Table of contents

A simple analytic formula for the free-free Gaunt factor averaged over a Maxwellian distribution is obtained by the integration of the asymptotic expansion of the classical expression for the Gaunt factor, in which the incident energy is replaced by a mean one. A comparison with the exact quantum mechanical calculation shows that the formula approximates the average free-free Gaunt factor within 1% for kT/Z² Ryd<or=0.3 and 0.4 (kT/Z² Ryd)³2/<or= omega /Z² Ryd<or=0.3. The average bound-free Gaunt factor is also considered.

The energies and widths of the positive-energy resonances in the Stark effect on the hydrogen atom are expressed in terms of the parameters of unstable classical trajectories. This approximation is in excellent agreement with the exact quantum results.

A previous Breit-Pauli R-matrix calculation for photoionisation of the ground state (6s²)¹S₀ of atomic barium (Bartschat el al., 1986) has been extended to the excited initial state (6s6p)¹P^o₁. Compared with photoionisation from the ground state, the resonance structure is extremely complicated. The theoretical results of approximately 120 Mb near the ionisation threshold are six times larger than those obtained in a Hartree-Slater approximation by Theodosiou (1986), but they are still four times smaller than the experimental data of Kallenbach et al. (1988). However, in view of the difficulties involved in the normalisation of the experimental data, the energy resolution of the experiment, a strong resonance found near threshold, and the qualitative agreement between the shapes of the theoretical and the experimental results, the reasons for the apparent differences in the absolute magnitudes are not clear at the present time.

It is well known that Floquet theory cannot be rigorously correct when a continuum is present. The author shows in the context of a model photodetachment problem, in what sense this theory does apply to real systems. The author finds that the wavefunction contains a term that is a rising exponential function of r out to a time-dependent cut-off radius, after which it drops rapidly to zero. Closely related analysis enables the author to evaluate the width and shift parameters of a decaying system to greater accuracy than that given by the Weisskopf-Wigner approximation.

By means of electron spectroscopy the authors have studied the electron spectra generated in collisions between negative hydrogen ions and xenon at energies up to 6 KeV. Much structure appears which, for electron energies above 6 eV, can be attributed to doubly excited autoionising states of xenon. Below 1.3 eV, the structure is due to a series of mono-excited Rydberg states, with a ²P_1/2 core, that decay to the ²P₃₂ continuum of Xe⁺.

Cross sections differential in both the ionized electron energy and angle have been calculated using the classical trajectory Monte Carlo method to investigate the binary peak structure in collisions of 1.4 MeV u^-1 U^q+ ions with Ar for charge states 11<or=q<or=92. To simulate the effects of the varying screening of the nuclear charge, model potentials have been utilized. At an ejection angle of zero degrees, the magnitude of the binary peak is found to rise at first with increasing projectile charge and then drop. The rise is due to the diminished screening of the projectile nuclear charge with increasing projectile charge state. The drop is ascribed to the influence of the varying screened projectile charge giving rise to a rapidly changing cross section at small angles. In fact, it is found that at 0' the binary peak is 10 times larger for the partially stripped U³²⁺ ion than for the fully stripped U⁹²⁺ ion.

The authors investigate sodium-proton charge transfer using 14-state adiabatic, molecular-basis classical trajectory calculations in which a common translation factor and dynamical radial and rotational coupling terms are included. The differential cross sections are evaluated by the eikonal procedure from S-matrix elements obtained over 283 impact parameters at each energy. A fine mesh of energy and angle resolves new effects in scattering from a target excited by a circularly-polarized laser source. One of them is ascribed to velocity matching. Calculations were carried out on an Intel iPSC/2 parallel computer using a version of the EIKONXS package which is described.

The authors attempt to construct an accurate total cross section for e^--H(1s) scattering in the intermediate energy range from 1.0 to 3.7 Ryd by adding together the best cross sections for elastic scattering, bound-state excitation and ionization that are presently available. For elastic scattering and n=2 excitation they have used the theoretical cross sections arising from intermediate energy R-matrix (IERM) calculations, for n=3 excitation experimental results have been implemented, while for excitation to n=4 and above an extrapolation procedure has been employed. For the ionization component the experimental cross section has been adopted. The constructed total cross section is found to be in very good agreement with the IERM theoretical total cross section obtained by applying the optical theorem to their elastic scattering amplitude. In addition, the authors have examined the accuracy with which the ionization cross section might be extracted from the total cross sections given the available knowledge about bound-state excitation cross sections.

The spin-dependent orientation parameters for singlet (L^s(inf) approximately=) and triplet (L^T(inf) approximately=) scattering are investigated for electron-sodium scattering using the two-potential localized exchange approach. The results of L^S(inf) approximately=, L^T(inf) approximately= and L perpendicular to for the 3²s-3²p transition at 11.36 and 20 eV incident electron energy, and of lambda , L^S(inf) approximately=, L^T(inf) approximately=, L(inf) perpendicular to , gamma and P_l for the 4²s-3²p transition at 10 and 20 eV incident electron energy are presented and compared with the available experimental data.

