Search

The KEK to Kamioka long-baseline neutrino experiment (K2K) has begun its investigation of neutrino oscillations suggested by atmospheric neutrino observations. Twenty-eight neutrino events have been detected in coincidence with the expected arrival time of the beam in the 22.5 kt fiducial volume of Super--Kamiokande, the far detector at 250 km distance. The expectation is 37.8+3.5-3.8, derived using measurements of neutrino interactions in a near detector and extrapolation using a beam simulation validated by a measurement of pion kinematics after production and focusing. The background is of order 10^-3 events., Comment: 6 pages, 3 embedded figures, LaTeX with RevTeX style, submitted to PRL. This version is As Submitted

The adsorbed state of ethylene on clean Pd(110) and Pd(110)(1 × 2)-Cs surfaces at 90 K and its thermal decomposition have been studied by the use of high-resolution electron energy loss spectroscopy (EELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). It is shown that ethylene on the Pd(110)(1 × 2)-Cs surface is more weakly bonded to the surface as compared with ethylene on the clean Pd(110) surface and that the dehydrogenation and CC bond scission processes are promoted by the pre-adsorbed Cs atoms.

The adsorbed state of ethylene on the Pd(110)(1{times}2)-Cs surface and its thermal decomposition have been studied by the use of high-resolution electron energy loss spectroscopy and thermal desorption spectroscopy. For a small exposure (0.1 Langmuir) at 90 K, ethylene is {pi}-bonded to the Pd(110)(1{times}2)-Cs surface. For a large exposure (2 Langmuirs), a small amount of physisorbed ethylene exists in addition to {pi}-bonded ethylene. The saturation coverage corresponds to 0.23 C{sub 2}H{sub 4} molecules per Pd atom of the unreconstructed Pd(110) surface. The physisorbed ethylene is desorbed by heating to 110 K. A part of the {pi}-bonded ethylene is desorbed intact at 200 K, and additionally at 270 K by a recombinative process. By heating to 300 K, the residual C{sub 2}H{sub 4} admolecules are decomposed, and methylidyne (CH) species are formed. Only carbon adatoms remain on the Pd(110)(1{times}2)-Cs surface by heating to 500 K. These results are compared with those for the Pd(110) clean surface, and the effects of Cs adatoms on the surface reactions are discussed.

The adsorption and thermal evolution of acetylene and ethylene on Pd(110) have been studied using high-resolution electron energy loss Spectroscopy, multiplexed thermal desorption spectroscopy and low-energy electron diffraction. For a small exposure at 90 K, acetylene is chemisorbed molecularly in a single state, and is located in the μ 2 -site with its CC bond axis inclined to the surface plane; one of the CH groups is hydrogen-bonded to the surface. For a large exposure, acetylene is adsorbed in several states. Ethylene is π-bonded to the Pd(110) surface at 90 K. The dehydrogenation processes of acetylene and ethylene on Pd(110) by the heat treatment are discussed.

We describe the construction and performance of a scintillating fiber detector used in the near detector for the K2K (KEK to Kamioka, KEK E362) long baseline neutrino oscillation experiment. The detector uses 3.7 m long and 0.692 mm diameter scintillating fiber coupled to image-intensifier tubes (IIT), and a CCD camera readout system. Fiber sheet production and detector construction began in 1997, and the detector was commissioned in March, 1999. Results from the first K2K runs confirm good initial performance : position resolution is estimated to be about 0.8 mm, and track finding efficiency is $98 \pm 2$ % for long tracks (i.e., those which intersect more than 5 fiber planes). The hit efficiency was estimated to be $92 \pm 2$ % using cosmic-ray muons, after noise reduction at the offline stage. The possibility of using the detector for particle identification is also discussed., Comment: In total 21 pages with 18 figures. THis is the proceedings for "The 7th International Conference on Instrumentation for Colliding Beam Physics", held on Hamamatsu, Japan, Nov. 15-19, 1999

The adsorbed states of K atoms on the Si(100)(2 × 1) surface and the interactions of the K-covered surface with several gases, i.e. H, O2, CO and HCOOH, have been studied mainly by the use of high-resolution electron energy loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). Experimental evidence is given for the existence of two chemisorbed states of K atoms on the Si(100)(2 × 1) surface. Two adsorbed states of O atoms were found on the Si(100)(2 × 1)-K surface. HCOO adspecies of an ioniccharacter exist on the Si(100)(2 × 1)-K surface.

