Ichiro Arakawa

We have studied electron stimulated desorption (ESD) and photon stimulated desorption (PSD) of positive ions from water absorbed on a surface of solid rare gases. In TOF spectrum, a series of ionized clusters are observed, which were thought to be protonated water clusters, (H₂O) _nH⁺. In both PSD experiments of H₂O/Ar, the desorption threshold of (H₂O) _nH⁺ was found to be coincide to 2p core ionization of Ar.

Luminescent phenomena during sliding friction between a diamond micro sphere and a flat quartz single-crystal disk under controlled ambient gas pressure of N<SUB>2</SUB>, Ar, and Kr were investigated. Spectra of the luminescence clearly indicated the gas discharge of each gas around the frictional contact. Spatial distributions of the gas-discharge luminescence were measured. The distributions at different sliding speeds were almost identical. The distributions depend strongly on the gas species and the pressure; the area of the discharge becomes larger with decrease of the gas pressure.

Absolute total desorption yields from the surface of solid krypton were measured by photo excitation at excitonic excitation energies and by electron excitation in the energy range between 70 and 320 eV. The absolute desorption. yields and their dependence on the film thickness and on the excitation energy were quantitatively reproduced by a simulation based on the desorption model of excimer dissociation followed by internal sputtering. (C) 2002 Elsevier Science B.V. All rights reserved.

Absolute yields of the photo-induced desorption at the surface of solid rare gases are studied in the excitonic excitation region. Both metastable and total desorption yields depend strongly on excitation energy and film thickness of rare gas solids. The absolute desorption yields and their dependence on film thickness are quantitatively reproduced by a simulation based on the diffusion of excitons in the bulk and the kinetic energy release by a cavity ejection mechanism and an excimer dissociation mechanism followed by internal sputtering. (C) 2003 American Institute of Physics.

An innovative technique to evaluate electric potential difference of triboelectricity between both dielectric surfaces was developed. Electric-discharge luminescence due to electrification during friction between a spherical diamond surface and a flat quartz one was observed using a microscope under various pressures of N₂ gas. Two-dimensional spatial distribution of the luminescence around a contact shows a strong luminescence immediately after the friction and ring pattern of luminescence outside of friction area. The distribution of the strong luminescence along sliding direction, i.e., breakdown characteristics depending on gap distance, was well explained by a semi-empirical equation of discharge-current and gap distance. The potential difference between the both sides was also evaluated. Using the relation between a potential difference and a charge density in parallel-plate capacitor, surface charge density in the discharge area was calculated.

If a solid Ne surface is irradiated by 20-200 eV electrons or by 55-75 nm synchrotron radiation, Ne-2* excimers in a (3)Sigma (u), State are desorbed to form a luminescent "plume" in front of the sample. The kinetic energy of the desorbed excimers was found to be (0.2+/-0.1) eV, which indicates that the cavity ejection mechanism is valid for the excimer desorption. The decay with time of the plume emission is not of single exponential type, because the emission lifetime of the desorbed excimers, which is of the order of 10(-6) s, depends on their vibrational level. Most of them are in the highest vibrational level since desorption takes place much faster than vibrational relaxation.

The layering transition of a physisorption system on a metal single crystal has been investigated by means of an ellipsometry and an eXtremely-low-current LEED (XLEED). Our XLEED system is operated at the primary electron current of about 0.1 pA, which minimizes the effect of desorption, defect formation, and charging. We observed the surface structure of Xe/Ag (111) by XLEED while monitoring the layer growth by the ellipsometry from a submonolayer film to a thick one. An equilibrium between the Xe film and coexisting three-dimensional Xe gas has been maintained throughout the experiment. From a monolayer film to a sufficiently thick film, the Xe overlayer has clear hexagonal structure whose directions of the unit vectors are coincident with those of the substrate. The Xe-Xe spacing in the monolayer film on Ag (111) is known to be a few per cent larger than that of bulk. Our interest is how the Xe-Xe spacing varies in the process of layer growth. We have made a systematic observation of the change of the Xe-Xe spacing in one and two monolayer films at pressures between 10^-7 and 10^-2 Pa and at temperatures between 50 and 100 K.

