Bernard Zygelman

Stimulated Radiative Association of He and H

The enhancement of the rate coefficient for the radiative association of He and H+ to form HeH+ arising from stimulated emission due to a blackbody radiation field is calculated. The effects on the fractional abundance of HeH+ in the early universe, in supernova ejecta, and in planetary nebulae are small. There may occur some enhancement in the abundance of HeH+ in quasar broad-line clouds.

Appearance of gauge potentials in atomic collision physics

Abstract The dynamical equations obtained from the method of perturbed stationary states (PSS) are shown to be formally equivalent to the dynamical equations of a particle with N internal degrees of freedom minimally coupled to U ( N ) static gauge potentials. Several examples are given that illustrate the appearance of non-abelian and magnetic monopole gauge potentials in simple systems. Advantages of expressing the PSS equations as a gauge theory are discussed.

Charge Transfer in Collisions of C+ with H and H+ with C

Charge transfer rate coefficients for collisions of C+ with H and H+ with C are presented for temperatures from 30,000 to 107 K and from 10 to 107 K, respectively. The rate coefficients were calculated from recommended cross sections deduced in a recent theoretical and experimental investigation that took into account previous measurements. Nonadiabatic radial coupling is the dominant mechanism for both reactions above ~50,000 K, but for lower temperatures the reaction of H+ with C proceeds primarily by radiative charge transfer. Implications, due to the magnitude of the rate coefficients, for various astrophysical environments are discussed.

Radiative Charge Transfer in Collisions of O with He

Radiative charge transfer has been investigated for collisions of O with He+ with both a fully quantum mechanical theory and an optical potential method. Cross sections and rate coefficients are presented for the process O(3P) + He+ → O+(4So,2Po,2Do) + He + ω and are compared to those of direct charge transfer. The relative collision energies considered range from 0.1 meV to ~3 eV with a semiclassical extension to 10 keV and temperatures between 10 and 2.0 × 106 K. The results demonstrate that radiative charge transfer is the dominant process over the energy and temperature region considered. Total emission spectra for the two strongest of the ten possible transitions are given for several collision energies, and the origin of resonance-like structures in the spectra is discussed.

Hydrogen–antihydrogen scattering in the Born–Oppenheimer approximation

We calculate the low-energy scattering between hydrogen and antihydrogen atoms in their ground states using the Born–Oppenheimer approximation. Improved results for rearrangement, direct annihilation and elastic scattering are presented. The elastic cross section agrees well with other calculations. For the rearrangement process we confirm a recent result (Armour E A G and Chamberlain C W 2002 J. Phys. B: At. Mol. Opt. Phys. 35 L489) that rearrangement to the N = 23 state of protonium has a larger cross section than rearrangement to the N = 24 state. For both rearrangement cross sections our results are smaller than those obtained by Armour and Chamberlain. The direct annihilation, and its influence on elastic scattering, is calculated using the scattering length for the strong force between the proton and antiproton. This approach gives strong-force effects qualitatively similar, but smaller than those obtained in another recent work (Voronin A Yu and Carbonell J 2001 Nucl. Phys. A 689 529c).

Electron capture in collisions of with H and with C

A comprehensive theoretical and experimental study of electron capture in collisions of with H and with C extending over the energy range to is presented. A variety of theoretical approaches were used including those based on quantal molecular-orbital close-coupling (MOCC), multielectron hidden crossings (MEHC), quantal decay and classical trajectory Monte Carlo techniques. Radiative charge transfer cross sections were computed using the optical potential/distorted wave (OPDW) and fully quantal (FQ) approaches. The MOCC, OPDW and FQ calculations incorporated ab initio potentials, nonadiabatic coupling matrix elements and transition moments computed at the configuration-interaction level. Ab initio potential surfaces in the plane of complex internuclear distance were obtained for the MEHC calculations. Merged-beam measurements were performed between and for the collision system. Diagnostics of the beam with a crossed electron beam could find no presence of a metastable component. The current results, in conjunction with previous measurements, are used to deduce a set of recommended cross sections.

Collisional properties of cold spin-polarized nitrogen gas: Theory, experiment, and prospects as a sympathetic coolant for trapped atoms and molecules

fields (10 mT to 2 T). The calculated dipolar relaxation rates are insensitive to small variations of the interaction potential and to the magnitude of the spin-exchange interaction, enabling the accurate calibration of the measured N atom density. We find consistency between the calculated and experimentally determined rates. Our results suggest that N atoms are promising candidates for future experiments on sympathetic cooling of molecules.

Ab initio study of charge transfer in low energy collisions with atomic hydrogen

Charge transfer cross sections for collisions of ground state and excited state with atomic hydrogen are presented for energies less than . The cross sections are calculated in a diabatic representation using a fully quantum-mechanical, molecular-orbital, close-coupled method. Completely ab initio adiabatic potentials and nonadiabatic radial coupling matrix elements obtained with the spin-coupled valence-bond method are incorporated. Inclusion of the and closed-channels results in oscillations in the charge transfer cross section for collision energies above the separated-atom energy of the lowest closed-channel . Rate coefficients for temperatures between 500 and 100 000 K and cross sections for collisions with isotopic D are presented. Results for the reverse process, charge transfer ionization, are also given.

Ab initio study of charge transfer in low-energy collisions with helium

Charge transfer cross sections for collisions of ground state and excited state ) with atomic helium are presented for energies less than 250 eV . Using a fully quantum mechanical, molecular-orbital, close-coupling approach, the cross sections are calculated in a diabatic representation. Completely ab initio adiabatic potentials and nonadiabatic radial coupling matrix elements obtained with the spin-coupled valence-bond method are utilized. Rate coefficients for temperatures between 100 and 100 000 K and cross sections for collisions with isotopic are presented. Results for the reverse process, , are also given.

State- and isotope-dependent charge transfer of with atomic hydrogen in astrophysical and fusion plasmas

State- and target-isotope-dependent cross sections for electron capture in collisions of with , , and are presented for the energy range 0.01 - . Results are given for capture via radial coupling into the , , , , , , and states and are obtained through a close-coupled, quantum-mechanical, molecular-orbital method. Fully ab initio molecular data determined with the spin-coupled valence-bond method are incorporated. Rate coefficients for temperatures between 1000 and K are also presented. Applications to astrophysical environments and laboratory plasmas are addressed. The importance of state-dependent parameters for the modelling of nebulae emission lines and for fusion plasma impurity diagnostics and the potential significance of isotope effects to models of the edge region of a tokamak device are briefly discussed.