Semiclassical analysis of the anisotropic total cross section in thermal energy collisions between a non-spherical and a spherical atom

Thermal energy atomic collisions of the typeA(² P _3/2)+B(¹ S ₀) are discussed to clarify the dependence of the orientational anisotropy Δσ in the total cross section σ on the relative velocityv. Simple expressions for Δσ/σ are derived by semiclassical treatments designed to be appropriate for large spin-orbit interactions. The Δσ/σ oscillates withv around its average value. The velocities giving the extrema in the oscillating curve are almost solely determined by the isotropic part \(\bar V\left( R \right)\) in the interaction potential, while the amplitude of the oscillation depends sensitively on the anisotropic interaction nearR _mi, whereR _mi is the location of the minimum in \(\bar V\left( R \right)\) . The oscillation-averaged Δσ/σ is independent of the relative velocity and proportional to the anisotropy in the van der Waals interaction.     

Velocity dependence of the total cross section for helium-argon,neon-krypton and neon-xenon scattering

Velocity dependences of the total collision cross sections for scattering of helium by argon, neon by krypton and neon by xenon were measured using a time of flight method of velocity selection. For neon scattering glory oscillations were observed from which values of ?r_m (the product of the well depth and the internuclear separation at the minimum) were determined for various Lennard-Jones potentials. Comparison is made with the results from diffusion experiments and differential scattering measurements.     

Measurements of the influence of the angular dependent intermolecular potential on the total collision cross section for molecular hydrogen and various scattering partners

For H₂ molecules colliding with Ar, Kr, Xe, CO₂, N₂ and SF₆ the total collision cross section is measured, applying state selection to the H₂ molecule. The preferential orientation of the H₂ molecule in the J = 1, m_J = 0 state is used to determine the influence of the angular dependent part of the intermolecular potential.     

Absolute total electron-scattering cross section of H2S

Measurements made on H₂S in the energy range 1.3–70 eV give total electron-scattering cross sections which exhibit two distinct features: a shoulder at about 2.3 eV and a very broad hump centered at 9 eV with a maximum value of 40 × 10⁻²⁰ m². These results are in agreement with those calculated by Jain and Thompson and are completely different from the experimental data of Sokolov and Sokolova.     

Determination of the H2-H2 potential from absolute integral cross section measurements

Absolute integral cross sections have been measured for n-H₂-n-H₂ at primary particle velocities between 1270 and 9650 m/sec with a 77 °K target. The number density of target particles was determined with an accuracy of 1% by the method of dynamic expansion. The velocity dependence of the integral cross section was also measured for the isotopic systems n-D₂-n-H₂ and n-D₂-n-D₂ at primary beam velocities between 280 and 1170 m/sec with a 15 °K target. All available integral cross section data are compared with literature potentials. A new potential is proposed, which exhibits one bound state for H₂-H₂.Dedicated to Professor H. Hartmann on his 60^th birthday.     

The total ionisation cross section and the large angle differential cross section for the system He(21S, 23S) + Ar,N2

We have measured the total ionisation cross section Q_ion(g) and the large angle differential cross section σ(θ, g) for the system He(2¹S, 2³S)+ Ar, N₂ at energies 0.05 < E_c.m. (eV) < 6. This energy range is covered by applying two different discharge sources for the production of metastable atoms. In the atomic beam the He(2³S) level is most abundant with relative populations C = 0.91±0.01 and C = 0.96±0.01 for thermal energy range and the superthermal energy range, respectively. A quench lamp is used for the quenching of the (2¹S) level population. In the thermal energy range, σ(θ, g) and Q_ion(g) are in fair agreement with experimental results of other authors and with calculated cross sections based on the optical potential given by Siska. In the superthermal energy range, the He(2³S)+Ar optical potential is modified to describe our experimental data. The slope of the repulsive branch of the real potential is increased for r < 2.85 Å; in the imaginary potential a saturation to a constant (or even decreasing) value for internuclear distances less than 2.5 Å is introduced.     