Absolute yields of photo-induced desorption at the surface of solid Ne have been measured at wavelengths of incident light between 25 and 100 nm. There is a strong dependence of the total desorption yield of Ne on both the excitation energy and the thickness of Ne films. On a thick him, the desorption yield is 1-2 atoms per photon by bulk exciton excitation and 2-10 atoms per photon by bulk ionization. These values are quantitatively explained by an internal sputtering mechanism. By the surface exciton excitation, the yield is 0.1-0.3 atoms per photon, which value means that the desorption probability of the surface exciton is almost unity. (C) 2000 Elsevier Science B.V. All rights reserved.

Long term behavioral characteristics of an axial symmetric transmission gauge, such as the outgassing rate and sensitivity factor, were examined over a period of 3500 h. It was found that the outgassing rate and the gain of the continuous-dynode-type electron multiplier, which was used for ion detection, were the main factors which determined the gauge characteristics. The outgassing rate of the multiplier as obtained from the manufacturer was very high but decreased to the acceptable level after about two days pf continuous operation in high and ultrahigh vacuum. The sensitivity factor of the newly fabricated gauge was very high but decreased with the decrement of the outgassing rate to the quasi-stationary state at which the factor tended to decrease gradually with operating time. When the gauge was exposed to atmospheric air, the sensitivity factor increased temporarily but decreased within the duration of the typical bakeout process. The increment was assumed to be due to the adsorption of gases on the multiplier surface. The lifetime of the electron multiplier estimated from the decay rate of the sensitivity factor during the acceleration test by hydrogen is about nine years for the operation of the gauge in the 10(-10) Pa region. It was also found that the gain of the multiplier was proportional to the soft x-ray background signal. The gradual variation in the sensitivity factor of the gauge caused by that of the gain of the multiplier over the period of operation can be compensated by this relation. (C) 1999 American Vacuum Society. [S0734-2101(99)05104-0].

We have developed an experimental apparatus for in situ observation of a physisorption system on a metal surface by an ellipsometry and an electron diffraction. In order to observe a physisorption system and a thick insulator film, we have developed an extremely low-current low energy electron diffraction (XLEED) apparatus equipped with a microchannel plate and a position-sensitive detector in place of a phosphor screen in an ordinary LEED optics. The ellipsometric adsorption isotherms of Xe/Ag (111) at substrate temperatures between 50 and 80 K were obtained in the wide pressure range between 10(-7) and 1 Pa. We observed the surface structure by XLEED while monitoring the layer growth by the ellipsometry from a submonolayer to a thick film where the equilibrium pressure was nearly equal to the bulk saturation vapor pressure. The Xe overlayer of a sufficiently thick film has clear (111) structure which keeps relative orientation to the Ag (111) substrate. (C) 1998 American Vacuum Society.

The effect of physisorbed hydrogen on the desorption of metastable particles induced by electronic excitation from the surface of rare-gas. Solids (Ar, Kr, and Xe) was investigated systematically. When the surface of a rare-gas solid was exposed to a very small quantity of hydrogen at a temperature of about 30 K, in all cases, the desorption of hydrogen metastables was observed, and new desorption species of Kr* via cavity ejection process appeared in time-of-flight spectra for Kr. For Ar, although no significant increase of desorption yield was found, the kinetic energy of Ar* desorbed through cavity ejection process was increased by hydrogen adsorption. These results will be discussed in terms of the change of electron affinity of the matrix and the interaction between hydrogen and rare-gas molecules in excited states. (C) 1998 American Vacuum Society.

We have developed an eXtremely-low-current Low Energy Electron Diffraction (XLEED) apparatus equipped with a micro-channel plate and a position-sensitive detector which reduces electron beam damage and charge-up effects and makes it possible to study the structure of physisorbed films. This XLEED system is combined with the ellipsometer system which monitors the thickness of a physisorbed film. We observed Xe overlayer on Ag (111) and grahpite (0001) surfaces. We found that a thick Xe films on Ag (111) showed a single crystal structure which keeps relative orientation to the substrate.

