Electron-stimulated desorption of lithium atoms from oxidized molybdenum surface

The yield and energy distributions of lithium atoms upon electron-stimulated desorption from lithium layers adsorbed on the molybdenum surface coated with an oxygen monolayer have been measured as functions of the impact electron energy and lithium coverage. The measurements are performed using the time-of-flight technique and a surface ionization detector. The threshold of the electron-stimulated desorption of lithium atoms is equal to 25 eV, which is close to the ionization energy of the O 2s level. Above a threshold of 25 eV, the yield of lithium atoms linearly increases with an increase in the lithium coverage. In the coverage range from 0 to 0.45, an additional threshold is observed at an energy of 55 eV. This threshold can be associated with the ionization energy of the Li 1s level. At the electron energies above a threshold of 55 eV, as the coverage increases, the yield of lithium atoms passes through a maximum at a coverage of about 0.1. Additional thresholds for the electron-stimulated desorption of the lithium atoms are observed at electron energies of 40 and 70 eV for the coverages larger than 0.6 and 0.75, respectively. These thresholds correlate with the ionization energies of the Mo 4s and Mo 4p levels. Relatively broad peaks in the range of these thresholds indicate the resonance excitation of the bond and can be explained by the excitation of electrons toward the band of free states above the Fermi level. The mean kinetic energy of the lithium atoms is equal to several tenths of an electronvolt. At electron energies less than 55 eV, the energy distributions of lithium atoms involve one peak with a maximum at about 0.18 eV. For the lithium coverages less than 0.45 and electron energies higher than 55 eV, the second peak with a maximum at 0.25 eV appears in the energy distributions of the lithium atoms. The results obtained can be interpreted in the framework of the Auger-stimulated desorption model, in which the adsorbed lithium ions are neutralized after filling holes inside inner shells of the substrate and lithium atoms.     

Electron-stimulated desorption of cesium atoms from cesium layers deposited on a germanium-coated tungsten surface

The yield and energy distribution of Cs atoms from cesium layers adsorbed on germanium-coated tungsten were measured, using the time-of-flight technique with a surface-ionization-based detector, as a function of the energy of bombarding electrons, germanium film thickness, the amount of adsorbed cesium, and substrate temperature. The threshold for the appearance of Cs atoms is ～30 eV, which correlates well with the germanium 3d-level ionization energy. As the electron energy increases, the Cs atom yield passes through a broad maximum at ～120 eV. For germanium film thicknesses from 0.5 to 2 monolayers, resonance Cs yield peaks were observed at electron energies of 50 and 80 eV, which can be related to the tungsten 5p and 5s core-level ionization energies. As the cesium coverage increases, the Cs atom yield passes through a flat maximum at monolayer coverage. The energy distribution of Cs atoms follows a bell-shaped curve. With increasing cesium coverage, this curve shifts to higher energies for thin germanium films and to lower energies for thick films. The Cs energy distribution measured at a substrate temperature T = 160 K exhibits two bell-shaped peaks, namely, a narrow peak with a maximum at ～0.35 eV, associated with tungsten core-level excitation, and a broad peak with a maximum at ～0.5 eV, deriving from the excitation of the germanium 3d core level. The results obtained can be described within a model of Auger-stimulated desorption.     

Mass Spectrometric Determination of Partial Ionization Cross Sections

A review of the measurement of partial electron impact ionization cross sections for rare gas atoms and molecules is presented, with particular regard to the low electron energy regime (<200 eV). Recently developed experimental techniques are surveyed. Predictions of quantal and classical approximations as well as some empirical formulae are compared with the experimental data. In conclusion a set of recommended atomic ionization cross sections is given for electron energies ranging from threshold up to 14 keV. Finally, the existence and measured appearance potentials for the production of doubly ionized rare gas dimers are discussed.     

Total electron scattering cross-sections for carbon monoxide at low electron energies

Absolute total electron scattering cross-sections for carbon monoxide have been measured at low electron energies using a photoelectron source. The measurements have been carried out at 22 electron energies varying from 0.73 to 9.14 eV with an accuracy of ±3%. The cross-sections obtained in the present experiment have been compared with other measurements and theoretical computations.     

Total electron scattering cross-sections for nitrous oxide at low electron energies

Absolute total electron scattering cross sections for nitrous oxide have been measured at low electron energies using a photoelectron source. The measurements have been carried out at 19 electron energies varying from 0.73–9.14 eV with an accuracy of ±3%. The cross sections obtained in the present experiment have been compared with other measurements.     