Variationally stable calculations for molecular systems: polarizabilities and two-photon ionization cross section for the hydrogen molecule

The variationally stable method of Gao and Starace [B. Gao and A. F. Starace, Phys. Rev. Lett. 61, 404 (1988); Phys. Rev. A 39, 4550 (1989)] has been applied for the first time to the study of multiphoton processes in molecular systems. The generalization in theory is presented, as well as the calculation of properties such as the static and dynamic polarizabilities of the hydrogen molecule and the generalized two-photon ionization cross section. The Schwinger variational iterative method [R. R. Lucchese and V. McKoy, Phys. Rev. A 21, 112 (1980)] has been applied in the achievement of the photoelectron wave function, while a Hartree-Fock representation has been used for the target. This research has been motivated by the scarceness of ab initio calculations of molecular multiphoton ionization cross sections in the literature.     

Calculation of cross sections for positron-Ar collisions

The three-body classical trajectory Monte-Carlo (CTMC) method is used to investigate positron-argon atom collisions. The total ionization cross sections are presented along with singly and doubly differential cross sections. The existence of the cusp-like peak in the triply differential electron and positron spectra at positron impact is predicted.     

Multipole interaction in high energy approximation. Calculation of the total collision cross section

For molecules of cylindrical symmetry the total collision cross section is calculated in the high energy approximation, taking into account various long range intermolecular interactions. Results are obtained for the limiting cases that the rotational motion of the molecules is fast or slow with respect to the duration of the molecular interaction. The assumption is made that either one multipole term of the intermolecular potential predominates or that all multipole terms can essentially be treated as a small perturbation of the van der Waals r^?6 interaction.     

Vibrational excitation cross section and V → T rate constants in molecular hydrogen

Vibrational excitation cross sections are computed for H₂ gas at various collision energies and between the lower-lying levels of the molecule. Only V → T processes are considered, while a quantum mechanical treatment via spherical potentials is implemented and applied. Various suggested potentials have been used and the corresponding results critically compared. The low-energy behaviour of the σ_{1 → 0} cross sections, and their orders of magnitude, appear to be in agreement with “experimental” deconvoluted data. Relaxation and excitation rates are computed and the K_1-0 (T) is examined over a wide temperature range to allow comparison with experiments. While the high temperature results are in fair agreement with the available data, the pure V - T mechanism of the present model seems to overestimate low-T rates as a consequence of the increase of (1 → 0) computed ross sections when bringing E_coll very near the lowest, υ = 1, threshold.     

Comparison of measured total integral cross sections for noble gas dimer-atom systems with IOSA calculations

Experimentally determined total integral cross sections for noble gas dimer-atom systems, involving the energetically accessible channels of excitation, exchange and dissociation are compared with theoretical values calculated using a simple formula. This simple formula is derived using the multichannel optical theorem and the infinite order sudden approximation (IOSA). The potential is well approximated by the sum of known pair potentials. Setting the IOSA parameters L and K equal to the angular momentum and the wave number of the initial channel, respectively, the largest error between the experimental and theoretical results is about 10%. We noticed that a proper choice of the relation between K and the collision energy greatly reduces this error; a possible explanation cannot be given.     

Observation of the orbiting backward peak in the differential cross section for Na(2P)-Hg collisions

A typical feature of collisions governed by attractive and repulsive forces is the orbiting phenomenon. Most clearly these collisions are visible as a backward peak in the differential cross section. In this work we report the first observation of this backward peak in the interaction of neutral atoms, specifically for Na(^2P interacting with Hg.     

Correlation of the total cross section for electron scattering by molecules with 10–22 electrons, and some molecular parameters at intermediate energies

Accurate experimental values of the total cross section (σ_T) for electron scattering by molecules (CH₄, NH₃, N₂, CO and CO₂) in the energy range 0.5 to 5 keV have been analysed to obtain correlations with molecular properties. A formula for σ_T, as a function of the number of target electrons and molecular polarizabilities, has been deduced. This empirical expression reproduces to within 6% our experimental results for the abovementioned molecules and has been used to obtain results for other molecular targets with 10 to 22 electrons. Total cross sections show reasonable agreement with the experimental and theoretical data for this energy range.     

