Enhanced creation of dispersive monolayer phonons in XePt(111) by inelastic helium atom scattering at low energies

Conditions likely to lead to enhanced inelastic atomic scattering that creates shear horizontal (SH) and longitudinal acoustic (LA) monolayer phonons are identified, specifically examining the inelastic scattering of (4)He atoms by a monolayer solid of XePt(111) at incident energies of 2-25 meV. There is strong inelastic scattering for both dispersive phonon branches (SH and LA) of the monolayer at incident energies below 8 meV. Several improvements enable more complete wave packet calculations of the inelastic scattering than in previous work. Long propagation times are made feasible by adding an absorbing potential at large distance. The times now extend to beyond 100 ps and enable a clarification of processes involving transient trapping of the He atoms. The wave packet is made more monochromatic by significantly increasing the spatial width of the initial Gaussian shape. The narrower energy distribution in the incident beam then enables a demonstration of strong energy dependence of the scattering over a scale of less than 0.3 meV.     

Calculation of inelastic helium atom scattering from H2/NaCl(001)

The one-phonon inelastic low energy helium atom scattering theory is adapted to cases where the target monolayer is a p(1 × 1) commensurate square lattice. Experimental data for para-H(2)/NaCl(001) are re-analyzed and the relative intensities of energy loss peaks in the range 6 to 9 meV are determined. The case of the H(2)/NaCl(001) monolayer for 26 meV scattering energy is computationally challenging and difficult because it has a much more corrugated surface than those in the previous applications for triangular lattices. This requires a large number of coupled channels for convergence in the wave-packet-scattering calculation and a long series of Fourier amplitudes to represent the helium-target potential energy surface. A modified series is constructed in which a truncated Fourier expansion of the potential is constrained to give the exact value of the potential at some key points and which mimics the potential with fewer Fourier amplitudes. The shear horizontal phonon mode is again accessed by the helium scattering for small misalignment of the scattering plane relative to symmetry axes of the monolayer. For 1° misalignment, the calculated intensity of the longitudinal acoustic phonon mode frequently is higher than that of the shear horizontal phonon mode in contrast to what was found at scattering energies near 10 meV for triangular lattices of Ar, Kr, and Xe on Pt(111).     

Reactive scattering of a helium seeded oxygen atom beam

A high pressure microwave discharge source operating with a dilute mixture of O₂ in He has been used to produce a supersonic nozzle beam of O atoms seeded in He. This source has been used to study the reactive scattering of O atoms with Cl₂ and CS₂ molecules at an initial translational energy E = 38 kJ mol^?1. Velocity distribution of reactive scattering were measured over a wide angular range by cross-correlation time-of-flight analysis. The O + Cl₂ reaction proceeds via a short-lived collision complex while the O + CS₂ reaction follows a stripping mechanism.     

Quantum and classical study of surface characterization by three-dimensional helium atom scattering

Exact time-dependent wavepacket calculations of helium atom scattering from model symmetric, chiral, and hexagonal surfaces are presented and compared with their classical counterparts. Analysis of the momentum distribution of the scattered wavepacket provides a convenient method to obtain the resulting energy and angle resolved scattering distributions. The classical distributions are characterized by standard rainbow scattering from corrugated surfaces. It is shown that the classical results are closely related to their quantum counterparts and capture the qualitative features appearing therein. Both the quantum and classical distributions are capable of distinguishing between the structures of the three surfaces.     

Determination of elastic shear constants of polyethylene at room temperature by inelastic neutron scattering

The elastic shear constants of both the amorphous and crystalline regions of polyethylene have been measured at room temperature. A newly developed method is used which allows the determination of elastic constants from the coherent inelastic neutron scattering of polycrystals. A deuterated and partially oriented sample is investigated on a triple-axis spectrometer and a time-of-flight instrument. The elastic constants of the crystalline regions of polyethylene are c₄₄ = 2.1 ± 0.3, c₅₅ = 2.2 ± 0.3, c₆₆ = 1.8 ± 0.2, and c′ = 1/4(c₁₁ + c₂₂ ? 2c₁₂) = 0.92 GPa. The shear modulus of the amorphous regions is obtained as G = 0.55 ± 0.03 GPa. In connection with other experimental results the elastic constant matrix is given and compared with theoretical estimates. With simple models, macroscopic moduli are calculated which are in good agreement with published experimental data.     

