Table of contents

The singular part of the free-free dipole matrix element is exactly calculated for the first time for the general case when a short-range and a long-range potential act on the particle simultaneously.

The multiphoton ionization of CO₂ has been studied using a laser of 794 nm wavelength and focused intensities in the region of 10¹⁶ W cm^-2. The kinetic energy distributions and ion yields obtained using time-of-flight spectroscopy coupled with covariance mapping can be interpreted most simply in terms of charge-symmetric fragmentation, the strongest channel being O²⁺+C²⁺+O²⁺. These results am compared with two previous experiments on CO₂, one using an intense laser art 248 nm, the other using Si⁸⁺ ion projectiles of 3 MeV u^-1 energy.

The method of potential envelopes is employed to approximate the bound-state energies of the Schrodinger Hamiltonian, - Delta - upsilon /(r²+ lambda ²)¹2/. It is shown that the simple formula E=min_r>0(P²/r²- upsilon /(r²+ lambda ²)¹2/) provides upper bounds to the energies if P=(2n+l-1/2) and lower bounds when P=(n+l), with accurate values found by direct numerical integration.

We report partial photoionization cross sections of the 6p²³P₀, and 6p²³P₁ states of barium in the continuum range 59 500-63 400 cm^-1 close to the 6p_{1/2, 3/2} thresholds. For a fixed geometry of the laser excitation, and fixed solid angle of detection, photoelectrons are measured in the 6s_1/2e(p) and 5d_{3/2, 5/2} e(p) continua. We focus on the 6p²³P₁ case, for which we use R-matrix method in combination with the multichannel quantum defect theory to compute the partial cross sections. Comparison with experiment is satisfactory and leads to the identification of new 6p₃2/nd; J=1, 6p_{1/2, 3/2} nd; J=2 and 6p_3/2ns; J=1 series.

The earlier work of Gokhale and Misra (1977, 1978) is extended for the calculation of the Sommerfeld screening parameters sigma ₁ and sigma ₂ for the S_1/2 levels. Calculation of these parameters has not been possible so far for certain S_1/2 levels owing to the non-availability of resolved experimental energy values for the succeeding P_1/2 P_3/2 levels. These latter values are now available through the recent work of Misra et al. (1990, 1992) and have been used here for the calculation of the two screening parameters.

Highly excited atom states carrying single or multiple vacancies in inner and/or outer shells still represent a subject of great relevance for both basic and applied research. Particularly, for inferring ionization cross sections or elemental abundances from measured X-ray line intensities, precise knowledge of the X-ray fluorescence yields appears indispensable. Based on previous exemplifying multi-centre calculations, the present paper attempts to include environmental and multiple-vacancy aspects in a dense approximate single-centre model for third-row main-group emitter atoms, which can readily deliver realistic transfer probabilities and fluorescence yields as parameters describing the cascading de-excitation of multiply ionized atoms in their chemical surroundings. In particular, the results of the present analysis justify in retrospect the tentative use of single-hole standard fluorescence yields in recent experimental work.

In studies of hydrogenic systems via the recursive residue generation method (RRGM) the major bottleneck is the matrix vector product HC, between the Hamiltonian matrix H and a Lanczos vector C. For highly excited states and/or strong perturbations the size of H grows fast leading to storage problems. By making use of direct methods, i.e. avoidance of explicit construction of large Hamiltonian matrices, such problems can be overcome. Utilizing the underlying analytical properties of the Laguerre basis e^{- lambda r}L_k^2l+2(2 lambda r) a direct RRGM (D-RRGM) for the unperturbed hydrogenic Hamiltonian is derived, changing the storage needs from scaling as N² to 4N where N is the number of radial functions for each factorized H₀(l,m) block with the possibility of parallel processing. A further computational simplification is introduced by putting the expression for the photoionization (PI) cross section in the rational form conventionally used in the representation of density of states (DOS). This allows the construction of the PI cross section directly from the tridiagonal Lanczos matrix avoiding the explicit calculation of individual eigenvalues and eigenvectors. To illustrate and verify the method the PI cross section for a hydrogen atom in a static electric field, for both pi and sigma polarization, was calculated for an electric field strength of F=5714 V cm^-1. Sufficiently large basis sets could be employed so that good comparison with experiment and other theoretical work was obtained, including the field-induced modulations near the zero-field ionization limit.

The discrete spectrum of the hydrogen atom moving across a strong magnetic field (B=7*10¹¹-7*10¹² G) is studied by expanding wavefunctions over a complete orthogonal basis, whose single term provides a correct description of an atomic state at large pseudomomenta K of the transverse motion. Wavefunctions, energies, atomic sizes and oscillator strengths of radiative transitions are calculated and analysed in a wide range of K values. All these quantities undergo radical changes when the atom moves across the field. The discrete spectrum remains infinite at arbitrary K, although the mean transverse velocity cannot exceed some maximum value for the bound states. Oscillator strengths change by orders of magnitude and some dipole selection rules are violated.

