Keiko Matsumura

Oxygen isotope exchange between O2 adsorbate and UV-irradiated MgO surfaces

Abstract By using thermal desorption (TD) gas analysis, oxygen-isotope exchange is found between 18 O 2 gas and UV-irradiated MgO powders baked in high vacuum. At a peak temperature ( T p ) 125 K, only 18 O 2 TD takes place. At 200 K, TDs of 18 O 2 and 16 O 18 O with a small amount of 18 O 2 are observed. In the temperature range 550–800 K, a group of three TD peaks composed of 18 O 2 , 16 O 18 O and 16 O 2 is obtained. The exchange ratios, R ( 16 O 18 O) and R ( 16 O 2 ) defined respectively as the ratios of 16 O 18 O and 16 2 O TD intensities to the sum of three TD intensities at the same peak, are: R ( 16 O 18 O) = 0.45 and R ( 16 O 2 ) = 0.05 for T p = 200 K, and R ( 16 O 18 O) = 0.20 and R ( 16 O 2 ) = 0.20 for T p = 690−750 K. A plausible exchange mechanism is discussed in terms of interactions of 18 O 2 adsorbates with low-coordinated 16 O − ions and 16 O 2 2- peroxy defects on MgO surfaces.

Possibility of hydrogen-deuterium exchange at surface irregularities of MgO: an ab initio MO study

Abstract Two models of irregular sites of MgO surfaces as candidates for the H2-D2 exchange reaction are investigated by the use of the ab initio method. After the heterolytic H2-chemisorbed states are analyzed, it is suggested that the exchange may take place at step sites on (001) surfaces. This comes from the steric condition that the (Mg-D Mg) configuration is readily converted to the (Mg D-Mg) one.

Calculation of Exoelectron Glow Curves

Glow curves of thermally stimulated exoelectron emission based on the Maxwell-tail theory and the Auger type recombination model have been calculated assuming that the concentration of free carriers is controlled through a recombination process. Some features of the curves have been investigated based on the emission model which assumes an isolated thin surface layer to be an effective region. The effects of the heating rate and of the initial concentration of traps give indications for clarifications of the mechanism of exoelectron emission.

An ab initio MO study of hydrogen adsorption on LiF surfaces

Abstract The adsorption energies ( E ads ) of a hydrogen molecule and an atom onto (001) and (110) surfaces are calculated by an ab initio MO with the STO-3G basis set. The surfaces are approximated by cluster models, (LiF) 9 for (001) and (LiF) 8 for (110). E ads is -0.18 eV for the H atom on the Li ion of both surfaces. E ads for H 2 is -0.02 eV and its dissociation does not take place on the surfaces. The state density after hydrogen adsorption has a new peak at the bottom of the valence band. The difference of E ads between the ionic surface and the lithium surface for H adsorption is also examined.

Kinetics of Exoelectron After-Emission from MgO

Having found a reproducible after-emission for MgO in a UHV-AES system, we studied its kinetics using two models:(I) a tunneling recombination of trapped electrons and holes and (II) a diffusion-controlled recombination of active species. A numerical analysis showed that only model II can explain the after-emission.

Molecular Parametrized CNDO Method for Hydrogen Adsorption on LiF Surfaces Compared with ab initio Approach

A comparative study on the hydrogen adsorption on LiF surfaces has been made by the use of the molecular parametrized CNDO (MBPCNDO) and ab initio calculations. The bonding parameters of LiF, LiH and FH in the CNDO scheme are modified and the clusters (LiF)9 for (001) surface and (LiF)8 for (110) including the appropriate Madelung potential are used as adsorbents. MBPCNDO reproduces well the shallow non-dissociative potential curves for the adsorption.

A Theoretical Study on the Hydrogen Adsorption at Anion Vacansies on LiF (001) Surface

The possibility of hydrogen chemisorption atop an anion vacancy of a LiF cluster is examined by the use of MBPCNDO MO. Only when an electron is localized at the vacancy, the hydrogen atom is trapped as an H - substitute at the surface site.

Molecular Orbital Studies of Hydrogen Chemisorption at Anion Vacancy Sites of LiF Surfaces

The possibility of hydrogen chemisorption onto anion vacancies on the (001) surface, edge and corner of LiF cluster is investigated with MBP CNDO MO. The clusters with an anion vacancy capture an electron to be localized at the vacancy and a singly occupied level (SOMO) of F-center character appears in the band gap. An approaching hydrogen atom can be bound tightly as sn H - substitute at each charged vacant site. The strong binding is caused by the overlap between H 1s orbital χ s (H) and SOMO and adsorption energy depends on the number of cations nearest to the vacancy.