Table of contents

Maxwell-average quantal close-coupling widths of the Na D lines perturbed by He are presented for the temperature ranges 8<or=T<or=4500K. The recent model potentials by Hanssen and co-workers (1979) were used. The agreement with the experimental data is satisfactory over the temperature range studied. At room temperature, detailed comparison between the rates computed by Wilson and Shimoni (1975) (Baylis, Krauss potentials) and experimental results provides a useful test of the potentials. The shape of the A² Pi potential seems to need some improvement between 6-11 au, whereas the B² Sigma potentials is probably good. At low temperatures, averaging over the Maxwellian distribution of velocities is shown to be necessary.

Using monochromatised synchrotron radiation in conjunction with a crossed-beam apparatus and a quadrupole mass spectrometer the authors have measured the relative cross sections q⁺ for single and q²⁺ for double photoionisation of Ba atoms as well as the ratio R=q²⁺/q⁺ in the wavelength range 60-54 nm with 0.09 nm bandwidth. Compared with other experimental data, measured with a bandwidth of about 0.2 nm, their data show much more structure. In the interesting region around 58 nm, the Ba²⁺ yield has been studied with a resolution of 0.015 nm; the comparison with photoabsorption data of Ederer et al. (1975) shows good agreement in the peak structure, and at the same time calls for additional experimental studies of the problem with the highest possible resolution.

Model configuration interaction calculations of the strong interaction between 3s3p⁶²S and the 3s²3p⁴nd²S series in Ar II are presented. It is shown that in a CI calculation of this problem the highest eigenvalue has no physical significance. The eigenvector corresponding to the highest eigenvalue has a large contribution from 3s3p⁶²S. It is shown that this is the reason that earlier calculations by other workers have predicted an intensity distribution among the satellites which does not agree with observations. The present calculations give good agreement with the observed photoelectron satellite intensity distribution but it is conjectured that a definitive calculation must include the effect of the epsilon d²S continuum.

Heterodyne Doppler-free spectroscopy, previously demonstrated on single-photon resonances (heterodyne saturation spectroscopy), has been extended to two-photon transitions in rubidium (5S-5D and 5S-7S). It provides a simple method of observing the two-photon dispersion lineshape directly. The noticeable sensitivity of this absorption technique, which has been used for an optical measurement of the hyperfine structure of the 5D level, is discussed in comparison with that currently obtained in two-photon-induced fluorescence.

The weak infrared absorption band nu ₅+ nu ₆ of ³²SF₆ has been observed with a diode laser. The R(J) lines show a simple manifold for the Coriolis splitting which is different from the usual analytic model. Some spectral parameters have been calculated.

The cross section for electron impact excitation of the auroral transition O²⁺(2p²¹D to 2p²¹S) at 4363 AA has been computed by means of the non-relativistic Coulomb-Born approximation. The contribution from high angular momentum partial waves (L>or=4) is used to supplement the recent close-coupling calculation of Baluja et al. (1981) in order to obtain a total collision strength at impact energies above the region of resonances. A simple graphical method based on the high-energy Born limit is given for interpolating collision strengths over the entire energy range. From the present results the authors conclude that at temperatures of astrophysical interest the previously published rate coefficient is underestimated by only a few per cent.

A method in which both the relativistic and core polarisation effects are treated in a uniform way within the Hartree-Fock computational scheme is presented. The method retains the simplicity of the standard Hartree-Fock formulation since only the potential function is modified by adding effective relativistic and polarisation potentials defined in earlier works. Calculations of the transition energies, fine-structure splittings and oscillator strengths in the alkali atoms are performed. The results indicate that the simple model describes with great accuracy both the relativistic and correlation effects and that it may be used to obtain a reliable description of the atomic spectra.

The Stark broadening for a confined atom is considered as an approximation for a pressurised system. The procedure consists of using the numerical solution of the two-dimensional Schrodinger equation using a finite-element package.