Transition rates have been calculated for all electric dipole allowed decays for Cr II of the 3d⁴4p levels arising from the closely spaced ⁶P^o, ⁶D^o and ⁴P^o terms, to all lower levels that arise from the ⁶S, ⁶D, ⁴P, and ⁴D terms of the 3d⁵ and 3d⁴4s configurations. We have applied a previously developed method of selecting a 'natural' basis for the algebraic expansion of the spectroscopic states that results in a very substantial reduction in the size of the problem, and hence contributes significantly towards rendering the problem at all tractable. To our knowledge, no previous ab initio calculations exist that explore these decays involving the even quartets with a serious consideration of both configuration interaction and relativistic effects. Our calculations include corrections to give the correct observed splittings for closely spaced levels in each J P subspace, and corrections for the difference between calculated and observed transition energies have also been applied. Based on the calculated transition rates, we have further determined the lifetimes of the 4P ^6,4P_J^o and 4P ⁶D_J^o levels of the Cr II ion, and compared them with available observed values. The agreement with the recent experiments of Schade et al. (1990) is reasonably satisfactory, whereas comparison with some of the older work shows larger discrepancies.

L X-ray production cross sections and relative intensities of Pb and Bi have been measured for proton ionization in the energy range 1 to 4 MeV. The measured cross sections are in good agreement with the theoretical results obtained from ECPSSR ionization cross sections of Cohen and Harrigan (1985) and different decay yield data. The experimental X-ray production cross sections for Bi, however, are found to be somewhat higher compared to the theoretical results obtained from RPWBA-DHS-BC ionization cross sections of Chen and Crasemann (1989) and decay yields of Chen et al. (1981, 1982). Unlike the earlier results of Xu and Xu (1992) none of the theoretical results on relative L_i X-ray intensities obtained by using different sets of decay yield data show agreement with the present experimental data in the entire projectile energy range. The centroid energies of L_alpha and L_beta peaks of both Pb and Bi remain the same with proton energy while the L_gamma centroid energy changes by about 50 eV. The experimental centroid energies are in good agreement with ECPSSR and RPWBA-DHS-BC theoretical predictions.

A new aspect of the space-charge effect is investigated. We have observed Ar⁺ double peaks in the time-of-flight mass spectra recorded during the tunnel ionization of the Xe-Ar gas mixture using CO₂ laser radiation. It is shown theoretically that this effect results from the Coulomb expansion of the two-component ion bunch produced in the focal volume. Calculations based on the proposed qualitative theoretical model agree well with the experimental data. This phenomenon is shown to be important in high-field ionization experiments in which multi-component ion bunches are produced.

Vibrational branching ratios have been measured for the H₂CO⁺ X ²B₂ state using photoelectron spectroscopy and monochromated synchrotron radiation. In the wavelength range encompassed by the present study the photoionization cross section is dominated by intense autoionizing resonances, most of which belong to Rydberg series converging onto the B ²A₁ threshold. Autoionization from these Rydberg states causes considerable variations to be observed in the vibrational branching ratios. The present data have been recorded over several Rydberg members, and the results indicate that autoionization tends to produce a vibrational distribution in the ion that is characteristic of the vibrational level in the neutral excited states.

The lifetime of the metastable autodetaching ⁴Pi_g state of the He₂^- ion has been measured by a time-of-flight technique to be 135+or-15 mu s, using a heavy-ion storage ring. The lifetime is nearly a factor of three shorter than the only available theoretical prediction. It is proposed that the lifetime should be assigned to the ⁴Pi_g^- Lambda doubling component.

A crossed beam method has been used to study the excitation of the n=2 states of H in He²⁺-H collisions by observing both spontaneous and electric field induced Lyman alpha radiation. Cross sections for H(2p) and H(2s) formation have been determined in the respective energy ranges 13.3-66.6 keV amu^-1 and 33.3-66.6 keV amu^-1 and shown to be in reasonable general accord with the predictions of recent close-coupling calculations.

We measured the electron capture to the continuum (ECC) peak in the ionization of He atoms by the collision of 50, 100 and 200 keV H⁺ and 100 keV amu^-1 He²⁺. We exhaustively scanned the double differential cross section between the angles -1.5 degrees to 10 degrees and within the velocities (1+or-0.15) upsilon _i where upsilon _i is the impact velocity. We propose a new method for analysing the ECC structure which is independent of the spectrometer transmission function modellization and enables a direct comparison with the available theories. Furthermore, we distinguish two regimes for the ionization process, depending on whether the electron velocity is larger or smaller than the projectile velocity.

An experimental study of the depopulation of Na(30s) Rydberg atoms under He⁺ ion bombardment has been undertaken to identify the range of collision velocities at which strongly coupled behaviour of the 30s and nearby levels at low velocity changes to predominantly dipole-allowed coupled at high velocity. Ion energies ranged from 700 to 3900 eV corresponding to collision velocities of 2.4 to 5.6 times the mean orbital speed of the excited 30s electron. The ratio of the cross section for the dipole-allowed transitions (30s to 30p and 30s to 29p) to the total depopulation cross section was determined for each collision velocity. (Other dipole-allowed transitions were observed to be negligible.) This ratio was found to increase from only 20% at a reduced velocity of 2.4 to 65% at a reduced velocity of 5.6. The ratio of the two dipole cross sections, sigma (30s-30p)/ sigma (30s-29p), was also determined and compared to calculations based on the dipole approximation with first-order perturbation theory.

