James S. Cohen

Theoretical absorption spectra for Ne+2, Ar+2, Kr+2, and Xe+2 in the near ultraviolet

Near‐ultraviolet absorption spectra for the I(1/2)u→II(1/2)g transition have been calculated for Ne+2, Ar+2, Kr+2, and Xe+2 using ab initio configuration interaction methods. The spectra are all similar except that the position of maximum absorption shifts to longer wavelength for the heavier rare gases. The absorption cross sections are tabulated at the wavelengths of the KrF, XeBr, XeCl, and XeF lasers.

Spin–orbit coupling and inelastic transitions in collisions of O(1D) with Ar, Kr, and Xe

Inelastic collisions of O(1D) with Ar, Kr, and Xe have been treated in the multistate Landau–Zener and the close‐coupling approximations. The coupling mechanism is spin–orbit mixing. The dependence of the spin–orbit matrix elements on internuclear distance R is calculated using accurate configuration–interaction wave functions and an effective operator composed of one‐electron, one‐center terms. The R dependence is found to be very significant. Cross sections for transitions to the individual triplet fine‐structure levels, as well as the total inelastic (quenching) cross section, are presented as a function of collision energy. The transitions occur primarily at curve crossings and the quenching rate constants were found to be significantly reduced by centrifugal barriers outside the crossing points. The calculated quenching rate constants at 300 K are (5.4±3.5) ×10−13, (6.0±0.7) ×10−12, and (3.0±0.2) ×10−11 cm3 molecule−1 s−1 for Ar, Kr, and Xe, respectively.

Chemiionization of the hydrogen molecule by He(2 3S) and He(2 1S) atoms: potential surfaces, autoionization widths, and cross sections

Potential energy surfaces have been calculated for He(2 3S) and He(2 1S) atoms on H2 using the stabilization method. Energies have been obtained for several He*–H2 separations X and relative orientations ϑ of the H–H and He–H2 axes; the H–H distance was fixed at 1.4 a0. Aside from shallow van der Waals minima at large separations, cuts of both surfaces at fixed ϑ are repulsive. The adiabatic He*–H2 interactions are resonances lying in the HeH+2+e continuum at all points; hence chemiionization can occur in collisions. The autoionization width Γ as a function of nuclear coordinates has been calculated in two ways: (i) by Fermi’s ’’golden rule’’ utilizing expansions in square‐integrable functions to avoid having to directly evaluate integrals over continuum orbitals, and (ii) by a new, less rigorous technique based on observations about the convergence characteristics of the stabilization procedure. The widths are found to fall off exponentially with He*–H2 separation and to be only weakly dependent on the a...

Chemi-ionization in atom--diatom collisions

Total cross sections and branching ratios are reported for the ionization processes resulting from collisions between He atoms and H2 molecules.(AIP)

Rapid accurate calculation of JWKB phase shifts

With a generalization of a procedure suggested by Pack, the JWKB phase shift has been evaluated using a number of Gauss–Mehler quadrature formulas. Extraordinary accuracy is obtained with convenient low‐order formulas employing the weight function (1−x)1/2 or (1−x)−1/2. The method has been tested with Lennard‐Jones (6–12), exponential, exp‐6, exp‐5, exp‐4, exp‐3, Morse, and Yukawa pontentials with excellent results in all cases. A change of variable is shown to make the method efficient even for high energies and small impact parameters. The present method is superior in speed and accuracy to the method proposed by Kennedy and Smith, which was formerly considered the best for numerical evaluation of JWKB phase shifts.

Capture of negative exotic particles by atoms, ions and molecules

This article describes the capture of heavy negative particles (μ−, π−, K−, ) by normal atoms, ions and molecules to form exotic systems. Capture by even the hydrogen atom presents great challenges for theoretical treatment. The wide variety of methods used are reviewed, including perturbative, two-state adiabatic and diabatic, time-independent quantum mechanical, time-dependent semiclassical and quantum mechanical and quasi-classical treatments. A few of these methods, as well as the Fermi–Teller model, have also been applied to heavier atomic targets. Most of the methods, other than the quasi-classical formulations, are not yet up to treating the dynamical electron correlation and multiple ionization found to be important in capture by multi-electron atoms, or the vibronic coupling found to be important in capture by simple molecules. The essential elements of potentially more rigorous quantum mechanical theories are characterized. The experimental data on capture states and relative capture probabilities in mixtures are also discussed. The connection of this experimental data to the theoretical capture calculations is fairly tenuous, but forthcoming experiments with antiprotons promise direct tests of some of the recent theoretical findings.

Interaction potentials for UF6 with itself and with rare-gas atoms

Abstract Potential curves have been calculated for the UF 6 -UF 6 and UF 6 -rare gas atom interactions using statistical models for the UF 6 charge density and the intermolecular interaction. Agreement with available experimental data is good.

QUASICLASSICAL MODELLING OF HELIUM DOUBLE PHOTOIONIZATION

We applied the method known as fermion molecular dynamics (FMD) to the description of a helium atom interacting with a short pulse of intense, long-wavelength laser radiation, for both linear and circular polarization. We describe the results of these calculations, insofar as they bear on the question of the mechanisms leading to double electron ejection. In the case of linear polarization, boomeranging trajectories leading to double ionization were observed at all laser intensities above threshold. The probability of occurrence was low, and almost independent of laser intensity. However, very near to threshold, boomeranging trajectories leading to double ionization were found to occur with a probability comparable to that for all other independent electron (sequential) processes. This produced a shoulder in the curve of double ionization probability versus laser intensity. The size of this shoulder was found to depend on laser wavelength and pulse length. No such trajectories were found for circular polarization.

Variational calculations of resonant states of H2

Variational calculations of several excited states of H2- are reported. The structure observed in e-H2 scattering around 11.5 eV is attributed to 2 Sigma g+ and 2 Pi u resonances. Further evidence is provided that the predissociation of the 2 Sigma g+ states causes the oscillations in the dissociative attachment cross section observed near 12 eV. The lowest 2 Delta g resonance is found to be near 11.7 eV.

Excitation and ionization accompanying the beta decay of T2

Abstract The probabilities for shake-up and shake-off excitation accompanying the beta decay of molecular tritium have been computed. The shake-off channels account for 15% of the excitation probability. Roughly 2/3 of the shake-off intensity is concentrated in a band of resonances occuring about 20 eV above threshold.