Electron energy-loss spectra have been measured on Ni(100) surfaces, clean and following oxygen and carbon monoxide adsorption, at primary energies of 40–300 eV. The observed peaks at 9.1, 14 and 19 eV in the clean-surface spectrum are ascribed to the bulk plasmon of the 4 s electrons, the surface plasmon, and the bulk plasmon of the coupled 3 d + 4 s electron, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Assignments of the loss structures on the gas-covered surfaces have been attempted.

The electronic state of oxygen adsorbed on a Cr(110) surface at 80 K has been studied by angle-resolved photoemission spectroscopy with synchrotron radiation. Dissociative chemisorption occurs up to \ensuremath{\sim}2 L, and then oxidation starts at \ensuremath{\sim}4 L, leading to the formation of a disordered ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$ layer. An emission at \ensuremath{\sim}7.5 eV below the Fermi energy, being attributable to a chemisorbed ${\mathrm{O}}_{2}$ molecule, was observed at above \ensuremath{\sim}20 L. Dispersion effects on the ${\mathrm{O}}_{2}$p--derived levels were found for the Cr(110)p(4\ifmmode\times\else\texttimes\fi{}2)-O and faceted Cr(110)-O surfaces produced by annealing.

Large shifts were observed in the positions of the $3p$ energy-loss features for clear Cr(110) and Fe(110) surfaces, which were in parallel with shifts for the ${M}_{2,3}\mathrm{VV}$ Auger "satellite," as the primary electron energy was varied. These observations are discussed in terms of the post-collision interaction and direct recombination process following the $3p\ensuremath{\rightarrow}3d$ transition.

The adsorbed states of hydrogen on Pd(1 1 0) surfaces at 100–300 K have been studied using high-resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). The observed losses are at 96, 118 and ∼ 165 meV for the (2 × 1)-H surface; 96 and ∼ 125 (shoulder) meV for the (1 × 2)-H surface formed at 100 K; ∼ 90 and 118 meV for the streaky (1 × 2)-H surface formed at 300 K. Tentative assignments of the losses are made. The adsorbed states of hydrogen are discussed correlating the EELS-LEED results with the thermal desorption and molecular beam diffraction studies which have previously been reported.

Chemisorption of methanol on the Si(111)(7×7) surface has been studied at ∼ 300 K using high-resolution electron energy loss spectroscopy. Methanol reacts with the Si(111) surface to form the stable surface species SiOCH3 and SiH. The methoxy species (CH3O) is bonded to the Si surface with a covalent bond formed between its oxygen end and the dangling bond of the Si(111) surface atom. A structural model for methanol chemisorbed on the Si(111) is proposed.

