Energy scaling for model hamiltonians in molecule-surface scattering

It is shown that scattering by model potentials for gas-surface interactions scales exactly with energy. The semiclassical sudden approximation preserves this property. Some applications are discussed in the framework of the sudden approximation.     

Electron-hole pair condensation in a graphene bilayer

The pairing of electrons and holes due to their Coulomb attraction in two parallel, independently gated graphene layers separated by a barrier is considered. At a weak coupling, there exists the BCS-like pair-condensed state. Despite the fact that electrons and holes behave like massless Dirac fermions, the problem of BCS-like electron—hole pairing in the graphene bilayer turns out to be rather similar to that in usual coupled semiconductor quantum wells. The distinctions are due to the Berry phase of electronic wavefunctions and different screening properties. We estimate the values of the gap in a one-particle excitation spectrum for different interlayer distances and carrier concentrations. The influence of the disorder is discussed. At a large enough dielectric susceptibility of the surrounding medium, the weak coupling regime holds at arbitrarily small carrier concentrations. Localized electron—hole pairs are absent in graphene, thus the behavior of the system versus the coupling strength is cardinally different from usual BCS—BEC crossover. The text was submitted by the authors in English.     

Quantum theory of molecule-surface scattering: Rotational transitions

The quantum theory of atomic scattering from hard surfaces, previously developed, is extended to cover molecular scattering, taking into account rotational transitions. Two methods (a perturbative and a nonperturbative approach) are considered: the former is applied to H₂ scattering, the latter to HD scattering from LiF. A comparison with recent experimental data gives satisfactory results when a potential well of 440 K and a corrugation amplitude of 0.17 Å are assumed for the H₂ — LiF interaction.     

Electron-hole pair production and the structure of ultraviolet isochromats

Isochromat spectra are generally considered to reflect the density of empty one-electron states in the anode material of the x-ray tube employed. Comparison of various measurements with theoretically determined density-of-states functions indicate that such a first-order picture applies only immediately above threshold. In this paper, we show that electron-hole pair production contributes considerably at higher energies above threshold. Taking this effect into account removes most of the existing discrepancy between theory and experiment in this region. Results are presented for the 5d transition metals hafnium, tantalum, and tungsten.     

Theory of molecule-surface scattering at thermal and hyperthermal energies

A theoretical model of molecule-surface scattering is developed which includes energy and momentum transfers between the surface and projectile for both translational and rotational motions and internal mode excitation for the projectile molecule. The translation and rotation motions are treated in the classical limit, while a quantum treatment for internal vibrational mode excitation is used. The results of calculations are compared with recent high-precision measurements of the scattering of a beam of C(2)H(2) molecules from a clean, ordered LiF(001) surface at energies of up to nearly 1 eV. The calculated results for angular distributions and rotational excitations are in good agreement with experiment.     

Off-resonant excitation of molecular vibrations in liquids,investigated by picosecond probe scattering

The coherent excitation of molecular vibrations in liquids via off-resonant stimulated Raman amplification is shown to consist of two parts with vastily different spectral and temporal properties. One component situated at the frequency difference _L –e _S of the driving fields follows almost instantaneously the system response function. The second part at the eigenfrequency e ₀ carries information on the vibrational dephasing. Detecting both frequency components simultaneously by coherent anti-Stokes probe scattering a novel 2-channel method is demonstrated. The vibrational dephasing time of thev ₃-mode of CH₃I is directly measured to beT ₂=2.6±0.15 ps. This result gives experimental support to the deconvolution techniques required in conventional spectroscopy.     

Simple surface interaction mechanism for magnetic substate excitation by inelastic scattering

A simple model is presented to explain the marked behaviour of the de-excitation γ-pattern in certain recent inelastic -particle scattering experiments from even nuclei (0⁺→2⁺ excitation). The model is a slight generalization of the well-known ring-locus model in that it is based on a ring locus of finite angular width. The dependence of the inelastic angular distribution and the reaction-plane γ-pattern on the parameters of the ring locus is discussed.     

Electron-hole mechanism for the decomposition of zinc sulfide in a hydrogen atmosphere

Chemical analysis is used to study the structural changes on the surface of a ZnS-Cu, Pb phosphor arising in the course of photoluminescence in molecular hydrogen and radical-recombination luminescence excited by hydrogen atoms. The photolysis of zinc sulfide and its decomposition by an atomic-molecular mixture of hydrogen were observed. A mechanism for the reduction involving electrons, holes, and chemisorbed hydrogen is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 42–46, October, 1976.     