Comparative study of measured and critically evaluated resonance integral to thermal cross section ratios,III

In the present paper, a tabulation is given of recommended Q₀-values [the ratio of the resonance integral (I₀) to the 2200 m·s^?1 cross-section (δ₀)] for 107 (n, γ) reactions of interest in NAA, including a revision and updating of formerly published results for 57 isotopes. The values were either critically evaluated from literature, or-in the majority of cases — experimentally determined according to the Cd-ratio method, with a correction for a non-ideal epithermal neutron flux distribution. These Q₀-measurements were performed at INW, Gent, at KFKI, Budapest, and occasionally at Risø. A comparison is made with results obtained by other workers or with values derived from δ₀'s and I₀'s quoted in recent compilations.     

Selection rules for self-formation in the molecular nanotechnology

Self-formation in molecular nanotechnology is based on interaction between organic and biological molecules. Selection rules of the distinct interactions of the symmetric and topologically similar molecule self-formation are investigated.     

Close-coupling elastic and inelastic integral and differential cross sections for Li+ + H2 collisions

The quantum mechanical close-coupling formalism is applied to the study of elastic and rotationally inelastic Li⁺ + H₂ collisions making use of the Kutzelnigg-Staemmler-Hoheisel potential energy surface. Integral and differential cross sections for j = 0 → 0 and j = 0 → 2 are obtained in the collision energy range 0.2 to 0.9 eV and for j = 1 → 1 and j = 1 → 3 at 0.6 eV. A rainbow structure is observed in both the elastic and inelastic angular distributions and a quenching of the fast oscillations is found in the cross sections for j = 1 initially compared to the case j = 0 initially. At 0.6 eV. the calculated quantum mechanical angular distributions are compared to those from a classical trajectory calculation using the same surface and to the experimental ones. The dynamics of rotational excitation in the Li⁺ + H₂ system is contrasted to rotational excitation in systems for which the atom-diatom interaction is predominantly repulsive.     

Anisotropic intermolecular potentials for NOAr and NOKr from total differential cross section measurements

Angular distributions of NO scattered off Ar and Kr in the thermal energy range are measured in crossed molecular beam experiments. Information on the anisotropy of the interaction potential is obtained from the damping of the rainbow structure within the framework of the infinite-order-sudden approximation. The derived potential energy surfaces are compared with the results of a previous experimental investigation and with the PES of the related O₂-and N₂-rare gas systems.     

Schematic model for the differential cross section in ion-pair formation

An analytic approximation to the differential cross section is derived for ion-pair formation in a model two-state (“turned-on Coulomb”) system. The result gives insight into the structure observed in recent experiments.     

H-D2 potential from absolute integral cross section measurements at thermal energy

The H-D₂ absolute integral cross sections were measured for hydrogen atom energies of 1.8–330 meV with high accuracy. A well defined potential was obtained for separations r ≧ 4a_o with the aid of a realistic three-parameter model for the potential.     

Quantum stereodynamics of Li + HF reactive collisions: the role of reactants polarization on the differential cross section

A complete quantum study for the state-to-state Li + HF(v,j,m) → LiF(v',j',Ω') + H reactive collisions has been performed using a wave packet method, for different initial rotational states and helicity states of the reactants. The state-to-state differential cross section has been simulated, and the polarization of products extracted. It is found that the reactivity is enhanced for nearly collinear collisions, which produces a vibrational excitation of HF, needed to overcome the late barrier. It is also found that LiF(v' = 0) products are preferentially forward scattered, while vibrationally excited LiF(v' = 1 and 2) are backward scattered. These results are interpreted with a simple reaction mechanism, based on the late character and bent geometry of the transition state, originating from a covalent/ionic crossing, which consists of two steps: the arrival at the transition state and the dissociation. In the first step, in order to get to the saddle point some HF vibrational excitation is required, which favors head-on collisions and therefore low values of m. In the second step a fast dissociation of H atom takes place, which is explained by the ionic Li(+)F(-)H character of the bent transition state: the FH(-) is repulsive making that H depart rapidly leaving a highly rotating LiF molecule. For the higher energy analyzed, where resonances slightly contribute, the orientation and alignment of product rotational states, referred to as reactants frame (with the z-axis parallel to k), are approximately constant with the scattering angle. The alignment is close to -1, showing that j' is perpendicular to k, while starting from initial states with well defined rotational orientation, as states with pure m values, the final rotational are also oriented. It is also found that when using products frame (with the z'-axis parallel to k') the rotational alignment and orientation of products varies a lot with the scattering angle just because the z' axis changes from being parallel to anti-parallel to k when varying from θ = 0 to π.