The initial stages of the interaction of oxygen with an Fe(100) surface have been investigated at 300 K mainly by electron-energy-loss spectroscopy (EELS) with in situ combined low-energy-electron-diffraction, Auger-electron and secondary-electron-emission spectroscopy, and workfunction\char22{}change measurements ($\ensuremath{\Delta}\ensuremath{\varphi}$). From all the results, three different stages of the oxygen interaction are distinguished: (i) dissociative chemisorption stage up to \ensuremath{\sim}3L, (ii) incorporation of O adatoms into the selvedge between 3 and 20 L, and (iii) oxidation above 20 L, leading to the formation of $\ensuremath{\gamma}$-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$. [1 langmuir (L)\ensuremath{\equiv}${10}^{\ensuremath{-}6}$ Torr sec.] Of special interest is the change in surface electronic properties from that characteristic of the metal to that of the oxide observed by EELS. An energy-loss peak characteristic of the chemisorbed oxygen was observed at 6 eV below 3 L, being ascribed to the transition between the bonding and antibonding orbitals, and the EELS spectrum in the oxide phase was characterized by the peaks due to the ${\mathrm{O}}^{2\ensuremath{-}} 2p\ensuremath{\rightarrow}{\mathrm{Fe}}^{3+} 3d$ charge-transfer transitions. The $\ensuremath{\gamma}$-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$-FeO phase transition at \ensuremath{\sim}570\ifmmode^\circ\else\textdegree\fi{}C was also confirmed by monitoring an energy-loss peak due to the $d\ensuremath{\rightarrow}d$ transition, which is spin allowed for an ${\mathrm{Fe}}^{2+}$ ion in FeO. It is shown that the Fe $3{d}_{yz,zx}$ electrons play a major role in the chemisorption bond (O adatoms located in the centered hollow or bridge site), and for the incorporation process the Fe $3{d}_{{z}^{2}}$ electrons are also involved in bonding by the symmetry breaking. A direct-recombination process following the $3p\ensuremath{\rightarrow}3d$ transition is proposed for the high-energy-side peak of the Fe ${\mathrm{M}}_{2,3}\mathrm{VV}$ Auger spectrum, which\char22{}though not yet unambiguously explained\char22{}has been tentatively assigned to an Auger transition from a doubly ionized Fe core level.

High-resolution vibrational electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED) have been used to study the interactions of hydrogen with the Ni(110) surface in the temperature range from 100 to 300 K. H2 is adsorbed dissociatively on the Ni(110) surface. At 100 K, the (2 × 1)-H and (1 × 2)-H structures are formed; the streaky (1 × 2)-H structure is formed by heating the H-covered surface to above ~ 220 K or by the exposure of the Ni(110) surface to H2 at 300 K. For the (2 × 1)-H surface which is characterized by the vibrational losses at 79 and 130 meV, H atoms are adsorbed in the low-symmetry short-bridge sites of the bulk-like Ni(110) surface. Adsorbed sites of H atoms for the low-coverage phases (found by molecular beam diffraction and video-LEED) are similar to those for the (2 × 1)-H structure. For the (1 × 2)-H surface with losses at 76, 117 and 150 meV, H atoms are considered to be adsorbed in the low-symmetry short-bridge sites and high-symmetry short-bridge sites of the second layer Ni atoms. The streaky (1 × 2)-H surface with losses at ~ 80, 117 and 140 meV consists of the undistorted and distorted patches whose local structures are similar to those of the (2 × 1)-H and (1 × 2)-H surfaces, respectively. The (2 × 1) → (1 × 2) phase transition is considered to be the first-order transition.

High-resolution electron energy loss spectra of hydrogen-covered Ni(110) surfaces both at 100 and 300 K are presented. The adsorbed sites of hydrogen atoms are discussed.

The adsorption of acetylene on a Si(111) (7 × 7) surface has been studied at 300 K using high-resolution electron energy loss spectroscopy. Acetylene is predominantly adsorbed non-dissociatively, and is rehybridized having a state of hybridization between sp2 and sp3. An adsorbed structure is proposed in which acetylene is di-σ bonded to two adjacent surface Si atoms saturating the dangling bonds.

High-resolution vibrational electron energy loss spectroscopy has been used to study the adsorbed state of NO on the Si(100)(2×1) surface. At 300 K, NO is adsorbed dissociatively on the Si(100) surface in the disordered structure, and the Si 3 N and SiOSi species are formed. By heating at 1200–1300 K, the O adatoms are removed from the surface and the silicon nitride is formed; the (2×1) structure is recovered, which is interpreted to indicate that the nitride is formed mainly in the subsurface region.

The interactions of a Pd(110) surface with (N2+N) have been studied by means of high-resolution vibrational electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). At 300 K, double- and triple-bonded N2 admolecules and N adatoms (located most probably in the long-bridge, short-bridge and on-top sites of the unreconstructed Pd(110) surface) are formed; some N atoms penetrate into the “bulk”. The double- and triple-bonded N2 admolecules are identified by the N-N stretching energies of 199 and 265 meV, respectively; the N adatoms in the long-bridge, short-bridge and on-top sites are identified by the Pd-N stretching energies of 55, 90 and ∼130 meV, respectively. By heating at ∼500 K, the triple-bonded N2 and most of the double-bonded N2 admolecules are desorbed. The Pd clean surface is recovered by heating at ∼900 K. The (2×3)-N structure is formed by a large (N2+N) exposure followed by heating at ∼500–800 K or by a large N2 exposure at ∼700 K. It is proposed that N atoms located in the long-bridge sites of the “distorted” Pd(110) surface form the (2×3)-N structure. Molecular nitrogen is not adsorbed on the (2×3)-N surface at 300 K.

