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This paper investigates the process of the capture by the surface defects of the photoexcited electron on Si(001) at 100 K by means of the time-resolved two-photon photoelectron spectroscopy. A rapid decrease in sub-ps of the electron population at the conduction band and the simultaneous increase and subsequent decrease of the electron population at the defect states are observed. They are interpreted as the capture of the hot electrons in the conduction band by the surface defect states. The electron–temperature-dependence of the surface recombination velocity, which determines the capture rate of the bulk electron by the surface defects, is estimated assuming the multiphonon emission capture mechanism.

The electron populations of the intrinsic surface unoccupied state ( D down ) and of the minimum of bulk conduction band (CBM) after fs-pulse excitation on the Si(0 0 1)-(2 × 1) surface were measured at two different excitation wavelengths ( λ ex ) by means of time-resolved two-photon photoelectron spectroscopy. The decay of populations of both states is faster for short-wavelength excitation at 377 nm than that at 754 nm, showing a prominent λ ex -dependent decay in sub-ns temporal domain. In this temporal domain, the populations of both D down and CBM are proportional with each other. The results show that the decay of D down population is governed by the de-population of bulk electrons near surface via a short lifetime at the D down state.

We used electron–ion coincidence spectroscopy to investigate the photostimulated ion desorption from clean and water-chemisorbed TiO2(1 1 0) and ZnO ( 1 0 1 ¯ 0 ) . Desorption of O+ from TiO2(1 1 0) and ZnO ( 1 0 1 ¯ 0 ) surfaces and that of H+ from water-chemisorbed TiO2(1 1 0) and ZnO ( 1 0 1 ¯ 0 ) surfaces were observed after O1s-excitaion. These are not due to the two-hole final state caused by the ordinal single-electron excitation and its normal Auger decay but mainly due to the n-hole (n > 2) final state which was provided by the multiple excitation/decay of the O1s electron. After metal-core excitation, meanwhile, ion (O+ and H+) desorption is observed from the clean and water-chemisorbed TiO2(1 1 0) surfaces, but not observed from clean or water-chemisorbed ZnO ( 1 0 1 ¯ 0 ) surfaces. The ion desorption following the metal-core excitation cannot be fully explained by a similar mechanism to the KF model where the inter-atomic Auger decay provides the change of the charging state of the oxygen. We therefore propose a new mechanism in which the charge transfer from the O2p orbital to the Ti3d orbital is induced by the Ti3p-core hole potential and is responsible for the creation and desorption of the O+ ions.

Electronic properties of individual CdSe colloidal nanodots have been investigated by conductive-tip atomic force microscopy (AFM). Submonolayer-thick films of the colloidal nanodots were fabricated on a self-assembled monolayer of alkanethiol molecules formed on Au(1 1 1) surfaces for single dot measurements. First, we simultaneously imaged the topography and conductivity of isolated single dots by AFM operating in contact mode with a conductive tip under appropriate bias voltages. In the current image, it is found that the dot regions have higher electric resistances due to tunneling resistance through the CdSe dots. We found a 10-nm scale electric inhomogeneity around the dots, which may correspond to the previously reported etch-pits of Au(1 1 1) surfaces formed during the deposition of the alkanethiol molecules. Then, current–voltage characteristics were measured with the conductive tip positioned on the single dots; large changes in the conductivity which suggest resonant tunneling through the quantized energy level in the dot were observed even at room temperature.

The adsorbed states of 1,1,1-trifluoro-2-propanol (TFIP) molecule on Si(1 0 0) c(4 × 2) have been investigated by high resolution electron energy loss spectroscopy. TFIP is dissociatively adsorbed on Si(1 0 0) as CF 3 CH(O–Si)CH 3 and H–Si at 100 K. These species are stable up to 300 K. At 400 K, CF bonds start to be dissociated. At 800 K, all adsorbed species are decomposed into SiH, SiF, SiF 2 and residual carbon species. At 1050 K, the silicon carbide is formed.

The electron dynamics of the Si(0 0 1)-(2 × 1) surface has been studied by means of two-photon photoelectron spectroscopy. The photoelectron peak from the unoccupied surface state (Ddown) of down-Si atoms of the dimers is clearly resolved spectroscopically and temporally. Time-resolved study shows that the Ddown state is populated within 10 ps of excitation, and de-populated primarily with a time constant of 210 ps at 300 K. The rate of populating Ddown states is excitation-intensity dependent, and has been ascribed to electron–electron scattering processes.