Elastic scattering of electrons from tetrahydrofuran molecule

Absolute differential cross-sections (DCSs) for elastic scattering of electrons from the DNA backbone sugarlike analogue tetrahydrofuran (THF) molecule were determined using a crossed beam measurements for incident energies from 20 eV to 300 eV and scattering angles from 10^o to 110^o. Using the relative-flow technique, elastic DCSs for THF relative to nitrogen have been obtained at incident energies of 20, 30, 40, 50 and 60 eV. In the energy region above 30 eV, the DCSs were measured independently as a function of both incident electron energy and scattering angle. Therefore, this set of relative DCSs has been calibrated to the absolute scale via normalization to a single point in the overlapping region. Additionally, both vibrational and electronic energy loss spectra for THF are presented and influence of energy resolution to the obtained DCSs is discussed.     

Total electron scattering cross sections for carbon dioxide at low electron energies

Absolute total electron scattering cross sections for carbon dioxide have been measured at low electron energies using a photoelectron source. The measurements have been carried out at 27 electron energies varying from 0.91–9.14 eV with an accuracy of ±3%. The cross sections obtained in the present experiment have been compared with other measurements and theoretical computations.     

The electron energy loss spectrum of tin at low primary beam energies

The electron energy loss spectra of clean and oxidised tin have been measured for primary energies in the range 100–1000 eV. The structure found for clean tin is similar to published spectra recorded with low primary energies except for the presence of a small peak at 4.7 eV. Differences between the reported volume plasmon energy recorded with low and high primary beam energies were noted and a model is presented to explain these differences in terms of plasmon dispersion.     

Electron stimulated desorption of cesium atoms from germanium-covered tungsten

The electron stimulated desorption (ESD) yield and energy distributions for Cs atoms from cesium layers adsorbed on germanium-covered tungsten have been measured for different Ge film thicknesses, 0.25-4.75 ML (monolayer), as a function of electron energy and cesium coverage Θ. The measurements have been carried out using a time-of-flight method and surface ionization detector. In the majority of measurements Cs is adsorbed at 300 K. The appearance threshold for Cs atoms is about 30 eV, which correlates well with the Ge 3d ionization energy. As the electron energy increases the Cs atom ESD yield passes through a wide maximum at an electron energy of about 120 eV. In the Ge film thickness range from 0.5 to 2 ML, resonant Cs atom yield peaks are observed at electron energies of 50 and 80 eV that can be associated with W 5p and W 5s level excitations. As the cesium coverage increases the Cs atom yield passes through a smooth maximum at 1 ML coverage. The Cs atom ESD energy distributions are bell-shaped; they shift toward higher energies with increasing cesium coverage for thin germanium films and shift toward lower energies with increasing cesium coverage for thick germanium films. The energy distributions for ESD of Cs from a 1 ML Ge film exhibit a strong temperature dependence; at T = 160 K they consist of two bell-shaped curves: a narrow peak with a maximum at a kinetic energy of 0.35 eV and a wider peak with a maximum at a kinetic energy of 0.5 eV. The former is associated with W level excitations and the latter with a Ge 3d level excitation. These results can be interpreted in terms of the Auger stimulated desorption model.     

Attenuation lengths of low-energy electrons in solids

A compilation is presented of measured attenuation lengths of low-energy electrons in solids in the energy range (40 to 2000 eV) normally employed in X-ray photoelectron and Auger-electron spectroscopy. The techniques used to obtain electron attenuation lengths are summarized, and it is pointed out that the accuracy of measurement needs both to be defined adequately and to be improved for more meaningful intercomparisons of data and theory. An approximate expression is derived to predict attenuation lengths using either dielectric data (derived from optical or electron-energy-loss data) or average excitation energies estimated from electron binding energies for given materials at electron energies greater than about 500 eV. Good agreement is found between the predictions of this formula and some measured attenuation lengths (e.g. for Al, C, Mo, W) but further work is required to validate the formula and to extend it to lower electron energies.     

Excitation of molecular vibrations by positron impact

Absolute cross sections for the vibrational excitation of CO, CO2, and H2 by positron impact are presented for incident positron energies from 0.5 eV to several electron volts. The measurements use a novel technique that exploits the adiabatic motion of a positron beam in a strong magnetic field. This work is the first systematic experimental study of vibrational excitation by positron impact, and extends to energies where positron measurements have traditionally been difficult. The measured cross sections are compared with available theoretical calculations.     