Adsorbed states of hydrogen on the Ni(110) surface have been studied using high-resolution vibrational electron energy loss spectroscopy and low-energy electron diffraction. Structural models for the (2 × 1)−H, (1 × 2)−H and streaky (1 × 2)−H surfaces are discussed.

The electronic energy-band properties of epitaxially grown YBa/sub 2/CuO/sub 3/O/sub 7-//sub x/(001) single-crystal thin films have been investigated by angle-resolved ultraviolet photoemission spectroscopy (ARUPS) with synchrotron radiation as the light source. The (001) surfaces exhibit sharp (1 x 1) low-energy electron-diffraction patterns. Our ARUPS data show a clear Fermi edge and dispersive nature of the valence bands, suggesting the validity of the band concept for this oxide. We determined energy-band dispersion along the GAMMA-X-M-GAMMA directions in the bulk Brillouin zone. The experimental results are compared with the existing ground-state band calculations.

The adsorption of formic acid (HCOOH) on the Si(100)c(4×2), Si(100)(2×1), Si(100)9° vicinal and Si(111)(7×7) surfaces has been studied by means of high resolution electron energy loss spectroscopy. Formic acid is partially dissociated to form the unidentate formate species (HCOO) and H adatoms on these surfaces at both 90 and 300 K. The formate species is bonded to the Si surface with a covalent bond formed between one of its oxygen atoms and the dangling bond of the surface Si atom. A condensed multilayer of molecular formic acid is formed after the Si surface is saturated with the formate species at 90–150 K. The thermal decomposition mechanism has been studied in detail. The decomposition temperatures of the formate species on Si(100) and Si(111) are ∼600 and 550 K, respectively. It appears that the surface steps do not play an important role in the Si–HCOOH interaction. As the formate‐covered surface is exposed to atomic hydrogen, methylendiolate (H2COO) is formed on the Si surface.

Electron energy-loss spectra of clean NiO (100), CoO (100), and MnO surfaces have been obtained using primary electrons of 100-900, 30-800, and 300-700 eV, respectively. The observed electronic transitions are interpreted on the basis of the primary electron energy dependence and comparison with optical data. The peaks below approximately 10 eV are due to the O2-2p to M2+3d and 4s(M=Ni, Co, and Mn) transitions. It is shown that the onset of the interband transitions is caused by the 2p to 3d charge-transfer excitons. The peaks between 10 and 50 eV are attributed to the one-electron excitations of O2-2p to M2+4p and M2+3d to 4p, and to the single- and multiple-bulk plasmon excitations associated with O2-2p, M2+3d, and coupled 2p and 3d electrons. The 3p-core excitation spectra are observed beyond 50-60 eV.

The adsorbed states of H 2 S on the Si(1 1 1)(7 x 7) surface have been studied using high-resolution electron energy loss spectroscopy. At 300 K, H 2 S is partially dissociated, and the SiH and SiSH species are formed. The SH species are adsorbed in a bent configuration. Results of heating experiments are presented, and the formation of SiSSi species is discussed.

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the band structure of the c(2×2) and (3×1) oxygen overlayers on Fe(110). The symmetries of the O-2p-derived states at the center of the surface Brillouin zone ( Γ ) were identified using polarized light. At Γ the p x p y - and p z -derived levels are at about 5.5 and 7.0 eV below the Fermi level, respectively, for both ordered overlayers. The p-states of the c(2×2)-O structure show very little dispersion (⪅0.1 eV) with k  . On the other hand, the c(3×1)-O overlayer exhibits considerable dispersion of ∼1.6 eV. The essential features of the measured dispersion are reproduced well by the dispersion predicted in a qualitative way for an isolated c(3×1) oxygen monolayer.

