequent to the excitation process is taken into account in the molecule-surface scattering.

Vibrational excitation dynamics in molecule-surface scattering

Abstract The excitation of intramolecular vibrations in the scattering of molecules from metal surfaces is investigated through the Keldysh Green function approach. The coupling between the vibrational degree of freedom and electron-hole pairs in the metal is considered for the channel of the energy transfer. It is shown that the vibrational excitation is considerably enhanced by the non-adiabatic effects appearing in the scattering of molecules from metal surfaces.

Thermal desorption of molecules coadsorbed on metal surfaces

Abstract On the basis of the absolute reaction rate theory, the thermal desorption spectra of the coadsorption system, NO + CO/Pt(111), are calculated with the aid of the lattice gas model. It is shown that the spectra for NO molecules strongly depend on the coverage of preadsorbed CO molecules on a Pt(111) surface. A good account of the experimental results is given for the thermal desorption spectra of NO molecules on a Pt(111) surface with pre-adsorbed CO molecules.

Electron correlation effects on charge transfer and energy dissipation in atom or ion scattering from metal surfaces

Abstract The charge transfer and the energy dissipation in the scattering of a particle from a metal surface is investigated within the framework of the time-dependent Newns-Anderson model laying stress on the effects of the intra-atomic Coulomb interaction on the particle. The final energy distributions of the electronic system are calculated by the perturbational expansion with respect to the electron hopping matrix element between the particle and the metal surface. The numerical results up to the fourth order terms are presented for the hydrogen scattering from the tungsten surface.

Kinetic oscillations in the catalytic CO oxidation on Pd(110)

It has been observed that the catalytic CO oxidation on Pd(110) exhibits kinetic oscillations shown by measurements of the reaction rate and of the work function change under certain conditions. In order to describe these phenomena, we write down a set of coupled phenomenological differential equations. The numerical results agree with experimental observations qualitatively.

Final energy distributions of the electronic system in atom or ion scattering from metal surfaces

Abstract The electronic energy transfer associated with the charge transfer in the scattering of particles from metal surfaces is investigated within the framework of the non-interacting time-dependent Newns-Anderson model in the wide-band limit. In this paper, the final energy distributions of the electronic system are shown to be decomposed into parts related to the initial as well as the final charge state of the particles (neutral atoms and positive ions). As a result of numerical calculations, several characteristic structures are found in the final energy distributions. Their origins are explained briefly.

Thermal Desorption of NO Molecules from Pt(111) Surface

On the basis of the absolute reaction rate theory, thermal desorption spectra of NO molecules from Pt(111) surfaces are calculated with the aid of the lattice gas model. It is shown that the occurrence of the conversion of the bridge bonded to the on-top bonded NO molecules may produce the variety of structures in the desorption spectra. The experimental findings are explained satisfactorily.

Vibrational excitation of diatomic molecules interacting with metal surfaces: heating vs. cooling

Abstract Vibrational excitation of diatomic molecules interacting with metal surfaces is investigated theoretically. Vibrational cooling of nitric oxide molecules in trapping-desorption process is explained satisfactorily by the simple calculation based on the kinetic theory of gas molecule-surface scattering and detailed balance relation. Vibrationally assisted sticking and vibrational heating of hydrogen molecules in associative desorption are explained by the coupled channel calculation based on the concept of a reaction path. It is suggested that the occurrence of vibrational heating (vibrational cooling) in desorption may be coordinated generally with the occurrence of vibrationally assisted (vibrationally hindered) sticking.

Charge and spin states of helium atoms approaching metal surfaces

Abstract Charge and spin states of metastable He atoms and positive He ions approaching metal surfaces are investigated with the aid of a simple time-dependent model Hamiltonian. With a simple decoupling approximation, a closed set of Heisenberg equations of motion is derived for the time-dependent quantities describing the charge and spin states of the He atoms. The numerical calculations in the case of simple time-dependence for the He atom-metal interaction, show that the main fraction of incident metastable He atoms changes into positive He ions near the metal surfaces if the work function of the surfaces is high and the Fermi level is located relatively far below the energy level for the 2s orbital of the He atoms. The singlet-triplet conversion of incident metastable He atoms as well as the conversion of the incident ions into metastable He atoms occurs efficiently near the surfaces if the work function gets lower and the Fermi level comes nearer to the energy level for the 2s orbital. However, if the work function gets even low and the Fermi level is located relatively far above the 2s level, the characteristic time for the singlet-triplet conversion becomes considerably long.