离子能量和沉积温度对离子束沉积碳膜表面形貌的影响

利用质量分离的低能离子束沉积技术,得到了非晶碳膜.所用离子能量为50—200eV,衬底温度从室温到800℃.在沉积的能量范围内,衬底为室温时薄膜为类金刚石,表面非常光滑;而600℃下薄膜主要是石墨成分,表面粗糙.沉积能量大于140eV,800℃时薄膜表面分立着高度取向的、垂直衬底表面、相互平行的开口碳管.用高分辨电子显微镜看到了石墨平面的垂直择优取向,离子的浅注入和应力是这种优先取向的主要机理. 关键词： 非晶碳 表面形貌 质量分离低能离子束     

The dissociative excitation of cobalt atom even sextet levels in collisions of electrons with CoCl2 molecules

The cross sections of dissociative excitation of 46 transitions from cobalt atom even sextet levels in collisions of electrons with CoCl₂ molecules were measured at an exciting electron energy of 100 eV. The dissociative and direct CoI excitation cross sections were compared. The results were used to calculate the total cross sections of dissociative excitation of CoI levels at an electron energy of 100 eV. Possible reaction channels at low electron energies are discussed.     

Inverse photoemission spectroscopy of the unoccupied electronic structure of Na/Cu(110)

The unoccupied electronic states of Na thin films on a Cu(110) substrate have been measured by inverse photoemission spectroscopy (IPES). The IPES spectrum provides the intensity of the unoccupied states, which decreases with increasing Na coverage at off-normal incidence of the electron beam. The IPES spectra at 17 and 19 eV incident electron energies show a shift towards the Fermi level with increasing Na coverage for the peak at ∼7.8 eV.     

Measurements of total cross sections for positron-K and-Rb scattering

A modification of the apparatus that we have been using to measure absolute total cross sections (Q _T′s) for positron and electron scattering by alkali metal atoms has enabled us to more easily obtainQ _T′s at low energies (<10 eV). Our positron-K and-RbQ _T measurements now extend from 1 to 98 eV and we have made the correspondingQ _T measurements for electrons in the same system using the same technique (a beam transmission technique). Our measured positron-K and-RbQ _T values show a peak in the vicinity of 6 eV and decrease substantially as the positron energy is reduced below 6 eV, in sharp contrast to the results obtained in recent 5-state close-coupling approximation calculations. At higher energies (above 10 eV), we have found that the same calculations are in quite good agreement with our measured positron-K and-RbQ _T′s. Our measurements of the correspondingQ _T′s for electrons below 10 eV do not show a peak, but rather continue to increase as the energy is decreased.     

Formation of singly charged selenium ions in e-Se2 collisions

The excitation of spectral lines of Se II in e-Se₂ collisions has been studied for the first time. Forty-eight excitation cross sections of Se II spectral lines have been measured at an electron energy of 50 eV. Three optical excitation functions have been recorded in an electron energy range of 0–100 eV. Possible reaction channels at low electron energies are discussed.     

Photoelectron branching ratios and partial ionization cross-sections for CO and N2 in the energy range 18–50 eV

Branching ratios for photoionization to various ion states of CO and N₂ have been measured in the energy range 18–50 eV using an electron—electron coincidence technique at an ejected electron angle of 90°. The branching ratios, which show a marked variation with energy are shown to be in good agreement with the results of conventional photoelectron spectroscopy at those energies where quantitative PES measurements have been made. The results indicate that the effect of the assymmetry parameter, β, is relatively insignificant in the case of CO and N₂. Partial oscillator strengths have been determined from the branching ratios using recent total photoabsorption data. In addition to the expected one electron ion states, others are observed arising from two electron processes and the variation of these cross-sections with energy loss is also reported.     

Polymer surface erosion under an oxygen plasma stream

Erosion of a polyimide, polyethylene, polyethylene terephthalate, polymethylmethacrylate in oxygen plasma streams with the particle average energy of 20–30 eV is investigated. The erosion yield of the materials with respect to the polyimide is measured by the gravimetric method. The surface texture formed under etching is investigated using a scanning electron microscope. It is shown that a relative erosion yield takes similar values at energies of 5 eV and 20–30 eV and surface reliefs contain similar elements.     

Ionization function of magnesium for electron impact

Ionization cross sections for electron impact are measured with crossed beams of Mg atoms and pulsed electrons. Total numbers of particles are determined by the light emission of excited atoms. The ionization cross section obtained for electron energies from 7 eV to 60 eV has its maximum valueq _i (max)=7.8 · 10^?16 cm² at 12 eV electron energy.