The radiative lifetimes of 6s⁴P, 6s²P, 5d⁴D, 5d⁴F and some 5p Kr II states were obtained using a conventional delay coincidence method with a pulsed electron beam. The lifetimes of 6s²P_1/2, 5d⁴D_1/2 and 5d⁴D_3/2 were measured for the firt time. The lifetimes of the upper levels of the transitions used in the He-Kr laser are shorter by factors of between 1.3 and 3.4 than those of the lower levels.

Profiles and linewidths of the 3d to f transitions in many lanthanides have been determined using synchrotron radiation emitted by the ACO storage ring and a new double-crystal monochromator. The results are compared with existing theory and a more detailed interpretation of the structure is given than has been possible to date.

The profile and the temperature dependence of the blue satellite of the Cs(6S_1/2-5D₅) transition perturbed by a rare gas is studied experimentally. By the use of the Szudy and Baylis (1975) quasimolecular model and various available ground-state potentials the profile of the satellite is calculated and the potential of the excited state is deduced from the fit with the experimental profile. The predicted temperature dependence of the calculation is then compared with experiment and the deviations are discussed.

Measurements of the energy and phase relaxation times T₁ and T₂ were made for the (3,3), (2,2), and (2,1) inversion transitions in ammonia gas at room temperature and at pressures between 10^-1 and 10^-4 Torr. At low pressures wall collisions contributed substantially to energy relaxation. The wall collision rate was calculated and found to agree with experiment. Relaxation rates by intermolecular collisions agreed generally with reported values. All measurements represented averages over M states in the (J,K) inversion manifold, and it was found that T₁=T₂ to good accuracy.

The absolute dipole oscillator strengths (cross sections) for the partial and total photoionisation of hydrogen fluoride have been obtained in the 5-60 eV photon energy range by magic-angle dipole (e, 2e) spectroscopy. Total photoabsorption oscillator strengths (cross sections) are determined up to 150 eV. Photoelectron branching ratios for the X² Pi , A² Sigma ⁺ and B² Sigma ⁺ ionic states of hydrogen fluoride are also reported. Comparison of experimental partial oscillator strengths (cross sections) for the X² Pi , A² Sigma ⁺ and B² Sigma ⁺ states, respectively, with sophisticated theoretical calculations show generally good agreement over the reported energy range.

The collision of sputtered B atoms with Ar atoms has been suggested on experimental grounds as giving a high yield of excited B (3²S) atoms. Ab initio CI calculations on the lowest three Sigma states of ArB are presented. They reveal that there are two avoided crossings around 3.4 au. The crossing to the 2² Sigma state (dissociating to B (e²S)) lies at about 6.6 eV. The non-adiabatic matrix elements of the delta / delta R operator are calculated and have the strongly peaked behaviour characteristic of an avoided crossing. This route to excited B atoms is thus confirmed, but the dynamical system in the crossing region is essentially a three-state rather than a two-state one.

An approximate relation between the relative sizes of cross sections from the ground state into different m states is derived. A previous deviation used pair potentials and the quantal impulse approximation. The present derivation makes different assumptions and shows the circumstances in which the relation becomes invalid.

The process of a multielectron transition in an atom taking place during scattering, from an atom, of a structureless charged particle moving at a velocity v is treated in the model of independent particles taking into account antisymmetrisation. In the impact parameter representation, expressions for the probabilities W( rho ) of multielectron transitions between various electronic subshells, multiple ionisation processes and also ionisation transitions leading to excitation of the atomic residual have been obtained. On the basis of the probability formulae, the cross sections for the ionisation of helium atoms resulting in the excitation of the atomic residual to the 2s, 2p, 3s, 3p and 3d states have been calculated. The cross sections obtained are compared with similar cross sections taken from the work of Dalgarno and McDonald and Mapleton.

The K-shell ionisation probability of argon has been measured for protons scattered over angles between 8 and 127.5 degrees and with initial energies of 500 and 750 keV. An increase of this probability has been found when going from scattering angles of 90 degrees and less to the largest observed scattering angle. The asymmetry parameter B in the angular dependence I( theta )=A(1+B cos theta ) has slightly larger, negative values than those measured previously by other workers for copper and aluminium.

