Plasmon-phonon mechanism of nonradiative transition 2S 1/2 → 2P 1/2 in grazing scattering of a hydrogen atom at the surface of a solid

The probability of nonradiative transition 2S _1/2 ?? 2P _1/2 in the case of grazing scattering of a hydrogen atom with an energy on the order of 1 keV at an Au and LiF surface is calculated in the semiclassical theory of nonadiabatic transitions. The transitions are induced by the interaction of the atom with the fields of surface plasmons and optical phonons, respectively. A formalism is developed which makes it possible to exceed the approximation based on the method of electrical images of charges. The value of the probability of scattering at LiF is high enough for experimental observation of the effect. The probability of this transition is determined for the motion of the atom parallel to the metal surface.     

The mechanism of near-UV photoemission from H2O- induced electron states on alkali fluoride (100) faces

In common with LiF (100) [1] NaF (100) may adsorb small amounts of H₂O from atmospheric air at room temperature, causing photoemission upon near-UV excitation.The model proposed ascribes the photoemission to photon-stimulated field ionization of H₂O molecules bonded to surface cation-vacancies. The Coulomb field of the vacancy, enhanced due to a lattice-depolarization effect induced by the H₂O dipole, makes field ionization by excitation possible.     

Positrons — an alternative probe to electron scattering

Interaction of positrons with atoms and molecules differs from electron interaction due to the difference in polarity of the charge. This makes positrons an alternative tool to study atomic and molecular structure. Recent measurements of the total cross-sections for positron scattering at low energies on He, Ar, H₂, N₂, C₆H₆, C₆H₁₂, C₆H₇N carried out at Trento University [Karwasz et al., Acta Phys. Pol. 127, 666 (2005)] are discussed and compared to electron scattering results. All measured total cross-sections exhibit an increase with decreasing positron energy in the limit of zero energy; H₂, N₂, Ar, show regions of constant cross-section which are a few eV-wide, characteristic of scattering on a hard-sphere potential. Helium shows two resonant structures much below the positronium formation threshold. They may be attributed to virtual positronium formation. In conclusion, positron scattering is complementary to electron scattering. The total cross-sections do not show Ramsuaer minima but constant values, and new resonances appear.     

Determining potential energy constants for atom- and molecule-surface interactions

Methods developed for diatomic molecule spectroscopy are adapted for use in analysing the energies of atom-surface bound states in order to determine certain features of the surface-averaged potential energy function. In cases for which appropriate data are available, these simple graphical methods can yield a model-independent estimate of the potential well depth e, a value for the coefficient (C₃) of the long-range z^?3 term in the atom-surface potential, and estimates of both the total number of bound states and the energies of any unobserved levels lying near the dissociation limit. Application of these techniques to the data for atomic hydrogen on (001)LiF and NaF and for atomic helium on (001)LiF yielded: ? = 18.6(±1.0), 18.2 (±3.6) and 8.6(±0.8) meV, and C₃ = 250(±90), 180(±110) and 95(±40) meV ų, respectively. Application of this approach to the data for molecular hydrogen on (001)LiF led to a new set of vibrational assignments and showed that ? = 37(±4) meV, and that the H₂ and D₂ data of O'Keefe et al. and the H₂ binding energies which Tsuchida obtained from the data of Frisch and Stern are all internally consistent.     

Ion-induced kinetic electron emission from 6LiF,7LiF and MgF2 thin films

<正>Secondary electron yields for Ar~+ impact on ~6LiF,~7LiF and MgF_2 thin films grown on aluminum substrates are measured each as a function of target temperature and projectile energy.Remarkably different behaviours of the electron yields for LiF and MgF_2 films are observed in a temperature range from 25℃to 300℃.The electron yield of LiF is found to sharply increase with target temperature and to be saturated at about 175℃.But the target temperature has no effect on the electron yield of MgF_2.It is also found that for the ion energies greater than 4 keV,the electron yield of ~6LiF is consistently high as compared with that of ~7LiF that may be due to the enhanced contribution of recoiling ~6Li atoms to the secondary electron generation.A comparison between the electron yields of MgF_2 and LiF reveales that above a certain ion energy the electron yield of MgF_2 is considerably low as compared with that of LiF.We suggest that the short inelastic mean free path of electrons in MgF_2 can be one of the reasons for its low electron yield.     