High-resolution electron energy-loss spectroscopy and monochromatic low-energy electron diffraction have been applied to the study of the Si(111)(7 × 7) surface and the thermally-quenched Si(111) (1 × 1) surface. For the (1 × 1) surface, the inelastic continuum, observed for the (7 × 7) surface, due to the Drude absorption of electrons in the dangling-bond surface states is not existent, which indicates that the surface-state band associated with the dangling-bonds of the (1 × 1) surface is insulating. The observed electronic transitions indicate that the (7 × 7) and (1 × 1) surfaces have similar local band geometries and that they differ only in long-range order. The (1 × 1) surface is considered to have a disordered structure. The defect model is favored for the (7 × 7) structure.

High‐resolution vibrational/electronic‐transition electron energy loss spectroscopy, low‐energy electron diffraction, and Auger electron spectroscopy have been used to study the oxidation of the Si(111) (7×7) surface at 300 K. From the initial stage (O2 exposure ∼1 L, fractional oxygen coverage θ∼0.2), an Si–Si bond breaking occurs, and atomic oxygen is adsorbed in the bridge site (between the first and second layers of Si substrate) as well as in the on‐top site. Some molecular species (superoxide‐like species) are also existent. With the increase in O2 exposure up to 100 L where θ∼1.5 is reached, the number of the Si–O–Si species in the selvedge region of Si substrate is greatly increased. By heating the surface pre‐exposed to 1–100 L O2 at 850 K, the superoxide‐like species are removed and the number of oxygen atoms in the on‐top sites is decreased. With the increase in O2 exposure (0→100 L) and by heating (300→850 K), the bond angle of the Si–O–Si species is increased towards that of the vitreous SiO2...

High-resolution vibrational electron energy-loss spectra of CO on an Ni(110) surface were studied at 300 K with the in-situ combination of LEED, Auger electron spectroscopy and work-function change measurement. The observed peaks are at 436 cm−1, 1855 cm−1 (shifting to 1944 cm−1 with increasing coverage) and at 1960 cm−1 (shifting to 2016 cm−1 with increasing coverage). The experimental results indicate that CO is adsorbed non-dissociatively at all coverages. Three adsorbed states of CO have been found. At fractional CO coverages less than θ ~ 0.9 where the disordered adsorbed structure dominates, CO is adsorbed in two inequivalent sites (short- and long-bridge sites) at random with its axis oriented perpendicular to the surface. At high coverages (θ > 0.9) where the (2 × 1) structure develops, our results indicate that the adsorbed CO molecules may occupy the distorted long-bridge sites forming zig-zag chains which lie essentially in the troughs of the (110) surface.

Chemisorption of ammonia on the Si(111)(7×7) surface has been studied by means of high resolution electron energy loss spectroscopy (EELS). Ammonia is decomposed on the Si(111)(7×7) surface at 300 K to form stable surface species SiNH2 and SiH. It is proposed that the amino species (NH2) is bonded to the Si(111) surface with a covalent bond formed between its nitrogen atom and the dangling bond of the Si(111) surface atom. Changes in the adsorbed state of ammonia by heat treatment are discussed.

