Shin-ichiro Tanaka

Orientation of oxygen admolecules on a stepped platinum(133) surface

Abstract The orientation of oxygen admolecules on a Pt(113) stepped surface was studied by near-edge X-ray absorption fine-structure (NEXAFS). Their adsorption and desorption were first characterized by thermal desorption combined with an isotope tracer. Direct desorption peaks from oxygen admolecules occur at 218 K ( α 1 -O 2 ), 182 K ( α 2 -O 2 ), and 162 K ( α 3 -O 2 ) the temperature range from 150 to 250 K. The orientation of oxygen admolecules yielding the above three peaks was examined separately with NEXAFS. All admolecules lie on the surface. α 1 -O 2 is parallel to the step edge, whereas part of α 3 -O 2 declines slightly from the bulk surface plane.

Ion desorption induced by core-electron transitions studied with electron–ion coincidence spectroscopy

Abstract The recent investigations of ion desorption induced by core-electron transitions using electron–ion coincidence (EICO) spectroscopy are described. In a study of CF 3 CH(OH)CH 3 chemisorbed on a Si(100) surface using photoelectron photoion coincidence (PEPICO) spectroscopy, excitation site-specific ion desorption is directly verified, that is, F + desorption is predominant for C 1s photoionization at the ue5f8CF 3 site, while H + desorption is predominantly induced by C 1s photoionization at the ue5f8CH 3 site. A study of condensed H 2 O using Auger electron photoion coincidence (AEPICO) spectroscopy showed that H + desorption is stimulated by O KVV Auger processes leaving two-hole states. The H + desorption probability is found to depend on the bonding character of the orbitals where holes are created and on the effective hole–hole Coulomb repulsion. AEPICO investigations of H + desorption induced by resonant core-electron excitations of condensed H 2 O clearly showed that one-electron–one-core hole or one-electron–two-valence hole states are responsible for the H + desorption mechanism. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a novel and powerful tool for the study of ion desorption induced by core-electron excitations. Furthermore, a comparison of PEPICO and photoelectron spectra showed that the surface core-level shift of condensed H 2 O is 0.7xa0eV. This study shows that PEPICO spectroscopy is also promising as a method to investigate the electronic structure of the specific sites responsible for ion desorption.

Ion desorption from molecules condensed at low temperature: A study with electron-ion coincidence spectroscopy combined with synchrotron radiation (Review)

This article reviews our recent work on photostimulated ion desorption (PSID) from molecules condensed at low temperature. We have used electron–ion coincidence (EICO) spectroscopy combined with synchrotron radiation. The history and present status of the EICO apparatus is described, as well as our recent investigations of condensed H2O, NH3, CH3CN, and CF3CH3. Auger electron photon coincidence (AEPICO) spectra of condensed H2O at the O:1s ionization showed that H+ desorption was stimulated by O:KVV Auger processes leading to two-hole states (normal-Auger stimulated ion desorption (ASID) mechanism). The driving forces for H+ desorption were attributed to the electron missing in the O–H bonding orbitals and the effective hole–hole Coulomb repulsion. The normal ASID mechanism was also demonstrated for condensed NH3. The H+ desorption at the 4a1←O(N):1s resonance of both condensed H2O and condensed NH3 was found to be greatly enhanced. Based on the AEPICO spectra the following four-step mechanism was propose...

Study of ion desorption induced by carbon core excitation for poly-methylmethacrylate thin film using electron–ion coincidence spectroscopy

We have developed a new electron–ion coincidence apparatus combined with synchrotron radiation in order to examine the various ion desorption mechanisms related to the Auger process induced by core excitation. Photon stimulated ion desorption (PSID) of a poly-methylmethacrylate (PMMA) thin film has been investigated by this apparatus. The PSID of PMMA induced by carbon core excitation has been examined using Auger electron yield, total ion yield, resonant Auger electron, and Auger electron–photoion coincidence (AEPICO) spectra. The spectrum of the total ion yield divided by the Auger electron yield shows that the desorption efficiency is largely increased at the resonant excitation of carbon 1s electron in the O–CH3 side chain to σ*(O–CH3) orbital. In AEPICO measurement, H+ and CHn+ (n=1–3) ions are observed at various resonant excitations. The AEPICO signal intensity depends on the Auger electron energy. Particularly, the CH3+ ion desorption in coincidence with Auger electron at 270 eV shows strong enhan...

Site-specific fragmentation caused by core-level photoionization: Effect of chemisorption

We used the energy-selected-photoelectron photoion coincidence (ESPEPICO) method to study site-specific fragmentation caused by C:1s photoionization of 1,1,1-trifluoro-2-propanol-d1 [CF3CD(OH)CH3,u2002TFIP-d1] on a Si(100) surface. High-resolution electron energy loss spectroscopy showed that TFIP-d1 is dissociatively chemisorbed like (CF3)(CH3)CDO–Si(100), and different chemical shifts at the three carbon sites were observed by photoelectron spectroscopy. The site-specific fragmentation evident in the ESPEPICO spectra of the sub-monolayer at room temperature indicates that the TFIP-d1 there has an O–Si bond oriented in the trans position with respect to the C–CF3 bond. Here we discuss the fragmentation processes in light of the results obtained with the ESPEPICO method and the Auger-electron photoion coincidence method.

