Table of contents

Observations of the 4d absorption spectra of atomic La and Tm are reported. The RPAE predictions for the 4d to epsilon f cross section in La I are currently closest to experiment. In Tm I a quite sharp autoionising resonance is observed. The differences between the two spectra are discussed with reference to ab initio calculations.

The lanthanides can be divided into two groups according to the occupation of the 4f subshell in the solid and in the atom. In the first group the 4d absorption spectrum in the atom and in the solid are similar. In the second group the atomic spectrum of the element with nuclear charge Z corresponds to the solid with nuclear charge (Z+1). Predictions are made for the 4d spectra of those lanthanides which remain to be observed.

It is shown that, besides the previously predicted photon antibunching effect, photon bunching may occur in resonance fluorescence. This effect can be observed by measuring the time-delayed coincidences of photons from the side components of resonance fluorescence in a strong driving field.

The authors study the time dependence of the two-photon ionisation rate for atomic caesium calculated using a sudden approximation. For pulse lengths similar to the ones used in the measurements by Granneman and Van de Wiel (1975) second-order perturbation theory gives results different from those obtained with the authors' model. Finally they utilise molecular data to resolve the discrepancy still outstanding between theory and experiment.

In a recent paper, Kobe (see Phys. Rev. Lett., vol.40, no.9, p.538 (1978)) has stated that the result obtained for non-resonant two-photon absorption depends on the choice of the coupling between the atom and the electromagnetic field (choice of gauge). The authors believe that his demonstration, even if mathematically exact, has no physical implications. They demonstrate that there are small terms due to relaxation processes omitted by Kobe which exactly compensate the difference between the values obtained, with the different couplings.

A set of coupled equations is derived from a generator coordinate version of the Born-Huang expansion (1956). The importance of coupling terms is discussed and a two-state approximation to atomic collisions is outlined.

Resonant two-photon ionisation of Na₂ and subsequent third-photon dissociation of Na₂⁺ has been studied with the use of a pulsed dye laser. Expressions for the time- and intensity-dependent populations of Na₂⁺ and Na⁺ are derived and fit to observed ion signal variations with laser intensity. This analysis yields the cross section for photodissociation of the ² Sigma _g state of Na₂⁺. The dissociation energy of this state is determined from the time-of-flight spectrum of Na₂⁺ photofragments.

A four-body classical-trajectory Monte Carlo method has been applied to Rydberg-atom-Rydberg-atom scattering problems. Numerical calculations were conducted over the velocity range v_e/3 to 3v₂ where v₂(au)=1/n and n is the principal quantum number of the Rydberg atom. Maximum ionisation occurs at approximately 1.25 v_e with the cross sections approximately scaling as pi n⁴a₀². At the lower velocities, a significant fraction of the ion production is due to negative-ion formation.

Measurements have been made of detachment from F^- by electron impact for energies between 13 and 1000 eV. The cross section attained a maximum value of 2.74*10^-16 cm² at 68 eV. Thomson's classical scaling law was used to compare measured detachment cross sections of H^-, C^-, O^- and F^-. The scaled cross sections differed by a factor of less than four.

Graphical methods of spin algebras are used to evaluate elements of the representation matrices of the spin permutation group for arbitrary transpositions. The matrix elements are evaluated directly; no matrix multiplication is required. The method is discussed for the Young-Yamanouchi basis and the Jahn-Serber basis.

Pair energies contributing to the all-external correlation energies of the 3d electrons are computed for the Zn²⁺ closed-shell ion by means of second-order perturbation theory starting with the sum of one-electron Hartree-Fock operators as the zeroth-order Hamiltonian. The variation-perturbation symmetry-adapted pair functions are taken in the form of partial-wave expansions. The results for the pair energies and total correlation energy are compared with related theoretical and semiempirical results. Our analysis indicates that nondynamical correlation effects are of little importance in the ground states of transition metal systems.

Absorption linewidths of the Na D2 line in shock-tube-produced atmospheres are reported. Densities range from 3.6*10¹⁹ cm^-3 to 1.2*10²⁰ cm^-3 with temperatures between 5000 and 10000K. Significant disagreement with previous work is found and discussed.