Effect of Intercalated LiF on Interface Contact Characteristics of MoS2 Field Effect Transistor: A First-Principles Study

Based on the first-principles calculation, the effect of intercalated LiF on the contact characteristics of the interface between Au electrode and MoS₂ layer is studied. It is found that adding LiF film can change the contact type between metal electrode Au and MoS₂ layer from Schottky contact to ohmic contact, which is accompanied by interfacial charge transfer from LiF layer to MoS₂ layer and the downward movement of d (d_xy and d_z2) orbital of Mo atom and p (p_x and p_y) orbital of S atom to Fermi level. And the interlayer spacing between LiF layer and Au electrode has a great impact on the interface contact characteristics. The electric field effect and stress effect of interface contact of Au, LiF and MoS₂ (Au/LiF/MoS₂)is more obvious than that of interface contact of Au and MoS₂ (Au/MoS₂). Au/LiF/MoS₂ shows ohmic contact with the interlayer spacing between Au layer and LiF layer less than 3.05 Å and with the electric field less than 0.15 VÅ⁻¹, respectively, while Au/MoS₂ still shows N-type Schottky contact. These findings are helpful to control the contact resistance and have guiding significance for high performance MoS₂ field effect transistor and other electronic components.     

Atom-surface scattering with velocity-selected H and D atomic beams from LiF and NaF (001)

Hydrogen and deuterium atoms with an energy between 20 and 140 meV have been scattered from LiF and NaF (001) cleavage planes in UHV. The atomic beam was nearly monoenergetic by use of a slotted disk type velocity selector with a velocity resolution of $\text{Δv}\text{v}\text{=12\%}$ (FWHM). The scattering showed diffraction with a pronounced specular beam. The energies of several bound surface states have been determined by analysis of selective adsorption minima. These energies together with relative intensities of diffracted beams and specular intensity versus incident energy measurements may be represented by an interaction potential of Morse-type with additional terms which are periodic parallel to the surface. The parameters of the Morse-potential have been found to be: depth D=17.8 meV and reciprocal range $\text{κ=1.04}\text{A}\text{?}{}^{\mathrm{?1}}$ for H₁/LiF (001) and D = 17.9 meV, k = 1.15 Å^?1 for H₁/NaF (001); the additional periodic terms have relative strength β₁₀=0.04 and $\text{β}{}_{11}\text{≈}\text{β}{}_{10}\text{2}$ in the case of H ₁/LiF, but β₁₁≈β₁₀=0.03 in the case of H₁/NaF. From the observed influence of surface temperature on the position of selective adsorption minima the coefficient of thermal expansion of the topmost surface layer turns out to be approximately twice the bulk value.     

Vibrationally inelastic scattering of H∼ ions from H2, N2, O2 and CO2

State-resolved measurements are presented for vibrational excitation of H₂, N₂, O₂ and CO₂ by H^? impact in the collision energy rangeE _cm=20–180 eV and for scattering in the forward direction (0±0.5°). The data obtained from the measurements are the relative intensities and differential cross sections for vibrational excitation up toν′=4, the transition probabilitiesP _0→ν′ and the vibrational energy transferΔE _vib. For the systems H^?-H₂, N₂, O₂ the vibrational inelasticity increases in the order H₂-N₂-O₂. The mechanism for vibrational excitation in these systems is due to transient charge transfer from the H^? ion into antibonding orbitals of the target molecule which provides a bond stretching force during the collision. For H₂ and N₂, the results are compared with corresponding measurements for H⁺ scattering where the interaction mechanism is quite different. In the case of CO₂, vibrational excitation in forward scattering is caused primarily by the long-range dipole interaction. The spectra are very similar for H^?-CO₂ and H⁺-CO₂. Finer details are attributed to the influence of transient charge transfer and valence interactions.     