The authors apply the first-order target continuum distorted wave (TCDW1) approximation to electron capture by positrons from helium into the ground state of positronium. Total cross sections for positronium formation are found to be in good agreement with the measurement of Fromme et al. (1986) above 160 eV. This is the energy range in which most theoretical results disagree with the measurements and indeed in previous classical trajectory Monte Carlo (CTMC) calculations it has been suggested that the experiments incorrectly describe large-angle ionization as charge transfer. The authors elaborate on this matter.

Oscillator strengths for allowed transitions involving the total symmetries ³P, ²S^o, ³D^o, ⁵S^o, ⁵P, ⁵D^o, ¹D and ¹D^o are calculated for atomic oxygen. The calculations are performed using both the R-matrix method and the method of configuration interaction. Comparison is made with previous theoretical results and with experiment and, where possible, recommendations are made with regard to accuracy.

The authors investigated the 3s²ns ²S, 3s²nd ²D and the 3s²nf ²F series in aluminium. These high levels were populated by three-photon two-step excitations using the 3s²4p ²P_3/2 as the intermediate state. They determined the positions of the series members accurately for n values between 28 and 60. The ²S and the ²F series are not perturbed. The quantum defect of the ²S series is confirmed to be 1.728 (2), the quantum defect of the ²F series is 0.041 (2). The perturbation of the ²D series is demonstrated by the variation of the quantum defect over the series, and by the fine structure (FS) of the ²D levels which is measured for n(37. The large positive splitting is shown to be explained by taking into account the fine structure of the 3s3p²²D perturber. Using simple arguments the percentage of admixture of 3s3p²²D to the 3s²nd ²D series can be determined while the FS of the perturber is estimated to be 20.5 cm^-1.

The J=1 odd-parity spectrum of barium is studied through a combination of the eigenchannel R-matrix and multichannel quantum defect (MQDT) methods. Short-range parameters used by MQDT to describe electronic channel interactions are determined by performing R-matrix calculations in LS coupling and the (LS/jj) frame transformation is used to include fine-structure effects. This approach has proven to be an efficient tool for describing lighter alkaline earths but its ability to handle Ba, where spin-orbit interaction is stronger, is analysed. The calculations reproduce well the perturbations of the principal Rydberg series 6snp ^1.3P₁ by eight low-lying doubly excited levels as well as most of the features observed in the photoabsorption spectrum 6s²-J=1' up to the 6p_3/2 threshold. Although some discrepancies were found, the considerable overall success obtained by describing spin-orbit effects in Ba with the (LS/jj) frame transformation instead of including them explicitly in the short-range calculation is a great surprise.

The moments S( mu ) for -6<or= mu <or=2 and L( mu ) for mu =0, 1 and 2 are calculated for the helium sequence for atomic numbers Z up to 30 under a screened hydrogenic model. In this model, one describes the atom by single-particle hydrogenic wavefunctions and treats the initial and the final state as characterised by two different effective charge parameters Z_i and Z_f, respectively. An asymptotic expansion is made of the differential oscillator strength of the screened hydrogenic model. Assuming the value 287.6 for the coefficient of the term in ^-7/2 for helium atom as given by Salpeter et al. (1962), the parameter Z_f is determined for the helium sequence. This approach has resulted in values which are in reasonable agreement with the various moment values of other authors.

Radiative lifetimes of eleven Cl I transitions in the visible range of the 4s-np and 4p-nd arrays have been directly measured. The results are compared with emission experiments and compilations of transition probabilities.

Experimental investigation of the alignment of the L₃ subshell vacancy state produced after photoionisation in lead by 59.57 keV photons has been reported. The results confirm the prediction of the calculations of Flugge et al (1972) that the atomic inner-shell vacancies produced after photoionisation are aligned and the X-ray emission from the filling of vacancies in states with J>or=3/2 is anisotropic, but contradict the predictions of Cooper et al. (1969) that after photoionisation of the inner shell, the vacancy state has an equal population of magnetic substates and the subsequent X-ray emission is isotropic.

The influence of the inner-shell electron dynamics of an atomic system on its nonlinear characteristics in a field of laser radiation is studied theoretically. The quasi-energy approach is developed to take into account the correlation interaction. The authors use the random phase approximation, in which the correlation effects are accounted for by considering the time-dependent interaction potential of a photoelectron with oscillations of the electron density of the inner electrons. In the following calculations the polarisational approximation for this potential was used. Explicit calculations in the case of linear polarisation of the laser field are made for negative ions of halogens and alkaline atoms in the central field approximation both with and without the account taken of the inner electron dynamics. The results show that there is substantial suppression of nonlinear characteristics when one takes the correlation interaction into account. This is in agreement with the results of several other studies.

