S. Geltman

That Bound-Free Absorption Coefficient of the Hydrogen Negative Ion.

The bound-free absorption coefficient of H/sup -/ is evaluated by using a set of successively improved bound- and freestate wave functions. Better agreement with experiment and greater theoretical consistency are shown to result. The best theoretical result is tabulated for use in astrophysical applications. (auth)

Free-free radiation in electron-neutral atom collisions

Abstract Free-free absorption coefficients are calculated for the electron-neutral atom systems involving He, C, N, O, Ne, Ar, Kr and Xe. The calculations are based upon model atomic potentials which have been adjusted to fit experimental scattering cross sections or electron affinities. Some angular distributions are presented and thermal averages are evaluated in the ranges λ = 0.5–20 μm and T = 500–20,000°K.

ELECTRON SCATTERING BY ATOMIC HYDROGEN USING A PSEUDO-STATE EXPANSION. I. ELASTIC SCATTERING.

A modified close-coupling expansion has been developed and applied to calculate elastic electron-hydrogen-atom scattering phase shifts. This expansion into a complete discrete orthonormal basis set is a variant of the more usual close-coupling expansion into eigenstates of the target Hamiltonian. We retain the first few hydrogen-atom eigenstates and augment these terms with pseudo-states chosen, in our particular case, to incorporate fully the ground-state polarizability of the hydrogen atom. The resulting phase shifts are compared, where possible, with those obtained in the most accurate variational calculations available, and they indicate a distinctly improved convergence over those obtained in the close-coupling expansion.

Ionization of a model atom by a pulse of coherent radiation

The time evolution of the ionization probability of a simple one-dimensional model atom under the influence of a harmonic electric field is studied by the numerical solution of the time-dependent Schrodinger equation. The conditions are clarified under which tunnelling or multiphoton absorption may be regarded as appropriate mechanisms. The standard tunnelling formulae are found to give poor quantitative estimates of ionization probability. The duration of the radiation pulse is found to be of critical importance, as multiphoton absorption is seen to require a certain minimum time to develop.

Laser-induced ionising collisions in alkali vapours

The semiclassical theory is developed for a simultaneous collisional and radiative process,, and applied to the laser-induced ionisation in a collision of two excited alkali atoms (in their lowest n2P states) in a laser field. Perturbation theory is used to evaluate the probability for this process in the case of straight-line paths at large impact parameter. The collision perturbation is taken to be the dipole-dipole interaction between the two atoms, which together with the dipole interaction with the radiation leads to the occurrence of s and d intermediate atomic states and p and f final states for the ejected electron.

ELECTRON SCATTERING BY ATOMIC HYDROGEN USING A PSEUDO-STATE EXPANSION. II. EXCITATION OF 2s AND 2p STATES NEAR THRESHOLD.

For pt.I. see abstr. A14168 of 1970. The pseudo-state modification of the close-coupling expansion is applied to the 2s and 2p excitation of atomic hydrogen by electron impact. Pseudo-states are used which ensure the implicit inclusion of all important excited state polarizabilities. A detailed comparison is made with results obtained from other modifications of the close-coupling expansion and the eigenphase minimum principle is used to determine the best result for each partial cross section. Comparison of the theory and experiment in the first electron volt above the n=2 threshold shows very good agreement in the ratio Q(1s-2s)/Q(1s-2p), but a 20% discrepancy exists between the individual cross section magnitudes when experiment is normalized to the Born approximation at higher energies.

Theory of laser-induced inelastic collisions

A theory of laser-induced inelastic collisions is presented, containing atomic selection rules and specific intensity and velocity dependences. It is applied to the case of the process Sr(5p1P0)+Ca(4s2 1S)+h(cross) omega (6408.6 AA) to Sr(5s2 1S)+Ca(4d1D), for which the intensity and wavelength dependence of the cross section is evaluated.

Atomic absorption of ultra intense laser radiation

The authors derive an expression for the rate of absorption by an atomic system (bound or free) of radiation from an ultra intense laser beam. The absorption characteristics are radically different from those of conventional weak-field absorption theory.

DOUBLE-PHOTON PHOTO-DETACHMENT OF NEGATIVE IONS

It is shown that the unique level structure of a negative ion leads to a considerable simplification in the theoretical evaluation of the magnitude of the transition probability. This simplification is used to obtain an expression for the double-photon photo-detachment probability in terms of the single-photon probability for a transition to the same final state. (C.E.S.)

Ionisation dynamics of a model atom in an electrostatic field

The time evolution of the ionisation probability of a simple one-dimensional model atom under the influence of an electrostatic field is evaluated by an exact solution of the time-dependent Schrodinger equation. It is shown that in the low-field, long-time limit the decay of the bound state is exponential in time with the decay constant identical to that obtained in the quasiclassical tunnelling theories. For shorter times, however, there are large departures from exponential decay, which lead to ionisation probabilities much higher than expected on the basis of the quasiclassical theories. The potential importance of this in the analysis of DC discharge phenomena is pointed out.