The orientation of oxygen ad-molecules was examined on stepped Pt(112)=(s)[3(111)×(001)] at around 110 K 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.

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 CF 3 site, while H + desorption is predominantly induced by C 1s photoionization at the CH 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 0.7 eV. 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.

Photo-stimulated desorption from the TiO 2 (110) surface was investigated by using electron–ion coincidence spectroscopy and synchrotron radiation. The desorption of O + ions was observed to be coincident with the emission of photoelectrons from the O 1s, Ti 2s, Ti 2p, Ti 3s and Ti 3p levels, and their satellite peaks due to the shake-up excitation. However, no ion desorption was observed in coincidence with the O 2s or valence photoelectrons. The O + peak intensities in the coincidence spectra are analyzed, and the result is unexpected from a simple expansion of the Knotek–Feibelman model. The shake-up excitation of O 1s levels yields O + desorption more efficiently than does the O 1s single-electron excitation.

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 K 3p 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 Zr 4d -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 Zr 4d orbitals, while the antibonding state has a dominant contribution of the K 4s 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 K 4s state, the observation of the antibonding state is direct evidence for the metallization of the K overlayer.

Core-level photoemission spectroscopy and angle-resolved photoemission spectroscopy with synchrotron radiation have been used to study the adsorption process of O 2 on the K-covered ZrC(111) surface at room temperature. Oxygen adsorbs dissociatively and the O atoms penetrate beneath the K overlayer to form a chemisorption bond with the substrate Zr atoms in the initial stages of adsorption. After about 70% of the Zr atoms on the substrate surface are bonded to the O atoms, the oxidation of the K overlayer proceeds subsequently. In addition, it is found that the K-promoted oxidation of the substrate is brought about when the K overlayer is oxidized to some extent.

Oxygen admolecules on stepped Pt(133)=(s)[3(111)×(111)] were characterized by means of thermal desorption spectroscopy and near-edge X-ray absorption fine structure. Three admolecule desorption peaks were found at 150 K (α3-O2), 175 K (α2-O2) and 220 K (α1-O2). The admolecules yielding α1-O2 lie along surface troughs, whereas those yielding α2-O2 and α3-O2 lie on declining (111) terraces and are largely rotated from the surface trough direction. The rotation angles were estimated to be 40±10° for α2-O2 and 60±10° for α3-O2. These values were determined by comparing experimental X-ray incident angle dependence of π* and σ* resonances with simple simulated calculations. These oriented admolecules can be selectively prepared on this surface.

The variation in interfacial reaction between Au islands and Si substrates and the fabrication of a Au/Si heterojunction on a nanometer scale are described. Morphologies and elemental distribution were drastically changed around the interface between Au islands and the Si substrate on changing the surface conditions of the substrate. In particular, the Au/Si bilayer structure was formed on the clean Si surface by thermal annealing instead of the well-known interdiffusion of the two elements. This structure was fabricated through liquid phase epitaxy (LPE), in which migrating Si atoms on the surface play an important role. These results suggest that the kinetics of the interfacial reaction are variable and controllable even within the same material system. On the basis of this phenomenon, mushroom-shaped bilayer dots of Au/Si and Ag/Si were fabricated on a nanometer scale by employing metal particles as the transport medium for Si growth.

Ion desorption induced by a resonant excitation of O 1s of condensed amorphous H2O has been studied by total ion and total electron yield spectroscopy, nonderivative Auger electron spectroscopy (AES) and Auger electron photo-ion coincidence (AEPICO) spectroscopy. The spectrum of total ion yield divided by total electron yield exhibits a characteristic threshold peak at hν = 533.4 eV, which is assigned to the 4a1 ← O 1s resonant transition. The AES at the 4a1 ← O 1s resonance is interpreted as being composed of the spectator-AES of the surface H2O, and the normal-AES of the bulk H2O, where the 4a1 electron is delocalized before Auger transitions. H+ is found to be the only ion species in AEPICO spectra measured at the 4a1 ← O 1s resonance and at the O 1s ionization (hν = 560 eV). The electron kinetic energy dependence of the AEPICO yield (AEPICO yield spectrum) at the 4a1 ← O 1s resonance is found to be greatly different from that at the O 1s ionization. The peak positions of the AEPICO yield spectrum at the 4a1 ← O 1s resonance are found to correspond to those of the spectator-AES of the surface H2O, which is extracted from the AES at the 4a1 ← O 1s resonance. Furthermore, the AEPICO yield is greatly enhanced at the 4a1 ← O 1s resonance as compared with that at the O 1s ionization. On the basis of these results, a spectator-Auger-stimulated ion desorption mechanism and/or ultra-fast ion desorption mechanism are concluded to be responsible for the H+ desorption at the 4a1 ← O 1s resonance. The enhancement of the H+ yield is ascribed to the OH anti-bonding character of the 4a1 orbital.