Surface analysis of LiF films by electron stimulated desorption and x-ray photoelectron spectroscopy

Electron stimulated desorption and X-ray photoelectron spectroscopy studies of evaporated LiF films show that the films are Li-rich in the surface region (Li/F atomic ratio ∼ 3:1) with a contaminant overlayer which includes hydrocarbons and water. Substantial amounts of hydrogen are also directly bonded to LiF surface. Exposure of the films to RF-activated Ar plasma results in reduction of the Li/F stoichiometry by a factor of ∼ 2 and the removal of carbon from the surface. H₂O and possibly other oxygen-containing species remain.     

Control of radiation spatial coherence under synchronous amplification in crystalline LiF:F 2 − amplifier

A technique of spatial coherence control, based on the synchronous amplification of a radiation in LiF crystals with F ₂ ^? color centers, is demonstrated. Spatial radiation distributions of stimulated Raman scattering (SRS) in oxide crystals were investigated under picosecond laser excitation. Low spatial radiation coherence was revealed for both the transient and quasi-stationary SRS. Spatially incoherent SRS was transformed to spatially coherent radiation as a result of phase—locked picosecond synchronous laser pumping of nonlinear Raman and LiF: F ₂ ^? crystals and the Stokes radiation amplification in the color center crystal.     

On the intrinsic nature of the blue luminescence in CaWO4

Luminescence and thermoluminescence properties of nominally pure LiF and CaWO₄ crystals are compared with those of the crystalline system LiF/CaWO₄. Evidence is found which clearly indicates that from the various emission bands of CaWO₄ crystals only the blue one is of intrinsic nature. Luminescence decay measurements are reported. Within the spectral range of the blue tungstate band in LiF/CaWO₄ four different decay times are obtained. This result is interpreted as arising from WO^2-₄ luminescence centres with different environments in the LiF host.     

Low energy atomic collisions

The Schrödinger equation for the system H⁺-H developed in a previous paper is considered using new expansion functions for electronic states obtained from H₂ ⁺ molecular ion electronic eigenfunctions by a unitary transformation. These new functions have the advantage of remaining orthonormal at all internuclear separations and asymptotically becoming symmetrized atomic hydrogen states. Although they are eigenfunctions of the H₂ ⁺ hamiltonian only in the limit of large internuclear distance, the effect of the H₂ ⁺ hamiltonian on these functions is readily found.

Due to coupling which remains non-zero in the limit of large interproton distance, each independent formal solution of the H⁺-H equations involves more than one expansion state in this limit. These solutions may be expressed asymptotically as column vectors multiplied by incoming or outgoing spherical waves.

The formal theory of scattering as developed by Gell-Mann and Goldberger has been utilized along with the projection formalism of Feshbach to obtain the correct asymptotic form of the scattering wave function. The procedure employed involves formulating the problem in terms of two-potential scattering and requires application of renormalization techniques for treating level shifts produced by the infinite-ranged coupling. This asymptotic form may be used in imposing scattering boundary conditions on numerical solutions of coupled equations for H⁺-H scattering.

Finally, it is shown that one cannot interpret coefficients of all outgoing spherical waves as scattering amplitudes. In addition, new interference phenomena are found to result from the presence of the infinite-ranged coupling. The present formalism is shown to reduce to the usual perturbed stationary-states method in the approximation that the infinite-ranged coupling is neglected.     

Observation of rotational resonances of ortho and para spin isomers of the H<Subscript>2</Subscript>O molecule in hexagonal ice using four-photon laser spectroscopy

Rotational resonances of ortho and para spin isomers of the H₂O molecule are observed in hexagonal ice using four-photon spectroscopy of coherent light scattering. It is experimentally shown that the resonant contribution to the four-photon scattering signal from para H₂O spin isomers in ice is about half as large as that in the liquid phase.     

50—5000 eV电子被C4H8O,C5H10O2, C6H5CH3和C4H8O2散射的总截面

使用电子被C, H和O原子散射总截面的实验数据, 利用修正后的可加性规则计算了能量为50—5000eV的电子被4个复杂大分子C₄H₈O, C₅H₁₀O₂, C₆H₅CH₃和C₄H₈O₂散射的总截面, 并将计算结果与实验结果及其他理论计算结果进行了比较. 结果表明, 即 关键词： 电子散射 可加性规则 总截面 几何屏蔽效应     

Dynamics of F2 centers in LiF: One phonon sideband and Raman scattering

The vibrational structure of the F₂⁺ emission in LiF was investigated, together with the resonant Raman scattering from the first excited electronic state. The one phonon sideband of the emission and the resonant Raman spectrum were found to be very similar, as expected for transitions involving non degenerate electronic states.     