We have developed an eXtremely-low-current Low Energy Electron Diffraction (XLEED) apparatus equipped with a micro-channel plate and a position-sensitive detector which reduces electron beam damage and charge-up effects and makes it possible to study the structure of physisorbed films. This XLEED system is combined with the ellipsometer system which monitors the thickness of a physisorbed film. We observed Xe overlayer on Ag (111) and grahpite (0001) surfaces. We found that a thick Xe films on Ag (111) showed a single crystal structure which keeps relative orientation to the substrate.

The electronic excitation on the surface of rare gas solids can lead the desorption of a variety of species. Close investigation of the desorption phenomena will reveal the dynamical aspect of the electronic excitation and its relaxation process at the surface. In the case of rare gas solids, the desorption of an excited neutral particle is closely related to the production of an exciton. © 1998 OPA (Overseas Publishers Association) N.V. Published by license under the Gordon and Breach Science Publishers imprint.

Desorption of long-lived neutral particles from solid Ne is studied using selective photoexcitation and time of flight (TOF) techniques. The luminescence due to direct light from the sample surface is blocked by rotating the sample. Two new prominent features are revealed in the TOF spectra measured with the special geometry in the study: a prominent long tail and a maximum at the delay time of 0.5 ms. From the analysis of the excitation spectra recorded by a time-window technique and the TOF spectra at selective excitation, it is considered that the new features are closely related to Ne-2* excimer desorption. An analysis of the probable mechanisms of long life-time emission is presented. (C) 1997 Elsevier Science B.V.

Absolute yields of the metastable excited atoms desorbed from the surfaces of solid Ne and Al by the creation of surface and bulk excitons have been measured using monochromated synchrotron radiation as a selective excitation source. We have obtained the absolute yields of (2.3 +/- 0.7) x 10(-3), (1.4 +/- 0.4) x 10(-3), and (7.8 +/- 2.3) x 10(-4) atoms/photon at the excitation of S1, B1 and S' exciton for Ne, respectively, and 1 x 10(-5) atoms/photon at S1 excitation for Ar. The probability for metastable atom desorption is found to be about 2 to 10% at the excitation of S1 exciton on the surface of solid Ne. (C) 1997 Elsevier Science B.V.

The effect of the physisorbed hydrogen molecules on the electron stimulated desorption of metastables from the surface of solid Kr was investigated. At the clean surface of solid Kr, the desorption of Kr metastables occurs only through an excimer dissociation mechanism. Physisorption of hydrogen molecules causes the following effects: (1) the total desorption yield of the metastable species increased by an order of magnitude or more; (2) metastable Kr, which is probably desorbed through the cavity ejection mechanism, emerges; (3) metastable Kr by excimer dissociation is also enhanced; and (4) a new metastable species, which seems to be composed of metastable hydrogen molecule, appears. (C) 1997 Elsevier Science B.V.

The characteristics of a new ionization gauge for pressure measurement in XHV were examined by experiments and computer simulations. The new gauge (axial-symmetric transmission gauge : AT gauge) has a Bessel-Box type energy filter between a B-A gauge type ionizer and a secondary electron multiplier as an ion collector to eliminate the limiting factors of pressure measurement in XHV, such as the X-ray effect and electron stimulated desorption (ESD) ions. The lower limit of the pressure measurement at present is estimated to be about 10^-12 Pa. The sensitivity factor of the AT gauge, which was further corrected for the effect of outgassing by the conductance modulation method (CMM), is almost constant in the pressure range from 10^-10 to 10^-6 Pa and is (2.3±0.1) ×10^-3 Pa^-1 for hydrogen.