Separation of the low-frequency mode from the inelastic continuum scattering of a sers active electrode

An I₂ vapor cell and an optical multichannel analyzer allowed the simultaneous detection (from −60 to +60 cm⁻¹) of inelastic scattering from a Ag electrode (electrochemically roughened in 0.1 M KI). Separation of the low-frequency mode, which peaks at ≈7 cm⁻¹, from the inelastic continuum, which increases in intensity as ∣Δω∣→ 0, was accomplished by dividing the data (spectra) by the Bose-Einstein function. The potential dependence of the low-frequency mode and its spectral profile were investigated.     

The helium atom in a semi-infinite space limited by a paraboloidal boundary

The ground state of the helium atom in a semi-infinite space limited by a paraboloidal boundary, with the nucleus at the focus, is evaluated variationally. The pressure and electric dipole moment of the semiconfined atom are evaluated as functions of the position of the boundary. Estimates of the boundary positions and pressure variations at the successive ionization thresholds are established. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65 : 269–275, 1997     

Excitation spectrum of a multielectron atom

The spectrum is derived by the transition-matrix method, which is suitable for one-particle and collective excitations. The configuration structure of the transition matrix is discussed, together with the relation to the Hartree-Fock approximation.     

Anharmonicity in a fragile glass-former probed by inelastic neutron scattering

Within the overall understanding of the glass transition, the relationship between microscopic dynamics and fragility is still to be clarified. Decalin is an organic glass former, for which a cis/trans mixture exhibits the highest known degree of fragility in a molecular system. It is therefore an ideal system for the investigation of microscopic dynamics in fragile systems. In the present study, the microscopic dynamics of pure cis-decalin has been measured by inelastic and quasi-elastic incoherent neutron scattering, giving the single particle self-correlation function. The fast relaxation dynamics and low-frequency vibrational modes are reported here. Both in the glass and in the crystal the vibrations show strong anharmonic behavior. In the glass phase, the short time microscopic dynamics evolve rapidly with temperature, however do not exhibit any significant change around the glass transition temperature T(g). The elastic intensity provides a measure of the mean square displacements which are comparable to those measured in other fragile glass formers, in particular, the archetypical fragile glass former orthoterphenyl. It appears that the microscopic relaxation gets unfrozen, relative to T(g), at much lower temperature than in other fragile systems.     

Proposed standard inelastic differential electron scattering cross sections; Excitation of the 21 P level in He

Differential and integral cross section data for electron-impact excitation of the 2¹ P level in He have been critically reviewed. Experimental and theoretical results have been compared and a set of differential cross sections at 20° scattering angle in the 25 to 500 eV impact energy range has been deduced based on all available information. It is proposed that this set of data represents the most accurate inelastic differential cross sections available at the present time and could be used as a secondary standard for normalization of cross sections.     

Potential-dependent measurements of the low-frequency mode,the inelastic continuum,elastic scattering and sers from a Ag electrode

An I₂ vapor absorption filter attenuated the elastically scattered light and allowed the separation of the low-frequency mode from the large inelastic continuum signal near 0 cm⁻¹. The potential dependence of the low-frequency mode, the inelastic continuum, the SERS of adsorbates, and the elastic scattering was measured from a Ag electrode. All the inelastic scattering contributions have similar potential dependences while the elastic scattering does not.     

The determination of tungsten in steels by neutron inelastic scattering

A simple, rapid and non-destructive method is described for determining tungsten in steels over the concentration range 0.08 to about 3% by mass. The method uses the low energy gamma-rays induced in¹⁸³W by neutrons from the 3 MeV bombardment of beryllium with deuterons. The precision is normally better than 10%.     