On rail vehicle vibrations induced by track unevenness: Analysis of the excitation mechanism

This paper deals with the analysis of the vibrations induced on the carbody of a rail vehicle by track unevenness. Attention is focused on the excitation mechanism of the carbody vibration modes, which has a strong influence on the vehicle's comfort. At first the problem is investigated through a simple three-degree-of-freedom analytical model, and the phenomenon of the critical velocities is analysed, pointing out how both rigid and flexible carbody vibration modes can be excited to a different extent, depending on the vehicle speed, and how they combine to produce the final carbody accelerations. Then the dynamic response of a real vehicle running on irregular track is simulated through a more detailed multibody model, suitable for quantitatively reproducing its dynamic behaviour in the 0-25 Hz frequency range. The 68 degrees-of-freedom of this model correspond to 35 rigid vibration modes of the vehicle components (carbody, bogie frames and wheelsets), plus the 33 carbody flexible modes which fall into the frequency-range of interest. In the last part of the paper, the obtained numerical results are compared to the experimental data collected during on-line tests, showing how the adopted numerical model accurately simulates the dynamic behaviour of the real vehicle at the different velocities, with very good agreement. The results presented in the paper demonstrate that the excitation of the flexible modes may have a decisive effect on carbody accelerations and that introducing carbody flexibility in the vehicle model turns out to be unavoidable for properly predicting a rail vehicle comfort performance.     

Femtosecond laser control of intramolecular vibrations in a liquid

Optical control of coherent intramolecular oscillations in chloroform CHCl₃ and dimethyl sulfoxide (CH₃)₂SO is attained experimentally under normal conditions by means of femtosecond polarization spectroscopy. Nonresonant excitation of the medium is accomplished by a sequence of two linearly polarized laser pulses. The state of the medium is probed by the third pulse via the optical Kerr effect. We show that control over the vibrational dynamics of molecules on a sub-picosecond scale can be achieved by varying the delay between the excitation pulses and their relative intensity.     

Selective excitation in kaon-nucleus inelastic scattering

The K⁺ meson (kaon) inelastic excitation of low-lying (E_x = 0–15 MeV) T = 0 collective states in ¹⁶O is theoretically studied as a function of energy and momentum transfer. The distorted wave impulse approximation is used to calculate angular distributions and total inelastic cross sections for exciting the first J^π = 2⁺, 3^?, 4⁺ and 5^? states at lab energies from threshold to 400 MeV. The distortions are represented in a Kisslinger-type optical potential constructed from elementary K⁺-nucleon amplitudes. Total nuclear elastic and reaction K⁺-nucleus cross sections are computed to demonstrate sensitivity to choice in K⁺-nucleon amplitudes. Fermi motion effects are also assessed using a simple averaging procedure. The weak absorption character of the kaon is reflected in the inelastic calculations which predict selective excitation of low spin states at low momentum transfer and high spin states at high momentum transfer.     

Hard pair production in large-angle Bhabha scattering

The cross section of hard pair production in large-angle Bhabha scattering is calculated in the leading and next-to-leading logarithmic approximations. Eight regions of the collinear kinematics, when the final particles imitate a process of the 2 → 2 type, and three semicollinear regions, when the final particles imitate a process of the 2 → 3 type, are considered. Analytical formulae for differential cross sections are presented.     

Influence of intramolecular vibrations in charge redistribution at the pentacene-graphite interface

We have investigated the relation between the intramolecular vibrational modes of pentacene and the charge redistribution at the pentacene-graphite interface by using high-resolution electron-energy-loss-spectroscopy. The three main vibrational peaks shift to lower energies as the pentacene film thickness decreases. In order to discuss this energy shift, we have calculated the vibrational energies of a free pentacene molecule by changing its charge state. We have also calculated the vibrational energies of a pentacene molecule adsorbed on a graphite sheet by changing the pentacene-graphite distance. Taking the experimental and calculation results into account, we conclude that the observed energy shifts result from an intramolecular charge redistribution. The present results indicate that the effect of an intramolecular charge redistribution is essential to discuss the origin of an energy shift observed in a vibrational study of an organic molecule/substrate interface.     

Two-phonon scattering with zero-point vibrations of atoms

We discuss a method of calculating two-phonon thermal diffusion scattering (I₂) in monatomic cubic crystals for temperatures below the Debye temperature. The quantity I₂ was calculated and measured for an Mo single crystal over a wide region of the reciprocal space at different temperatures. It is shown that good agreement between the calculated and experimental results can be obtained only when the quantum correction to the phonon energy is included.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 82–86, February, 1989.