Auger-electron-photoion coincidence (AEPICO) studies of photon-stimulated ion desorption (PSID) for condensed acetonitrile induced by carbon core excitation have been performed to elucidate the desorption mechanism related to the Auger process. We have detected only the H+ ion in AEPICO spectra. The total ion yield spectrum divided by the total electron yield shows that the desorption efficiency is largely increased at the resonant excitation to CH∗. We have also measured the Auger electron spectrum and the AEPICO yield spectrum at the CH∗ excitation. The AEPICO yield spectrum shows enhancement at 245–250 eV electron energy. This seems to be related to the spectator resonant Auger stimulated ion desorption. That is, H+ desorption is enhanced due to a two-hole-one-electron state at which the electron is in an anti-bonding CH∗ orbital and the two holes can be in a 1π bonding orbital localized on the CH3 group. We have also measured similar spectra for other resonant excitation ( π∗ CN , σ∗ CC , σ∗ CN ). The results are discussed in connection with the bonding/anti-bonding character and localized character of the excited state.

Auger electron-photoion coincidence (AEPICO) spectroscopy, which has been recently developed and proved to be a very powerful technique for investigating the dynamics of desorption induced by the core-level excitation, is applied to the investigation of Auger-stimulated ion desorption from the chemisorbed-water-Si(100) surface induced by O 1s excitation. It is shown that the fast relaxation of the excited state with a core hole and an excited electron takes place before the core hole decay, and that the desorption yield is enhanced by the shake-up (and/or shake-off) excitation. The relative cross-section for Auger-stimulated ion desorption is estimated, and is shown to increase as holes are created at deeper levels of the valence bands as the final state of the Auger decay. A comparison is also made with condensed H2O.

Photoelectron-photoion coincidence (PEPICO) spectra of condensed Si(CH3)4 were measured with C 1s and the Si 2p photoelectrons using synchrotron radiation. H+ was found to be the only species desorbed in both cases. Furthermore, the H+ yield in coincidence with the C 1s photoemission was found to be three times as large as that with the Si 2p photoemission. These results indicate that the Auger-stimulated ion desorption mechanism is responsible for the H+ desorption, that the neutralization and recapture probabilities of the polyatomic ions are much larger than those of H+, and that the C 1s photoemission leads more efficiently to H+ desorption than the Si 2p photoemission.

Scanning tunneling microscopy (STM) has been used to study the initial growth stages of calcium fluoride (CaF 2 ) on Si(001)-(2 × 1) surfaces in situ. At high coverage, the deposited CaF 2 molecules nucleate into small hemispherical islands at room temperature, which are of the order of 1 to 3 nm in diameter and cover the Si(001) surface homogeneously. Annealing this surface at 520°C produces a drastic change in the surface morphology. Due to the increased mobility of the deposited CaF 2 molecules, they rearrange into long and narrow islands typically of a few nm width and whose length is often more than four times the width. The long, narrow islands grow three-dimensionally, and their long axes are aligned along [110] or [11¯0] of the substrate Si lattice. Increasing the substrate temperature to about 610°C induces a shape transition of the islands from long and narrow to two-dimensional layers. The STM image shows that the layers consist of rows aligned with respect to the underlying Si(001) lattice. Considering the coverage of the deposited CaF 2 molecules, the row-like layers are not reconstructed Si(001) surfaces induced by CaF 2 adsorption, but are layers that are formed initially in the CaF 2 heteroepitaxial growth on a Si(001) surface. STM images clearly show that the terraces are completely covered with row-like layers at 670°C. The formation of the row-like layers can be attributed to the chemical reaction between CaF 2 and Si. The perpendicular spacing between adjacent rows varies from 0.8 to 2.4 nm. Thus, it is suggested that the inhomogeneous row-like layers will prevent growth of CaF 2 films with high crystalline quality on Si(001).

