Yoshitada Murata

Alkali-metal adsorption on metals

Abstract The interplay between microscopic electronic structure and ordering phenomena is surveyed for alkali-metal monolayers on metal surfaces. First, the electronic structure of alkali-metal atoms adsorbed on surfaces is discussed on the basis of experimental results obtained with a variety of surface science techniques as well as theoretical works including those using a jellium model, the Anderson-Newns formalism, and a local-density-functional theory. The focus of interest here will be brought into drastic changes in electronic properties observed when coverage increases from zero to one monolayer. Then we turn to a variety of structural phases and transitions in alkali-metal monolayers. The microscopic mechanism and driving force of the ordering is discussed in terms of the electronic structure of adatoms.

Unit for the Precise Measurement of Electron Diffraction Intensities by Gas Molecules

An apparatus for the precise measurement of electron intensity diffracted from gas molecules has been constructed. Care has been taken to define camera distances precisely by installing a table, to which the nozzle and the photographic plate can be fixed firmly, and to eliminate extraneous scattering by designing a suitable electron-optical system and by inserting a brass ring with a folded wall covered with soot into the sector race. A critical examination by the use of carbon dioxide has shown that the structure can be determined with the accuracy of one thousandth of an angstrom with the present apparatus.

UV-laser-induced desorption of NO from Pt(111)

Abstract Ultraviolet-laser-induced desorption of NO molecules from a Pt(111) surface is investigated at 80 K by state-selective detection. We find two distinctive desorption channels are operative on an as-adsorbed surface and a surface annealed at 220 K. Rotational energy distributions are Boltzmann on as-adsorbed Pt(111) and non-Boltzmann on annealed Pt(111) with inverted population of spin-orbit states. Rotational, vibrational, and translational temperatures of NO from an as-adsorbed surface are increased with the incident photon energy. The effect of the adsorption state on desorption and the desorption mechanism are discussed.

Photo-stimulated desorption of NO from a Pt(001) surface

Abstract Desorption of NO molecules chemisorbed on a Pt(001) surface at 80 K induced by ultraviolet laser radiation is investigated by a resonance-enhanced multiphoton ionization technique. The desorption yield of neutral NO is proportional to pump laser fluence, which is indicative of a single-photon process. The translational, rotational, and vibrational temperatures of desorbed molecules are approximately 650, 300, and 1200 K, respectively. The substantial difference of these values from sample temperature implies that desorption is not thermally driven, but induced by electronic excitation. Polarization and wavelength dependences of a pump laser are examined to characterize the electronic excitation relevant to desorption. No significant difference is observed for either s- and p-polarization and at λ = 193, 248, and 352 nm. The incidence angle dependence of the desorption yield is similar to that of the substrate ultraviolet light absorption. These results suggest that the initial step of photodesorption is substrate valence electron excitation.

Interaction of reactive ions with Pt(100). I. Neutralization and surface trapping

Scattering of (10–400 eV) O+, O+2 , C+, CO+, and CO+2 ions from a Pt(100) surface has been studied. Below 100 eV, the peak position of the angular distribution for survival ions was shifted parallel to the surface, and the lobe width was very narrow in comparison with noble‐gas ions. This suggests that surface trapping due to chemical interaction takes place at very low energies. The importance of collisional neutralization at high energies is demonstrated by the result that the yields of reactive ions decrease steeply with increasing incident energy. The scattering ion yields of O+ and O+2 ions, especially, were very low, being less than 1/100 and 1/10 of those of nitrogen ions, respectively. These results show that a chemical interaction effect is significant for the scattering of oxygen ions.

Development of electron-ion coincidence spectroscopy for the study of surface dynamics combined with synchrotron radiation

Energy-selected electron-ion coincidence spectroscopy for the study of surface dynamics combined with synchrotron radiation (SR) was developed. The equipment consists of an electron gun, a cylindrical mirror analyzer (CMA), and a time-of-flight (TOF) ion mass spectrometer. A sample surface was excited by SR, and energy of the emitted electron was analyzed by the CMA. The TOF spectrum of the desorbed ions was measured taking the energy-analyzed electron signal as the starting trigger. The ions coincidently desorbed with the electron gave a characteristic peak in the TOF spectrum. The apparatus was evaluated on the basis of photoelectron–photoion coincidence (PEPICO) and Auger electron–photoion coincidence (AEPICO) measurements of H2O condensed on gold foil. The results demonstrate that PEPICO and AEPICO combined with SR are powerful methods for investigating the ion desorption induced by core-level excitations.

Interaction of reactive ions with Pt(100). II. Dissociative scattering of molecular ions near the threshold energy region

Dissociative scattering of N+2, CO+, and CO+2 ions from Pt(100) has been studied at low energies. For dissociated N+ emergence, the threshold of incident kinetic energy was found to be 40 eV. The threshold of dissociated CO+ emergence in the CO+2 incidence was clearly observed at 25 eV. The threshold of dissociated C+ emergence in the CO+ incidence was observed at 70 eV. Correlation between the dissociation energy of a free molecule and the threshold of incident kinetic energy is clearly discernible. The angular distributions show that the dissociation product appears at larger scattering angles than that for the parent molecular ions scattered nondissociatively. These experimental results are consistent with the model that dissociation is due to translational‐rovibrational energy transfer above the dissociation limit at the impulse collision with the surface.

Adsorption and desorption of NO and CO on a Pt(111)-Ge surface alloy

Abstract Adsorption of NO and CO on Pt(111) alloyed with a few per cent of Ge is investigated by reflection—absorption infrared spectroscopy and thermal desorption spectroscopy. Both molecules exclusively occupy the on-top site in contrast to bridge and on-top adsorption on clean Pt(111). The adsorption energy of NO is dramatically reduced compared with that on clean Pt(111). Photodesorption of CO observed on the clean Pt(111) is noticeably suppressed on the Pt(111)Ge surface alloy, while NO desorption is induced by photon irradiation. The rotational and translational temperatures of photodesorbed No are similar to those on clean Pt(111). The change in chemical properties of Pt(111) for molecular adsorption is discussed in terms of d-band filling of the substrate.

Alloyed interface formation in the AuSi(111)2 × 1 system studied by photoemission spectroscopy

Abstract Room temperature deposition of gold (Au) on the Si(111)2×1 cleaved surface has been studied by photoelectron spectroscopy using synchrotron radiation. Binding energies of Si(2p) and Au(4f) and the splitting of the Au(5d) signal are changed with increasing coverage in a different manner below and above one monolayer coverage and reach to saturation values at 20–100 monolayers. The Au(4f) binding energy and the Au(5d) splitting at saturation are clearly different from those of pure Au metal. The origin of alloying in the AuSi(111)2×1 system is discussed from these results. A new model, “chemical bonding model” is proposed to explain the initial stage of the metallic overlayer formation in the AuSi(111) system.

Photodesorption of CO and CO+ from Pt(111): Mechanism and site specificity

Ultraviolet photodesorption of CO and CO+ from Pt(111) at 80 K is investigated by (2+1) resonance‐enhanced multiphoton ionization and reflection absorption infrared spectroscopy. Desorption of CO and CO+ occurs at the on‐top site as single‐photon and three‐photon processes, respectively. The rotational, vibrational, and translational temperatures of desorbed CO are approximately 130, 3700, and 2000 K, which are considerably higher than the sample temperature. The threshold energy of neutral CO desorption lies between 2.3 and 3.5 eV suggesting that an unoccupied 2π state is responsible for the desorption.