Table of contents

Multielectron calculations for the 3p photoabsorption cross section of free Mn⁺ (3p⁶3d⁵4s⁷S) are performed for the first time. The essential influence of a strong mixing between the 3p to 4s and giant 3p to 3d resonances on the 3p absorption cross section is discovered. Theoretical arguments that the available experimental data for the Mn⁺ 3p absorption spectrum could belong not to the 'pure' Mn⁺ absorption but to the mixed absorption of Mn⁺, excited Mn⁺* ion and neutral Mn atom are put forward.

Experiments have been performed investigating the interaction of an intense CO₂ laser pulse of nanosecond duration with diatomic molecules O₂, CO and NO as well as triatomic CO₂. New measurements relating to the tunnel ionization behaviour of noble gas atoms have been performed. These results contain less data scatter than previous work and thus provide a useful reference with which to compare the molecular results. Comparison with theory shows that in the absence of vibrational resonance with the CO₂ laser (in the weak field approximation), these 'simple' molecules are tunnel ionized as if they were structureless atoms with an ionization potential equivalent to that of the molecular ground state.

In this review, the authors examine theoretical models of atoms dressed by intense fields. They review the breakdown of lowest-order perturbation theory and those 'essential states' methods adopted to include Rabi frequencies, Stark shifts, induced widths and continuum dressing. Methods more suitable for super-strong fields are described, such as Floquet and Volkov methods and the direct numerical integration of the Schrodinger equation. Such methods are used to provide completely non-perturbative strong field descriptions of atomic dynamics. The authors conclude with a brief examination of the relativistic effects expected to be important when high intensity ultrashort pulse lasers are employed in strong field physics.

It has been shown that recently by Lindgren (1990) that gauge independent energy shifts in the effective potential formalism can only be obtained when all the diagrams to the same order of perturbation are taken into account. In this work, the authors extends Lindgren's proof in two generalized gauge conditions. It is shown explicitly that in the generalized Landau gauge and axial gauge, by neglecting radiative corrections, the energy shifts calculated in these gauges equal the Feynman gauge results to the leading relativistic order.

Very accurate energy values of hydrogen in a magnetic field are obtained by a method which is a mixture of two methods: (i) expanding the wavefunction in a power series, which is very accurate near the origin, thus computing the first few points, and (ii) a point method which starts with these few points and shoots outwards. At low magnetic fields, the accuracy is 14 significant figures; if a computer having 32 digits were used, the accuracy would be 26 figures. The best results in literature have 16 accurate figures. At high fields the accuracy decreases rapidly, and the discrepancy with other results increases to 2% for unknown reasons. Errors were very carefully estimated and the results were checked by many aspects. In one dimension, the method is also excellent.

The authors present calculations of autoionization widths using a fully numerical multiconfigurational Hartree-Fock method (MCHF). Energy positions and autoionization rates are given for several doubly-excited states lying below the n=2 threshold in He I and H^-. Particular attention is given to orthogonality considerations in the computation of the autoionization widths. The method is extended to the 1s2p²²D, 1s2s(³S)(3d ²D and 1s2s(³S)we 4d ²D states of Li I. To the authors' knowledge, the MCHF value, 12.5 meV, is the first result obtained for the autoionization width of the 1s2s(³S)4d ²D state. Comparison is made with other theoretical results and with experimental values and we point out several inconsistencies between the two sets of results.

Using the truncated diagonalization method with a second-order energy correction in a B-spline basis, energies, autoionization and radiative rates are obtained for 2lnl' (n<or=7) and 3lnl' (n<or=5) singlet terms in the doubly excited He-like carbon ion and compared with experimental as well as other theoretical results where available. Branching ratios for radiative decay are determined in order to estimate the importance of radiative decay in the interpretation of ejected-electron spectra. For a proper determination of cross sections for resonant transfer excitation or double capture into 2lnl' states, it is found that it is necessary to take radiative decay into account. For the determination of cross sections for double capture into 3lnl' levels, the probability of radiative decay cannot be neglected entirely, although it is much less significant than for 2lnl'. For the latter configuration radiative decay is the dominant decay mode for most terms with n equal to or larger than 6.

A high-resolution X-ray emission spectrum of molecular oxygen is presented. Comparisons with ab initio calculations are made regarding relative intensities, energies, and band profiles for the outer valence, ² Pi _g, ⁴ Pi _u, ² Pi _u, ⁴ Sigma ^-_g, ² Sigma ^-_g, levels. The importance of lifetime-vibrational interference is discussed.