Electron-Ion Coincidence Spectroscopy as a New Tool for Surface Analysis –an Application to the Ice Surface

Electron-ion coincidence (EICO) spectroscopy [K. Mase, M. Nagasono, S. Tanaka, M. Kamada, T.Urisu and Y. Murata: Rev. Sci. Inst. 68 (1997) 1703] has recently been developed to investigate the process of ion desorption induced by the core level excitation. In the present study, we apply EICO spectroscopy to determine the O1s level of condensed H2O (ice) at 100 K. The kinetic energy of O1s photoelectrons which gives the highest coincidence yield of H+ desorption is shifted by about -0.7 eV compared to the O1s peak observed in the conventional core-level photoelectron spectroscopy. It is ascribed to a core-level shift in the O1s level from which hydrogen ions desorb. The results indicate the advantages and the possibilities of the EICO spectroscopy for surface analysis.

Orientation of oxygen ad-molecules on stepped platinum (112)

Abstract The orientation of oxygen ad-molecules was examined on stepped Pt(112)=(s)[3(111)×(001)] at around 110xa0K by means of near-edge extended X-ray absorption fine-structure (NEXAFS). The π ∗ resonance was always major when the electric vector of the incident X-ray was in a plane perpendicular to the step edge. On the other hand, when the vector was in the plane parallel to it, the σ ∗ resonance was major at the normal incidence and was attenuated with increasing incidence angle of the X-ray whereas the π ∗ resonance was enhanced. It was concluded that oxygen was mostly oriented along the step edge.

An Electron-Ion Coincidence Spectroscopy Study of Ion Desorption Induced by Core-Electron Transitions of Surfaces

Ion desorption induced by core-electron transitions is studied for various surfaces by electron-ion coincidence spectroscopy combined with synchrotron radiation. In a study of F+ desorption from Si(100) terminated by fluorine using photoelectron photoion coincidence spectroscopy, the site-selective F+ desorption is directly verified; that is, F+ desorption is induced by Si 2p photoionizations at the ≡SiF, =SiF2, and –SiF3 sites. An Auger electron photoion coincidence study of a CaF2(111) film epitaxially grown on a Si(111) surface also presents direct evidence of F+ desorption, in this case induced by F 1s surface core exciton. These investigations demonstrate that electron-ion coincidence spectroscopy combined with synchrotron radiation is a powerful tool for studying ion desorption induced by core-electron excitations.

Ion desorption induced by core-level excitation of H2O/Si(100): Evidence of desorption due to the multielectron excitation/decay

This work is an investigation of the desorption by Ou200a1s excitation of ions from Si(100) reacted with water. Photoelectron, photostimulated desorption, and electron-ion coincidence spectroscopy are used to observe the process. When the incident photons have energy levels which are near the 1s threshold of O, they induce Auger decay that is accompanied by shakeup/off excitation and cascade Auger decay, and they are shown to be the main factor responsible for desorption in this case. When the photons have energy levels which are above the shakeup threshold, most of the desorption that occurs is a result of the shakeup excitation that accompanies the core excitation. In both cases, the desorption is induced by the respective multihole final states. The ion desorption yield for the two-hole final states of the normal process of Auger decay is small. The results are discussed, with the help of the Auger electron spectra, mainly in terms of the lifetime of the final state of Auger decay.

Potassium adsorption on the polar ZrC(111) surface: photoemission spectroscopy study

Abstract Core-level photoemission spectroscopy and angle-resolved photoemission spectroscopy with synchrotron radiation have been used to study an adsorption mechanism of K on a ZrC(111) surface. Coverage-dependent measurements of the work function and the Kxa03p core-level photoemission reveal that the adsorbed K atoms are partially ionized in the initial stages of adsorption, and that the metallic overlayer is formed at high coverages through a continuous depolarization of the overlayer with increasing coverage. Valence-band photoemission measurements show that the Zrxa04d -derived surface state on ZrC(III) is hybridized with the 4s states of the adsorbed K atoms to form occupied bonding and unoccupied antibonding states at low coverages. This, together with the fact that adsorbed K is ionized in the initial stages, suggests that the bonding state should be mainly composed of the Zrxa04d orbitals, while the antibonding state has a dominant contribution of the Kxa04s orbitals. As the coverage increases, the antibonding state shifts downwards and becomes an occupied state around the Γ point in the high coverage region where the metallization of the overlayer is brought about. Since the antibonding state can be viewed as a Kxa04s state, the observation of the antibonding state is direct evidence for the metallization of the K overlayer.