The sensitivity factor of an axial symmetric transmission gauge (AT gauge) was determined by the conductance modulation method (CMM) in the pressure range from 10(-10) to 10(-6) Pa. The gauge has a Bessel-box type energy filter between a Bayard-Alpert gauge type ionizer and an ion detector to eliminate the limiting factors of pressure measurement in extreme high vacuum such as the soft x-ray effect and the electron stimulated desorption of ions. The lower limit of the pressure measurement of the gauge is estimated to be about 10(-12) Pa or lower. The sensitivity factor showed a slight pressure dependence caused mainly by outgassing from the gauge itself. The sensitivity factor of the AT gauge, which was further corrected for the effect of outgassing by the CMM, is almost constant in the pressure range from 10(-10) to 10(-6) Pa and is (2.3 +/- 0.1) x 10(3) Pa-1 for hydrogen. (C) 1997 American Vacuum Society.

We have developed a hot filament ionization gauge equipped with a Bessel box-type energy filter for pressure measurement in XHV. In order to find an optimum geometry of the gauge, we examined the influence of the geometrical configuration of the electrode of the ion source on the characteristics of the gauge. First, the influence of the cover electrode of the grid type electron collector was examined. Contrary to our expectation, the experimental results indicate that the cover electrode does not seriously influence the sensitivity of the gauge, but influences the separation of the gas phase and ESD ions. The energy spectra of ions produced inside the grid without the cover electrode show two well-separated peaks of the gas phase and the ESD ions. In case of the grid with the cover electrode, however, the signals of the gas phase and ESD ions were overlapped. Second, the influence of the filament position was examined by comparing two positions relative to the grid. One is near the end of the grid which is close to the energy filter, and the other is near the opposite end. The spectra show that the well separated peaks were observed for both positions, but higher sensitivity was achieved for the lower position. Copyright (C) 1996 Elsevier Science Ltd.

Metastable atom desorption from a pure solid Ne and a solid Ne-Kr alloy has been systematically studied using a monochromatic synchrotron radiation. Refined time-of-flight spectrometer enabled us to investigate the desorption mechanism in more detail. Metastable Ne atoms desorbed through cavity ejection mechanism by the excitation of the first order surface, and the first and the second order bulk excitons are found to have almost the same kinetic energy, E(k) = 0.18 +/- 0.02 eV, while by 2p(5)3p type surface exciton, E(k) = 0.21 +/- 0.02 eV. By the excitation of Ne bulk exciton in Ne-Kr alloy, effective excitonic energy transfer process has been observed, which results in the decrease of the Ne* desorption yield by the bulk excitation.

Metastable-particle desorption from Ne-Ar and Ne-Kr solid alloys stimulated by photon excitation were studied using synchrotron radiation as excitation source. Excitation of Ne excitons by optically allowed transitions led mainly to desorption of the other components of the alloy revealing a very efficient energy transfer process. On the other hand, the excitation of the partially allowed electronic transition at the surface (2p(5) 3p) of Ne showed almost no electronic energy transfer to the matrix and desorption of Ne metastables was observed. A sharp enhancement of the Ar metastable signal with respect to pure solid Ar was also observed in the study of Ne-Ar alloys in the energy range below the excitation of Ne excitons. Different mechanisms for desorption are suggested according to the energy region of excitation.

A new type of hot filament ionization gauge with an ion energy analyzer was developed for pressure measurement in ultrahigh and extremely high vacua. The new gauge has a Bayard-Alpert gauge type ionizer, a Bessel Box type energy analyzer, and a secondary electron multiplier of pulse counting mode for ion detection. The Bessel Box type energy analyzer is composed of a cylinder electrode with a center disc and two end plates. The center disc, which is placed perpendicularly to the cylinder axis, shields the ion collector from direct incidence of X-rays generated at the surface of the ionizer. The energy analyzer focuses the ions from the limited region in the grid cage, whose potential is about 30 V lower than that of the grid, on the ion collector. Therefore, the energy spectrum obtained by the energy analyzer showed that the two peaks of gas phase ions and electron stimulated desorption (ESD) ones were completely separated with an energy difference of 30 eV. The residual X-ray photocurrent was equivalent to a pressure about 4 x 10(-11) Pa (N-2 eq.). The sensitivities of the gauge for argon and hydrogen were constant between 5 x 10(-10) and similar to 10(-6) Pa and were 8.2 x 10(-3) and 3.2 x 10(-3) Pa-1, respectively. The counting rate of the gas phase ions was 1200 counts s(-1) at 2 x 10(-10) Pa(N-2 eq.) with 30 mu A of emission current. The lower limit of pressure measurement of this new gauge is estimated to be 3 x 10(-12) Pa or less.