Mössbauer characterization of biotites from zoned pegmatites

We have measured total cross sections for 5–302 eV positrons and 31–302 eV electrons scattered by atomic hydrogen using a beam-transmission technique. Atomic hydrogen obtained from a radio frequency (rf) discharge source flows into an aluminium scattering cell maintained at about 150 K to minimize recombination. Absolute total cross sections are obtained by making relative measurements for positrons and electrons scattering from H₂ and a known mixture of H and H₂, and then normalizing the measurements for positron-H₂ scattering to prior absolute measurements. Our total cross section measurements for positrons and electrons scattering from H are found to be merged to within 5% for energies from 31 to 302 eV.     

A perturbation theory study of H2 on LiF(0 0 1)

In response to recent helium atom scattering (HAS) results, Monte Carlo (MC) simulations and perturbation theory have been performed for H₂ on LiF(0 0 1). MC simulations predict that H₂ molecules form a series of structures, p(2 × 2), p(8 × 2), p(4 × 2) with coverages Θ = 0.5, 0.625 and 0.75, respectively, that are stable up to 8 K. This is in partial agreement with the HAS results that report c(2 × 2) and c(8 × 2) structures; they agree in terms of coverage and stability, but disagree in terms of symmetry. To reconcile the results of the simulations and experiments, the orientation of the adsorbed H₂ molecules was studied using perturbation theory. These calculations show that the adsorbed H₂ molecules are azimuthally delocalized and that the structures are c-type rather then p-type. The calculations also indicate that p-H₂ and helicoptering o-H₂ prefer cationic sites, while cartwheeling o-H₂ prefers anionic sites.     

Angular characteristics of processes of electron detachment from negative ions and hydrogen atoms in gases

The results of experimental measurement of spatial-angular distributions of hydrogen particles (H⁻, H⁰, H⁺) obtained in scattering of a collimated ribbon beam of H⁻ ions and H⁰(1s) atoms in He, Ar, Kr, Xe, H₂, O₂, and CO₂ gas targets for certain values of energy in the range from 0.6 to 15 MeV are reported. The experimental setup and the measurement procedure with an angular resolution of 5×10⁻⁶ rad are described. The angular characteristics of measured distributions, i.e., full width at half maximum and standard deviation, were determined. It is shown that the shape of distribution for a beam of hydrogen atoms produced by neutralization of H⁻ ions in a gas target varies with the type and thickness of the target, and the angular spread is smallest for the H₂ target. The variations in the shape of distribution are due to the contribution of scattering processes without changing the charge of particles.     

On the problem of water adsorption on alkali halide cleavage planes,investigated by secondary ion mass spectroscopy

In the epitaxial growth of thin metallic films, atomic beam scattering and other work on (001) cleavage planes of alkali halides, the problem of water adsorption (physically or chemically) and its consequences often arose. In this paper (001) planes of LiF, NaF and NaCl were investigated in a UHV apparatus by secondary ion mass spectroscopy (SIMS) under a variety of conditions: air and vacuum cleaved, with and without heat annealing, with and without exposure to water at various crystal temperatures. The main results are: cleaved crystal surfaces are free of water and hydroxide layers even in the submonolayer region under UHV conditions. Water vapour at a partial pressure of 10^?9 Torr will only adsorb on cooled cleavage planes: for LiF below 200 K, for NaF below 250 K and for NaCl below 260 K, the adsorption energy for the fluorides being in the 200 meV region. Thicker layers of adsorbed water on LiF (001) will grow according to the Weber-Volmer mechanism. The formation of hydroxide layers on the fluorides is possible only under extremely forced conditions. Direct evidence is given that the “active sites” for the adsorption of H₂O consist in surplus Li metal atoms on the surface. Heat treatment in UHV at temperatures up to the melting point for several hours removes these active sites and yields crystal surfaces which do not adsorb water even if cooled. Cleavage planes of the water soluble NaCl showed a somewhat different behaviour.