High-resolution translational-energy spectrometry has been applied to study excitation of CO molecules in 1.8 keV collisions with H₂⁺, N₂⁺, CO⁺, CO₂⁺, O₂⁺ and CS₂⁺ projectiles. Transitions to the optically forbidden triplet states a³ Pi and a'³ Sigma ⁺, optically allowed states A ¹ Pi , B ¹ Sigma ⁺ and G ¹ Pi , and ionizing transitions to the low-lying states of CO⁺, X² Sigma ⁺, A ² Pi and B ² Sigma ⁺, are observed. Oscillator strengths for electronic transitions to individual states are obtained by simulating the measured energy-loss spectra with calculations using either spectroscopically determined or calculated Franck-Condon factors. Wavefunction overlap effects manifest themselves in the observation that the cross section for collisional excitation of CO is found to be sensitively dependent on whether the molecular projectile ion possesses Sigma or Pi symmetry. Excitation cross sections are at least a factor of 10 larger in the case of Sigma projectiles.

New data from a superelastic scattering experiment on e^--Na atom collisions at 10, 15, 20, 25 and 30 eV incident energy are presented and compared with existing experimental data and theories based respectively on the distorted wave second-order Born approximation, 15 state close-coupling and coupled channel optical potential calculations. Overall, fair to good agreement is found between theory and experiments for individual atomic scattering parameters. The loss of coherence previously observed by some workers at small scattering angles (5-20 degrees ), is confirmed at 20 eV and also observed at 25 eV. This loss of coherence is poorly modelled by existing theories.

Absolute elastic cross sections for e-disilane collisions have been measured by using incident kinetic energies of 2, 3, 4, 5, 7.5, 10, 15, 20, 40, and 100 eV and recorded over a scattering angular range of 10-130 degrees . These cross sections have been integrated by employing a nonlinear phase shift fitting procedure to generate a list of integral elastic and momentum transfer cross sections for the same energy range. The angular distributions have been found to agree reasonably well with a continuum multiple scattering calculation using only local potentials and also with a multichannel Schwinger variational calculation. Limited experiments on vibrationally inelastic scattering reveal the existence of a shape resonance with a peak at approximately 2 eV that is characteristic of trapping by a valence orbital with Si-H antibonding character analogous to that encountered in e-monosilane scattering.

The authors report experimental measurements of electron spectra resulting from the scattering of low energy (6.2-32 eV) electrons by helium atoms through an angle of 9 degrees in the presence of a high intensity ( approximately 10⁸ W cm^-2)CO₂ laser. The intensities of the additional peaks caused by the presence of the laser which occur separated from the elastic scattering peak by multiples of the photon energy are much greater than expected on the basis of calculations, using the Kroll-Watson approximation (1973). The intensity of these free-free transitions also increases with decreasing electron energy, again in disagreement with the calculations. The authors propose that the disagreement between the experimental results and those calculated is due to the electrons scattering from a helium target polarized by the laser field.

The authors discuss the decay modes of the 'exotic' He atom obtained by replacing one electron in ordinary He by an antiproton. They use the Born-Oppenheimer approximation as a first step in the description of this three-body Coulomb system, and concentrate on states with angular momentum approximately 20 to 40. The authors find that within the Born-Oppenheimer approximation strong correlations between the motion of the antiproton and electron are critical in determining the radiative lifetimes of "vibrationally" excited states. These correlations vastly increase the lifetimes of states formed in the potential determined by the electronic ground state. Going beyond the Born-Oppenheimer approximation, and including states based on excited electronic configurations leads to a new decay mechanism, a radiative transition with simultaneous electron excitation. Surprisingly this is the dominant radiative decay mode for highly excited states of antiprotonic helium. The inclusion of pi states can further alter the picture; strong angular coupling between the 2p sigma and 2p pi states leads to a complete breakdown of the Born-Oppenheimer approximation for states of total angular momentum <or approximately=20 and provides a spectacularly virulent example of Lambda doubling, so strong that the ground vibrational states of angular momentum <or approximately=20 are actually in an excited electronic state well. The auhtors discuss the implications for the long lived states observed experimentally in He.

Within a one-dimensional semiclassical model with a Morse potential the possibility of infrared multiphoton dissociation of vibrationally excited nitrogen oxide was studied. The dissociation thresholds of typical driving forces and couplings were found to be similar, which indicates that the results were robust to variations of the potential and of the definition of dissociation rate.

The theory of a non-linear degenerate parametric oscillator including the quantification of pump and signal mode is presented in the framework of quantum electrodynamics. The expressions for the dispersions of the signal mode quadratures are obtained at arbitrary temperature and dampings of the pump and signal mode.

High-order harmonic generation is observed in an argon jet using elliptically polarized picosecond dye laser pulses. The experimental data appear to be in near agreement with the perturbation theory. However, there is a general trend of small deviation from the theory. We speculate that such a deviation could be due to the laser induced coupling of upper states in or near the continuum which relaxes the selection rules leading to an enhancement of harmonic emission.