Cross sections within the range 3-25 keV for the electron detachment process H^-+X to H(n,l)+X( Sigma )+e (where Sigma denotes all final bound and continuum states) have been determined for the formation of H atoms in the 2s state for targets of H and H₂ and in the n=2 and n=3 sublevels for targets of He, Ne, Ar and N₂. In the case of H₂, total cross sections for Lyman alpha and Balmer alpha emission (which include the contribution from dissociative excitation of H) have also been determined.

The authors present here the results of the first detailed theoretical calculations of differential cross sections for free-free transitions which involve electrons at real target-atom scattering resonances in a laser field. The scattering resonances included, which may occur in the initial or final state (or both) of the free-free absorption, are the lowest ¹S^e, ¹P^o, ³S^e and ³P^o Feshbach resonances of hydrogen occurring at incident electron energies just below the n=2 excitation threshold. They evaluate differential cross sections involving initial ¹S^e and ³P^o resonances and radiation at the CO₂ laser wavelength (h(cross) omega =0.117 eV), the double resonances between ¹S^e and ¹P^o (h(cross) omega =0.630 eV) and between ³P^o and ³S^e (h(cross) omega =0.383 eV), and the processes going from ¹S^e and ³P^o resonances to a final state above the n=2 threshold by means of radiation at the Nd laser wavelength (h(cross) omega =1.17 eV). The latter process has three possible final atomic states (1s, 2s, 2p) and corresponds to the joint electron-phonon excitation of the hydrogen atom. It is found that the presence of the electron scattering resonances can enhance the background free-free absorption in the forward direction by as much as four orders of magnitude, but has a relatively minor effect in the backward direction.

The triple-differential cross sections for the electron impact ionisation of helium have been calculated in the Coulomb-projected Born (CPB) and polarised CPB approximations. The results have been compared with various recent coplanar and non-coplanar measurements. The inclusion of the target polarisation changes the cross sections very little. The overall trend of the results is found to be comparable, looking at the uncertainties of available experimental data, with that of Bransden et al. (1978).

The Born approximation was used to investigate the influence of a dense plasma on the inelastic scattering of electrons by one-electron ions. Scaled collision strengths Z² Omega for 1s to 2s, 1s to 2p and 2s to 2p transitions in an ion of arbitrary nuclear charge Z were computed for a Debye-Huckel model of the screened Coulomb interaction. Over a wide range of screening lengths, the effect of the plasma environment is to appreciably reduce cross sections at all energies.

Using the formalism of discrete state-continuum interaction as introduced, for example, by Feshbach and Fano, formally exact operator expressions are derived for the cross sections of resonant electron scattering, dissociative attachment and related processes. Concentrating on the fixed-nuclei limit, a rigorous description of resonances, virtual states and bound states and their connection at threshold is developed. In principle the approach is to impose the threshold law on the decay width of the resonance and to take proper account of the energy-dependent level shift. The analytic structure of the fixed-nuclei S matrix for electron scattering is obtained by analytic continuation into the complex momentum and energy planes. At energies sufficiently close to threshold the results are of universal validity, depending only on angular momentum and the leading multipole moments of the molecular charge distribution. The approach is exemplified for s-wave and p-wave scattering by short-range potentials, where it reproduces the well-known results of general analytic S-matrix theory. New results are then obtained for the behaviour of resonances and bound states of polar molecules close to threshold.

P-wave phaseshifts for positron-hydrogen elastic scattering are calculated using a new adiabatic approximation in which the length of the radius vector from the proton to the positron is fixed but its direction is allowed to vary. This adiabatic approximation makes possible the full inclusion in the calculation of virtual states in which angular momentum is transferred to the target H atom. The results obtained agree qualitatively with the highly accurate results of Bhatia and co-workers (1974) and are much closer to them than the results obtained using the usual adiabatic approximation in which the radius vector from the proton to the positron is fixed.

With the help of the functional integration method the authors obtain the exact results for the population of the ground state of an atom driven by the chaotic laser field. They investigate a large class of field models and the role of the laser line shape.