The initial stages of the interaction of oxygen with an Fe(110) surface have been studied at 300 K by electron energy-loss spectroscopy with in-situ combined low energy electron diffraction, Auger electron spectroscopy and work-function change measurement. From all the results, four different stages of the oxygen interaction are distinguished: (I) a first dissociative chemisorption up to 3 L, characterized by the c(2×2)-O structure, (II) a second dissociative chemisorption between 3 and 7 L, characterized by the c(3×1)-O structure, (III) incorporation of O adatoms into the selvage between 7 and 30 L, and (IV) oxidation above 30 L leading to the formation of FeO(111), characterized by the diffuse hexagonal diffraction pattern. The sticking probability was found to be initially near unity and fall off rapidly to a minimum value of ≈0.05 at ≈1 L. Particular emphasis was placed upon the investigation of the change in surface electronic properties from those characteristic of them metal to those of the oxide. In stage (I) an energy-loss peak, being attributed to the transition from the 2p orbital of the chemisorbed oxygen, was observed at 6.0 eV, while in stage (II) two additional peaks of the same origin appeared at 7.5 and 9.3 eV due to the formation of the O 2p band. The energy-loss spectrum in the oxide phase was characterized by the peaks at 4.8 and 7.5 eV due to the O2− 2p → Fe2+3d charge-transfer transitions and by a peak at 2.4 eV due to the ligand-field d → d transitions of an Fe2+ ion in FeO. It is shown that the Fe 3dyz,zx and 4sp electrons play a major role in the chemisorption bond (O adatoms located in the long-bridge site), and that for the incorporation process the Fe 3dy2 electrons are also involved in bonding by the symmetry breaking. The change in the Fe 3p-excitation spectrum during oxidation was also investigated. The differences between the experimental results on Fe(100) and (110) surfaces are summarized.

High‐resolution electron energy loss spectroscopy (EELS) has been applied to the study of the Si(111)(7×7)–H2O(D2O) system. At 300 K, H2O(D2O) is partially dissociated on the Si(111) surface to form the SiOH(SiOD) and SiH(SiD) species. Angle and primary‐electron‐energy dependences of the vibrational loss intensities were measured. Relative contributions to the vibrational excitations of the dipole, impact, and resonance mechanisms were estimated. The O–H(O–D) stretching and Si–O–H (Si–O–D) bending vibrations are partly excited by the resonance mechanism in the primary energy region of Ep ≂2–7 eV. EELS spectra of the Si(111) surface exposed to H2O(D2O) at 300 K and of the same surface heated to ∼700–900 K are presented, and surface reaction mechanisms are discussed.

Electron energy-loss spectra of CO-adsorbed Ni(100) surfaces have been observed with concomitant measurements of LEED, AES, work function change, and electron impact desorption. Five CO-induced loss peaks are observed for the first time. The loss peaks at 5.8 and 8.6 eV are ascribed to the charge-transfer transitions ( Ni d π → CO 2 π ∗ and Ni d σ → CO 2 π ∗ , respectively), and the weak peaks at 11.8, 12.6 and 16.0 eV to the intra-molecular transitions ( 5σ → 2π ∗ , 1π → 2π ∗ and 4σ → 2π ∗ , respectively). The COCO adsorbate interaction is shown to shift the loss energy of the 12.6 eV peak. The adsorbed state of CO consistent with the results of LEED, AES, work-function change and electron impact desorption is discussed.

Angle-resolved photoemission from epitaxially grown $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}(001)$ single-crystal thin films reveals a clear Fermi edge and fine structure near the edge. Some of the valence-band photoemission features show $k$-space dispersion. These new results suggest that the electronic structure of this oxide can be treated within the framework of the one-electron band picture.

The adsorption of oxygen on a Pd(110) surface has been studied using in situ combined techniques of low-energy electron diffraction (LEED) and vibrational electron energy loss spectroscopy (EELS). Changes in the adsorbed structure of oxygen with increasing exposure are discussed.

The adsorbed state of formic acid (HCOOH) on the Si(111) (7×7) surface has been studied by means of high-resolution electron energy loss spectroscopy (EELS). Formic acid is partially dissociated to form the unidentate formate species (HCOO) of a neutral character and H adatoms on the Si(111) surface at 300 K.