The origin of the Ne metastables desorbed by excitonic excitations was investigated. The measurement of the angular and kinetic energy distributions of Ne metastables and their excitation energy dependence revealed that there are, at least, five components desorbed through different desorption kinetics: excimer dissociation species from the higher-order exciton, cavity ejection from three kind of surface excitons, and ejection of metastables from a bulk exciton in an underlying layer. The metastable desorbed through the cavity-ejection mechanism by the excitation of the first-order surface exciton showed a narrow angular distribution along the surface normal with an FWHM between 30 degrees and 90 degrees, which depends on the velocity of a desorbed Ne metastable.

The adsorption/desorption hysteresis which appears in the adsorption isotherm for Kr physisorbed on exfoliated graphite has a very characteristic feature. A sharp transition at the termination of the hysteresis in the desorption branch of the isotherm suggests a first-order phase transition. The temperature dependence of the hysteresis for Kr and Xe on exfoliated graphite was investigated. The origin of the hysteresis of Kr is thought to be capillary condensation accompanied by a liquid-solid phase transition.

We have developed a supersonic cluster beam source for the use of an electron-cluster crossed-beam experiment to study the electronic excitation processes in rare gas clusters. A cryopump is used for the differential pumping system instead of a diffusion pump, which is used in conventional cluster beam sources. The performance of this differential pumping system is examined. The size distribution in the cluster beam is estimated by measuring the kinetic energy and velocity distribution as a function of the stagnation pressure.

Electron stimulated desorption of metastable species from the surface of solid Ne was investigated. Ne metastables through various desorption channels were identified in the kinetic energy distributions which were obtained by time-of-flight measurement. The energy distribution is identical to that obtained by photo-stimulated desorption experiments. Two dominant species observed in the time-of-flight spectrum showed different characteristics in the angular distribution of desorption. The lower kinetic energy one, which has a kinetic energy of approximately 0.18 eV and was attributed to a ''cavity-ejection'' mechanism, had a sharp angular distribution along the surface normal with FWHM of approximately 30-degrees. The other one, which has approximately 1.4 eV and was attributed to a ''excimer-dissociation'' mechanism, did not show a preferential direction of desorption.

When low-energy electrons or photons (10-1000 eV) impinge on a surface of a rare-gas solid (RGS), desorption of ions, neutrals, and molecules can occur together with other processes. The desorption is due to an initial electronic excitation and a subsequent transformation of the excitation energy into kinetic energy of a desorbed atom. This phenomenon is called DIET (desorption induced by electronic transitions) : DIET by electron impact and photon impact are well known as ESD (electron-stimulated desorption) and PSD (photonstimulated desorption), respectively. Few studies of DIET of ions from RGS have been reported. In the investigation of ESD of H₂ physisorbed on condensed Xe, Arakawa and Tuzi found that ESD of ions (Xe⁺, Xe²⁺) and neutrals from condensed Xe could occur. Sakurai et al. reported the PSD of ions from the surface of solid Ar, Kr, and Xe. Dujardin et al. reported that electronically excited ions (Ar^+*) and doubly charged ions (Ar²⁺) might be primary excited states which lead to desorption of Ar⁺ from solid Ar. Menzel reported that the desorption of singly and doubly charged Kr ions occurs due to core ionization. They suggested that the kinetic energy of a desorbed ion was caused by Coulomb repulsion between adjacent ions which were formed by charge exchange between an initial multiply excited atom and its ground-state neighbor or by the deexcitation of doubly charged ionic dimers.