Table of contents

A new Floquet-based method is introduced which can be used widely in calculations on atoms and molecules in strong microwave or laser fields. Accurate calculations using this new method take several orders of magnitude less CPU time than with the standard Floquet approach and are less expensive in terms of computer memory than with the single Brillouin zone method. Illustrative results have been obtained for an experimentally examined threshold microwave spectrum of potassium; novel structure in this has been found and understood theoretically.

We present new experimental measurements and theoretical calculations of R-matrix and unitarized first-order many-body theory for electron-impact excitation of krypton. The usefulness of differential cross section ratios in providing sensitive tests of electron scattering models for the excitation of the configuration of the heavy rare gases is demonstrated. In addition to differential cross sections alone, these ratios provide interesting physical insights into the details of the collision process. Comparisons of the measured ratios with predictions from the present and other available calculations show some agreement, but also reveal that significant improvements of these models are required.

The optogalvanic (OG) reaction induced by ion transitions in an Ar/Mg hollow-cathode discharge is interpreted as conductivity due to production of additional secondary electrons. The irradiated ion extracts secondary electrons more effectively from the Mg cathode surface. This OG mechanism has not been considered to date. The observed additional polarization (self-alignment) of the Ar I 794.8 nm spectral line is taken as a confirmation of this process.

We have investigated vibrational effects on the cross sections for rotational transitions within the vibrational ground states of and HD, induced by collisions with H atoms. We find that the shift of the mean intramolecular distance from the position of the potential minimum is significant in the case of H scattering on , where the interaction potential is only very weakly anisotropic at low collision energies.

The `hollow atom' (HA) is the latest and probably most exotic creation of atomic collision physics. HA are short-lived multiply-excited neutral atoms which carry a large part of their Z electrons (Z, the projectile nuclear charge) in high-n levels while inner shells remain transiently empty. This population inversion arises for typically 100 fs during the interaction of a slow highly charged ion with a solid surface. Despite this limited lifetime, the formation and decay of a HA can be conveniently studied from ejected electrons and soft x-rays, and the trajectories, energy loss and final charge state distribution of surface-scattered projectiles. For impact on insulator surfaces the potential energy contained by HA may also cause the release of target atoms and ions. This topical review gives a short historical account of relevant experimental methods and studies in this field, presents a now widely accepted scenario for HA formation and decay, discusses some results from recent studies of the authors and concludes with an outlook on open questions and further promising aspects in this new field of atomic collisions.

We present ionization probability and lineshape calculations for the two-step three-photon ionization process, , of the ground state of hydrogenic atoms in a non-monochromatic laser field with a time-dependent amplitude. Within the framework of a three-level model, the AC Stark shifts and non-zero ionization rates of all states involved were taken into account, together with spatial and temporal inhomogeneities of the laser signal. In contrast with the usual perturbative technique, the time evolution of the atomic states was simulated by directly solving the system of coupled time-dependent inhomogeneous differential equations numerically, the equations being equivalent to the appropriate non-stationary Schrödinger equation. Particular numerical results were obtained for typical parameters of the pulsed laser field that are employed in a new experiment to measure the 1S-2S energy separation with muonium at the Rutherford Appleton Laboratory. The shifts and asymmetries of the photoionization lineshapes revealed may be of relevance for ultra-high-precision experiments in hydrogen in CW laser fields.

The ion chemistry in tetraethylgermanium has been examined by Fourier-transform mass spectrometry under single-collision conditions in the pressure range. The cross sections for electron impact ionization of are measured from threshold to 70 eV. The molecular ion and 15 fragment ions from the electron-molecule collision are observed with a total cross section of at 70 eV. All fragment ions, except , are found to react readily with to yield , with rate coefficients in the range of 2-. Small yields of digermanium cluster ions are observed at higher reactant pressures.

We investigate the spin-polarized electron impact ionization of laser-excited alkali atoms. The triply differential cross section is decomposed into orientation and alignment parameters which can be directly measured. This paper extends a theoretical frame recently developed for unpolarized electrons.

A three-state target elastic positronium close-coupling approximation (CCA) is employed to investigate Ps-He scattering in the energy range 0-200 eV with and without electron exchange. Low-lying phase shifts below the first excitation threshold and the total integrated cross sections using both the models are reported. Estimation of integrated excitation cross sections for and using CCA are presented for the first time. The present total cross sections are in good agreement with the measured data in the incident Ps energy range 20-30 eV.