The clean and hydrogen-covered Cr(110) surfaces have been investigated at 300 K by electron energy-loss spectroscopy (ELS) and secondary-electron emission spectroscopy (SES) (primary energy: Ep = 60−300 eV) with the in-situ combined supplementary techniques. The clean surface SES spectrum has peaks at 7.3, 9.2, 11.4, 13.2, 15.2, 17.6 and 20.6 eV above the Fermi level, which are thought to correspond to maxima in the unoccupied density of states. Only the 11.4 and 15.2 eV peak intensities are strongly reduced by irreversibe hydrogen adsorption. The clean surface ELS spectrum is characterized by peaks at 2, 3.5, 5.5, ∼ 10, 23, 35, 42, 48 and 55 eV. The ELS spectrum is interpreted by comparison with the optical data, the band calculation and the SES results. The 2 and 3.5 eV loss peaks are attributed to the interband transitions between d bands, and the 5.5 and ∼: 10 eV peaks to those associated with sp-like bands. By hydrogen adsorption, the damping and blurring of only the 5.5 and ∼: 10 eV peaks were observed, and a hydrogen-induced peak was found at ∼: 16 eV. These results are considered to indicate that the CrH bonding occurs mainly via sp bands; the role of d bands, however, cannot be neglected. Hydrogen-exposure dependence of the ELS spectrum is found to be practically independent of primary energy, in contrast to the Ni(100)H2 system, indicating that hydrogen is rapidly penetrated into the Cr selvedge lattice.

The excitation angle (β) and emission angle (θ) dependences of the Ni M2,3VV (61 eV) and Ni L3VV (850 eV) Auger emissions from clean polycrystalline Ni surfaces, and the S L2, 3 M2, 3M2, 3 (150 eV) Auger emission from S-adsorbed poly-Ni surfaces have been investigated. In the case of Ni (61 eV) and S Auger emissions, the β-dependence shows the 1 cos β distribution, while a significant deviation from 1 cos β is observed for Ni (850 eV) Auger emission. The cosθ distribution and the intermediate between isotropic and cosθ distributions are observed for Ni (61 eV), and for Ni (850 eV) and S Auger emissions, respectively. Those results have been found to be in fairly good agreement with the calculations based on the simple continuum model without consideration of the diffraction effect and the inherent anisotropic emission.

The discrete variational (DV) Xα cluster model is used to study the chemisorptions on (001) surfaces of bcc V, Cr and Fe. The adatom induced levels and the electronic charge densities are calculated and compared with experimental data such as UPS and LEED analysis. The oxygen induced levels and the change in valence-state density explain well the UPS spectra of oxygen on Fe surface. The results suggest that the four-fold hollow site of (001) surface is more stable than the on-top site for oxygen chemisorption, and that the first layer distance of the surface is expanded by chemisorption, consistent with the experiment.

The initial stages of the interaction of oxygen with a Cr(110) surface have been investigated at 300 K by LEED, AES, electron energy loss spectroscopy (ELS), secondary electron emission spectroscopy (SES) and work-function change measurement (Δφ). In the exposure region up to 2 L, the clean-surface ELS peaks due to interband transition weakened and then disappeared, while the ∼5.8 and 10 eV loss peaks attributed to the O 2p → Cr 3d transitions appeared, accompanied with a work-function increase ( Δφ = +0.19 eV at2L). In the region 2–6 L the work function decreased to below the original clean-surface value ( Δφ min = −0.24 eV at6L), and five additional ELS peaks were observed at ∼2, 4, 11, 20 and 32 eV: the 2 and 4 eV peaks are ascribed to the ligand-field d → d transitions of a Cr 3+ ion, the 11 eV peak to the O 2p → Cr 4s transition, the 20 eV peak to the Cr 3d → 4p transition of a Cr 3+ ion and the 32 eV peak probably to the Cr 3d → 4f transition. A new SES peak at 6.1 eV, being attributed to the final state for t he 11 eV ELS peak, was observed at above 3 L and identified as due to the unfilled Cr 4s state caused by charge transfer from Cr to oxygen sites in this region. In the region 6–15 L the work function increased again ( Δφ max = +0.32 eV at15 L), the 33 and 46 eV Auger peaks due to respectively the M 2,3 (Cr)L 2,3 (O)L 2,3 (O) cross transition and the M 2,3 VV transition of the oxide appeared and the 26 eV ELS peak due to the O 2s → Cr 4s transition was also observed. Above 10 L, the ELS spectra were found to be practically the same as that of Cr 2 O 3 . Finally, above 15 L, the work function decreased slowly ( Δφ = +0.13 eV at40L). From these results, the oxygen interaction with a Cr(110) surface can be classified into four different stages: (1) dissociative chemisorption stage up to 2 L, (2) incorporation of O adatoms into the Cr selvedge between 2–6 L, (3) rapid oxidation between 6–15 L leading to the formation of thin Cr 2 O 3 film, and (4) slow thickening of Cr 2 O 3 above 15 L. The change in the Cr 3p excitation spectrum during oxidation was also investigated. The oxide growth can be interpreted on the basis of a modified coupled current approach of low-temperature oxidation of metals.