Photostimulated ion desorption induced by core-level excitations was studied for condensed NH3 and ND3 by using Auger electron-photoion coincidence (AEPICO) spectroscopy with synchrotron radiation. Series of AEPICO spectra for condensed NH3 and ND3 were measured as a function of the electron kinetic energy around the N KVV Auger electron energy range. AEPICO yields of both H+ and D+ were enhanced at the electron kinetic energies corresponding to the N KVV Auger transitions, which provide clear evidence of an Auger-stimulated ion desorption mechanism. We analyzed the fine structure of the AEPICO yield spectra on the basis of the assignment of Auger final states, and suggested that Auger final states containing two holes in the same valence orbital efficiently lead to ion desorption. The AEPICO yield of D+ was found to be one-third to one-half the yield of H+. We concluded that the isotope effect is derived from the reneutralization and recapture of D+ on the surface because of the slow desorption velocity of D+ compared with that of H+.

The initial stages of CaF 2 growth on Si(111) at a high substrate temperature have been studied, in situ, with an ultra high vacuum scanning tunneling microscope (UHV-STM) from submonolayer range up to a monolayer. The STM images directly demonstrate that the initial growth mode changes from a three-dimensional island formation to a wetting heteroepitaxial layer growth with increasing substrate temperature. At a substrate temperature of about 470°C, islands of characteristic shape, with steps arranged in the [110] direction, is observed on the Si(111) surface. This island formation initially occurs both at steps and on the flats of Si terraces. At a higher temperature of around 680°C, a submonolayer grows epitaxially from Si step edges, exhibiting a well- ordered row-like structure along the [110] direction. The empty state image of this row-like structure has a 3 X 1 periodicity at 680°C. Further deposition of CaF 2 results in covering the Si surface uniformly with the heteroepitaxial monolayers.

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.

The adsorption state of K on NbC(100) has been studied with core-level photoemission spectroscopy (CLPES) and thermal desorption spectroscopy (TDS). The adsorption mechanism of K on the NbC(100) surface is found to be very unique as compared with those of many alkali-metal surface systems. The TDS study shows that the desorption energy of K increases with the increase of K coverage ( θ K ). The work function measurements show that the polarization of the NbC(100) surface after K adsorption is small as compared with those for many alkali-metal surface systems. The lineshape analysis of the K 3p photoemission spectra shows that the valence electronic density of states for adsorbed K is nearly independent on the coverage. Considering these results, it is proposed that the K-adsorbed layer is grown via the island formation mechanism.

Thermally cleaned InP surfaces in an arsenic flux have been observed using an ultra-high-vacuum scanning tunneling microscope (UHV-STM). The STM image of the reconstructed In-stabilized surface showed 1.6 nm periodic lines in the [110] direction. The width of these lines was about 0.8 nm, each line comprising two rows when the sample was heated at 510°C for about one minute in an arsenic flux. On the other hand, the lines were about 0.4 nm wide and comprised a single row when the sample was heated at a higher temperature for a longer time. This result suggests that the In-stabilized surface comprises one or two In-In dimers and missing dimers per 4 × 2 cell, and that the dimer density depends on the cleaning conditions. For samples annealed at 480°C in a vacuum of 10 −5 Pa for about ten minutes after cleaning, the surface image showed 4 × 2 In-stabilized domains and 2 × 4 As-stabilized domains. This structure is thought to be formed by a desorption of surface atoms during annealing.

Interaction of Cs and oxygen on the ZrC(1 1 1) surface at room temperature has been investigated by core-level photoemission spectroscopy and X-ray absorption spectroscopy utilizing synchrotron radiation. Oxygen adsorbs dissociatively on the Cs-covered surface, and the dissociated O atoms interact directly with the substrate Zr atoms beneath the Cs overlayer. The O atoms are also bound to the Cs atoms so that the Cs cations are formed. However, there is no spectroscopic evidence for the formation of Cs oxides such as Cs2O, Cs2O2 and CsO2. Although oxidation of the ZrC bulk does not proceed, a part of the Zr atoms in the substrate surface becomes highly charged cations, which cannot be obtained when the clean ZrC(1 1 1) surface is exposed to O2 at room temperature.