The authors report above-threshold ionisation in krypton irradiated by a two-colour radiation field ( lambda ₁=1064 nm, lambda ₂=532 nm), where the phase difference between the two colours can be precisely controlled. The ionisation rate depends not only on intensity but also on the relative phase between the two lasers, as do the relative intensities and widths of the electron spectral peaks.

The hypervirial perturbative method based on the use of the hypervirial and Hellmann-Feynman theorems to calculate the energy corrections is applied to the eigenvalue equation of Lennard-Jones (n, 6) potentials. The method is used to calculate high accuracy numerical energy eigenvalues and to derive analytic expressions which provide very good estimates for the eigenenergies. The results are compared with those from other procedures such as the shifted 1/N expansion method, the numerical integration of the eigenvalue equation and semiclassical WKBJ calculations.

Plots of fragment-ion current as a function of 'off-plane' displacement (scatter profiles) can be partitioned into fragmentational and collisional contributions. For reactions CH₄⁺ to CH_n⁺ (n=0-3) occurring in 8 keV collisions with Ar atoms, the latter are dominant with mean displacement values which correlate with the known mean energy depositions. The sensitivity of such curves to the reaction energy requirement should allow the technique to distinguish between ions of different internal energy or structure, or to determine breakdown diagrams.

A molecular treatment of the Li⁺+H^- mutual neutralisation process for centre of mass energies in the range 25-5000 eV yields cross sections in very good agreement with absolute experimental data. Comparison is made of the relative accuracy and computational effort of ab initio and model potential methods in the evaluation of the relevant molecular data.

Differential cross sections for single and double electron capture by C⁴⁺ ions on helium target are measured at energies of 6 and 9.6 keV. These data are interpreted using a simple four-state model. Concerning the double capture, the theoretical analysis shows that a simultaneous two electron transition induced by electron-exchange interaction provides a possible alternative to transitions purely induced by electron-electron interaction usually considered.

Results are presented for the triple differential (e, 2e) cross sections in helium in the region encompassing the (2s²)¹S, (2s2p)³P, (2p²)¹D and (2s2p)¹P resonances at incident energies of 100, 200, and 400 eV for scattering angles of 13' at 100 and 200 eV and 3' and 16' at 400 eV. Measurements were taken with an energy resolution of 150 meV over an extended angular range for the ejected electrons, encompassing both the binary ( phi _e- phi _s= pi ) and recoil ( phi _e- phi _s=0) regions. The data are analysed in terms of Shore's parametrisation to obtain direct (e, 2e) cross sections f_r and resonance parameters a_r and b_r for the (2s²)¹S, (2p²)¹D and (2s2p)¹P resonances as a function of ejected electron momentum. The data show very strong correlations between the resonance amplitudes and the direct ionisation amplitudes.

The electron impact excitation cross section for resonance transitions of Be⁺ and Mg⁺ is calculated by the diagonalisation method. The energies, effective quantum numbers, total and partial widths of resonances in cross sections are obtained. The partial and differential cross sections were also obtained. The total cross sections were compared with experiment and other calculations.

Elastic, inelastic and total cross sections for X-ray and electron scattering calculated using high quality SCF molecular wavefunctions for silane (SiH₄) are presented. The effects of basis set choice and free rotation on these cross sections are investigated. The effect of molecular binding and various other trends and systematics in the cross sections have been examined with the help of difference functions obtained by subtracting the appropriate independent atom model (IAM) cross section from the molecular cross section. The probability density for the interelectronic distance (radial intracule density) is obtained and analysed.

The total vibrationally inelastic and momentum-transfer elastic e-C₂F₆ and e-C₃F₈ cross sections are obtained by the best fit of the measured electron drift velocity and diffusion/mobility ratio in pure C₂F₆ and C₃F₈ as well as in their mixtures with Ar.

Polarisational bremsstrahlung in the high-energy collision between an atom of nuclear charge Z₂ and an ion of charge Z₁>>Z₂ is studied. The elastic polarisation bremsstrahlung is shown to dominate in the absolute radiation spectrum in a wide region of the photon frequencies. Simple expressions for the absolute radiation spectrum are obtained in the range of the photon frequencies, which is located below the region spanned by the process of radiative electron capture.

The bremsstrahlung spectrum for 600 eV electrons on Xe in the photon energy range 70-200 eV is calculated with the 'atomic' bremsstrahlung mechanism taken into account. It is the first time that such a process has been considered beyond the Born approximation for projectiles. Results are compared with previous theoretical calculations and experimental data and it is shown that 'atomic' bremsstrahlung is essential to reproduce the shape of the experimental spectrum.