The adsorbed state of ethylene on Pd(110) at 90 K and its thermal decomposition in the temperature region between 90 and 600 K have been studied by the use of high resolution electron energy loss spectroscopy (EELS), low‐energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). At 90 K, ethylene is π bonded to the Pd(110) surface and is adsorbed almost disorderedly. The c(2×2)‐C2H4 patches are formed near the saturation coverage (which corresponds to 0.58 C2H4 molecule per surface Pd atom). By heating the C2H4‐saturated Pd(110) surface to 260 K, some C2H4 admolecules are desorbed intact and the remaining admolecules rearrange their adsorbed sites to form the c(2×2)‐C2H4 structure. At above 300 K, almost all the C2H4 admolecules are dehydrogenated, and the ethynyl (CCH) species, H adatoms and unstable dehydrogenated species [possibly, vinyl (CHCH2) species] are formed; the C2H4 desorption occurs by the recombination of H adatoms and dehydrogenated species. The remaining H adatoms are d...

The electronic state of the hydrogen-adsorbed Cr(110) surface at 80 K has been investigated using angle-resolved photoemission spectroscopy. We observed a p(2\ifmmode\times\else\texttimes\fi{}2) low-energy electron-diffraction pattern at \ensuremath{\sim}2 L, and a ``streaked'' (1\ifmmode\times\else\texttimes\fi{}1) pattern along the [11\ifmmode\bar\else\textasciimacron\fi{}0] azimuth at above 10 L. The photoemission results show that for the p(2\ifmmode\times\else\texttimes\fi{}2) phase an H 1s--derived state, having a nearly constant binding energy of \ensuremath{\sim}5.5 eV below the Fermi level, is observed in the outer part of the surface Brillouin zone, that is, outside the Cr s-p band projections. [Here 1 langmuir (L)\ensuremath{\equiv}${10}^{\mathrm{\ensuremath{-}}6}$ Torr sec.] On the other hand, for the streaked (1\ifmmode\times\else\texttimes\fi{}1) phase, an H 1s--derived state, split off from the Cr bulk bands, is found at the binding energy of 7.8 eV at \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} and shows considerable dispersion of \ensuremath{\sim}2.8 eV in the [001] azimuth being consistent with the periodicity of the substrate structure. In addition, a H-induced surface state is found at 2.5 eV at \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} for both phases. The nature of the Cr-H chemisorption bond is discussed.

The combined techniques in situ of Auger electron spectroscopy, electron energy loss spectroscopy, electron impact desorption, and work-function change measurement have been applied to the study of the adsorption of HCl and HBr on thermally cleaned Si(111) surfaces. Major results are summarized as follows: (1) HCl shows a fast adsorption to the saturation coverage of θs ⋍ 0.3 (estimated using the continuum approximation) by the exposure of about 1 L at room temperature. (2) The average sticking probabilities for HCl and HBr are ∼0.7. (3) Two adsorbed states of HCl or HBr at room temperature are discriminated. For HCl, the first state is characterized by the emission of ∼1.2 eV ions and the electronic transition at 8.4 eV, which is subsequently converted to the second state characterized by the emission of ∼3.2 eV ions and the electronic transitions at 7.0 and 8.4 eV. Heating the sample at ∼800 K causes the desorption of hydrogen and the appearance of the Cl-related peaks at 6.0, 7.0, and 9.0 eV in the loss spectra. For HBr, the first and the second states are characterizied by the emission of ∼1.2 and ∼3.2 eV ions, respectively. The electronic transition is observed at 7.8 eV in both states. (4) It is proposed that HCl and HBr are adsorbed as molecules initially, which are subsequently dissociated into atoms spontaneously at room temperature.