Three-photon bimolecular absorption is a process which, through the simultaneous absorption of three laser photons, results in the electronic excitation of two molecules or atoms. Following a recent treatment of the selection rules, a quantum electrodynamical treatment is employed to describe the main features associated with the process and to produce an equation for the excitation rate. The dependence on photon polarization is investigated, and the magnitude of the excitation rate is estimated. It is shown that two different mechanisms operate, each with different selection rules and different dependences on intermolecular separation. In each case an energy mismatch is conveyed from one molecule to the other by means of virtual photon coupling. Some possible schemes for experimental study of the process out outlined.

The authors have used the light-induced diffusive pulling (LDP) method to measure the ground state momentum transport cross sections and the relative difference of the excited and ground state cross sections with respect to the ground state for potassium with several inert gas atoms. For the LDP measurements, the excited state cross section is a weighted average of the potassium 4²P_3/2 and 4²P_1/2 fine structure levels, which are mixed by collisions with the inert gas atoms. The authors have also used a light-induced drift (LID) experiment to measure the ratio of the differences in the excited and ground state cross sections for the individual fine structure levels ( Delta sigma /J=3/2): Delta sigma (J=ff) for potassium-Ne and potassium-Ar. For the case of potassium-Ar, the authors have combined this ratio with the results from the LDP measurements to obtain absolute transport cross sections for the individual 4²S_1/2, 4²P_1/2 and 4²P_3/2 levels.

The authors present a theoretical study of state selectivity for electronic processes occurring in the course of collisions between ions and atoms. This analysis is performed within the semiclassical atomic basis impact parameter treatment in combination with the eikonal method to obtain angular differential cross sections and alignment and orientation parameters. The authors apply the formalism to H⁺-Na collisions with the 0.75-0.50 keV energy range for which new experimental data are available, cf the companion paper (Richter et al., 1993) and the comparison there with the current authors' theoretical predictions. The authors report and discuss the orientation of the final H(2p) and H(3p) capture states for an initial Na(3s) state and for initially oriented Na(3p_+or-1) states.

For pt.V see ibid., vol.24, no.8, p.L213-8 (1991). The complete density matrix describing the charge transfer process H⁺+Na(3s or 3p) to H(n=2)+Na⁺ has been measured for scattering angles theta (0.2' and an H⁺ energy of 1 keV. At this energy the collision velocity is about half the mean velocity of the Na(3p) valence electron. Pronounced right-left scattering asymmetries has been observed both for initially oriented as well as for aligned Na(3p) states. Very good agreement is found with recent elaborate theoretical results of Dubois et al. (1993). Measurements at 0.75 keV and 2 keV show similar trends. The findings are discussed in terms of simple models such as velocity matching and propensity rules for orientation.

Differential, integral and momentum transfer cross sections are calculated for the vibrationally elastic scattering of electrons from H₂O. An ab initio static potential is used for the interaction. Effects of electron exchange and target polarization are taken into account approximately. In particular, the contribution from the dipole potential is evaluated carefully so that an accurate cross section is obtained for the forward scattering. Calculations are carried out for electron energies of 6-50 eV and a detailed comparison is made with experimental data. The differential and momentum transfer cross sections calculated are in good agreement with experimental results. The theoretical value of the integral cross section deviates greatly from the measured one.

The author derives equations giving the differential cross section for excitation of vibrations by resonant electron impact in polyatomic molecules. The description 'lowest order' applies because many of the quantities are represented by Taylor's expansions in the normal coordinates with the series truncated after the first term that produces excitation. The discussion shows the importance of the phases of the scattering amplitude matrix in influencing the angular scattering in vibrational excitation. In particular, the linear Jahn-Teller effect between a doubly degenerate resonant state and a degenerate vibrational mode produces more scattering in the backward direction than in the forward direction in the case of the v=0 to v=1 excitation. The theory is applied to methyl chloride, CH₃Cl, giving a comparison of the results with experiment.

The author reports finite-field self-consistent field (SCF) and fourth-order many-body perturbation theory (MP4) calculations of the electric quadrupole and hexadecapole moments of ethane (H₂C2CH₂). The best values (with C2C on the z axis and the molecule on the xz plane) are Theta _xx1.197, Theta _yy=-2.434 and Theta _zz=1.237 ea₀². Phi _xxxx=-14.91, Phi _yyyy=16.89 and Phi _zzzz=-14.05 ea₀⁴.