The coupling of two autoionizing states or of a discrete and an autoionizing state by a strong laser field is studied analytically as well as numerically. The motion of the complex energies is traced as a function of the field strength for different field frequencies and atomic parameters. Most interesting is the critical region where a crossing (or an avoided crossing) of the trajectories occurs. At this critical field intensity, level repulsion in the complex plane occurs. With further increasing intensity, the complex energies move differently. When the resonances are coupled mainly via one common continuum, resonance trapping dominates, i.e. a short- and a long-lived resonance state are formed (level repulsion along the imaginary axis). When, however, the direct coupling dominates, level repulsion along the real axis takes place. Population trapping (defined by a vanishing decay width of one of the states at finite intensity) results from the interplay of the direct coupling of the states and their coupling via the continuum. We also studied the corresponding variation of the cross section for ionization of a laser-driven atom by the probe field.

This paper develops a theoretical framework for studying simultaneous observation of a photoelectron and a fluorescent photon emitted sequentially from a linear molecule rotating according to Hund's scheme (a) or (b). The correlation function obtained herein is completely general, independent of any dynamical models, and can be specialized to any experimental geometries suitable for such coincidence studies. Parity-adapted wavefunctions for all of the molecular states involved in such a two-step process have been used. Coherence effects arising due both to the M as well as degeneracies have been taken into account properly. Several photon-propagation and electron-detection configurations have been suggested for which general coincidence functions derived in this paper become particularly simple. Some of those phenomena are predicted which are either absent in or different from non-coincident observation of a photoelectron or of fluorescence. It has been suggested that in view of some recently performed experiments, the proposed rotationally resolved photoelectron-fluorescence coincidence studies in linear molecules are feasible even with presently available experimental techniques.

We present calculations of state selected (vibrational and electronic) cross sections for single charge transfer in , and DT collisions in the energy range . Cross sections for transfer dissociation and vibrational excitation reactions are also presented. We have employed ab initio electronic wavefunctions of the triatomic system, the eikonal method to treat the ion-diatom relative motion, and the sudden approximation for vibration and rotation of the diatom.

A method for calculating state-to-state rotational three-dimensional (3D) total cross sections from two-dimensional (2D) close-coupled equations is described. The idea is to replace the S-matrices in the 3D cross sections by those obtained from the 2D calculations. The correctness of the approximation is proved from the quantum 3D hard shape model. Classical trajectory calculations are performed for very high collision energy (0.2 eV) and compared with the proposed method showing satisfactory agreement. The accuracy of the method is checked on a few examples by comparing the results with the standard close-coupling calculations and classical trajectories. The method is a powerful tool for studying systems in which a large amount of energy is transferred between translation of atoms and rotations of molecules.

Natural radiative lifetimes of 10 levels of singly ionized ytterbium have been measured with time-resolved laser spectroscopy. Free ions were produced in a laser-induced plasma. A stimulated Brillouin scattering technique has been used to produce laser pulses as short as 1 ns for the excitation of short-lived states. Pseudo-relativistic Hartree-Fock calculations, taking into account the core-polarization effects, have also been performed. A good agreement of the measured and the calculated lifetime values has been achieved.

Electron-impact excitation of the ground state of hydrogen into the states is studied by propagating the R-matrix in the two radial dimensions. Integral cross sections and resonance parameters are presented for collision energies from 0.75 Ryd (the n = 2 threshold) up to 0.96 Ryd (the n = 5 threshold). Below the n = 3 threshold, there is very good agreement with results from other methods and with experimental data. Between the n = 3 and n = 4 thresholds, integral cross sections are in better agreement with those of a coupled pseudostate method than with those of the J-matrix method, while resonance parameters are in good agreement with those of a 15-state R-matrix calculation.

Cross sections and rate coefficients have been computed for rotational transitions within the vibrational ground state of HD, induced in collisions with molecules. Where possible, our results are compared with previous calculations by Schaefer; the overall level of agreement is found to be good. Rate coefficients are available for kinetic temperatures and rotational levels of the HD molecule .