Elastic and inelastic electron tunneling spectroscopy of a new rectifying monolayer

Rectification of electrical current was observed in a Langmuir-Schaefer monolayer of fullerene-bis[ethylthio-tetrakis(3,4-dibutyl-2-thiophene-5-ethenyl)-5-bromo-3,4-dibutyl-2-thiophene] malonate, Au electrodes at room temperature (there are two regimes of asymmetry, at lower bias, i.e., between 0 and +/-2 V, and at higher bias), and also between Pb and Al electrodes at 4.2 K. The latter experiment was coupled with second harmonic detection of the second derivative of the current with respect to voltage (d2I/dV2). The d2I/dV2 spectrum shows intramolecular vibrations, and also two antisymmetric broad bands, centered at +/-0.65 V, due to resonant electron tunneling between the Fermi level(s) of the electrodes and the lowest unoccupied molecular orbital of the molecule.     

Calculation of the electric susceptibilities of the helium atom

We calculated the static and the dynamic dipole polarizabilities and the static quadrupole polarizability of the helium atom. The results are α = 0.2070 × 10^?24 cm³ for the static dipole polarizability and γ₄ = 0.1038 × 10^?40 cm⁵ for the static quadrupole polarizability.     

Excitation and decay processes in helium clusters studied by fluorescence spectroscopy

Excitation and decay processes of helium clusters are investigated with fluorescence methods. The results differ remarkably from that obtained for the heavier rare gas clusters. They are discussed in view of the unusual structural and electronic properties of helium.     

Oscillating term of the inelastic scattering factor

Formulas have been derived to calculate the factor of inelastic scattering of fast electrons by a molecular gas S(s) with inclusion of the chemical binding effect. In contrast to the free atoms approximation, the function S(s) has an oscillating term.Institute of High Temperatures, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 3, pp. 36–40, May–June 1993Translated by L. Smolina     

Bound states of helium atom in dense plasmas

We have obtained the bound 1s^{2 1}S, 1s2s ^1,3S, and 1s2p ^1,3P states energies of helium atom in dense plasma environments in accurate variation calculations. A screened Coulomb potential to represent the Debye model is used for the interaction between the charged particles. A correlated wave function consisting of a generalized exponential expansion has been used to take care of the correlation effect. The 1s^{2 1}S, 1s2s ^1,3S, and 1s2p ^1,3P states energies along with the ionization potential, the energy splitting between the 1s2s ³S, and 1s2s ¹S states, transition energies between the ground state and low‐excited states of He estimated for various Debye lengths, are reported. The results show high degree of accuracy even under strong plasma conditions. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Quantum dynamics of inelastic scattering with a moving grid

The time‐dependent discrete variable representation (TDDVR) of a wave function with grid points defined by the Hermite part of the Gauss–Hermite (G‐H) basis set introduces quantum corrections to classical mechanics. The grid points in this method follow classical trajectory and the approach converges to the exact quantum formulation with sufficient trajectories (TDDVR points) but just with a single grid point; only classical mechanics performs the dynamics. This newly formulated approach (developed for handling time‐dependent molecular quantum dynamics) has been explored to calculate vibrational transitions in the inelastic scattering processes. Traditional quantum mechanical results exhibit an excellent agreement with TDDVR profiles during the entire propagation when enough grid points are included in the quantum‐classical dynamics. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Semiclassical initial value calculations of the collinear helium atom

Semiclassical calculations using the Herman-Kluk initial value treatment are performed to determine energy eigenvalues of bound and resonance states of the collinear helium atom. Both the eZe configuration (where the classical motion is fully chaotic) and the Zee configuration (where the classical dynamics is nearly integrable) are treated. The classical motion is regularized to remove singularities that occur when the electrons collide with the nucleus. Very good agreement is obtained with quantum energies for bound and resonance states calculated by the complex rotation method.