Starting from the quantum one-perturber theory, the 'exact resonance' approximation is used. This accounts exactly for the long range dipole interaction. The validity of this approximation is analysed. In contrast to usual dipole semiclassical approaches, it is found that dipolar quantum calculations give an asymmetry to the wings that is in good agreement with measurements of Preston (1977). This is because the full quantum approach provides a proper dynamical treatment of the electron-atom interaction.

From a comparison of self-absorption of resonance radiation in pure ²⁰Ne gas and in a fifty-fifty mixture of ²⁰Ne and ²²Ne gas, an upper limit is found for the isotope shift of a number of neon resonance lines. It appears that these shifts are considerably smaller than the full widths at half maximum of the single Doppler-broadened absorption lines at room temperature. An upper limit for the difference in energy between the ground level of ²⁰Ne and that of ²²Ne could be obtained using published values for excited-state level shifts. This upper limit (5 GHz) indicates that the difference in normal mass shift and the difference in specific mass shift of the ²⁰Ne and ²²Ne ground levels are of different sign and equal magnitude.

Recent calculations by Rajnak and Fred (1977) show that the transitions studied by Gagne et al. (1976) have upper levels of mixed configurations. The amount of mixing and the probability of mass shifts due to 5f electrons is discussed.

The complex rotation method is applied to calculate autoionisation states (¹S^e, ¹P^o and ³P^o) of the helium isoelectronic sequence (Z=2-10) below the n=3 hydrogenic thresholds. The Hylleraas-type wavefunctions are employed and up to 161 terms for S states and 165 terms for P states are used, so that several lower-lying states are determined. For He and Li⁺ states, comparisons are made with other calculations and experiments. The width of the lowest ¹P^o state of helium below the n=3 threshold is determined to be (191+or-7) meV which differs quite significantly from the experimental value of (132+or-14) meV of Dhez and Ederer (1973). Furthermore, it is concluded that the width computed by the complex rotation method would not reduce to the experimental value even if more terms in the wavefunction are used.

Phaseshifts for e^-+He⁺ elastic scattering in the ³P channel are calculated using perturbation theory to take account of correlation. The position and width of the 2s2p ³P autoionising state are determined and a calculation of the photoionisation oscillator strength for the 1s2s ³S-1s epsilon p ³P transition is carried out. The dipole-length, dipole-velocity and dipole-acceleration transition operators are used giving excellent mutual agreement.

Multiphoton electron-ion inverse bremsstrahlung is studied using the correspondence principle. It is shown that the existing quantum-mechanical theory of this phenomenon is only valid in the low-frequency limit omega tau <<1. A different approach is suggested which enables the author to obtain the multiphoton cross sections for all values of omega tau . Using these cross sections, the absorption coefficient of laser radiation in an isotropic plasma is found.

In the VUV range, the spectra of krypton, at pressures of a few hundred torr, consist of continuous emissions that are characteristic of Kr₂* molecules. A time resolved study of the second continuum of krypton ( lambda =145 nm) has allowed the authors to establish a scheme of the formation and decay kinetics of the excited species that is responsible for this emission. The radiative molecular state has a lifetime of tau ₂=250+or-20 ns, it is formed by two- and three-body collisions from an intermediate molecular state at a frequency 1/ tau ₃=18.6 p²+3.9*10³ p s^-1 (p in torr).

Certain theoretical predictions are presented for the preferential population of final states with angular momentum l' in collisions involving an initially excited atom. Varying l', the authors find that the maxima of both the inelastic form factors and cross sections for the nl to n'l' transitions in hydrogen, induced by collision with electrons and heavy particles, in general oscillate on a background which rises as l' is increased, until they both attain a pronounced peak at a unique value l'_max which is strongly dependent on only the initial principal quantum number n and which is fairly insensitive to changes in l and n'. An expression for l'_max is derived. For l'>l'_max, the form factors and associated cross sections exhibit a dramatic decline, resulting in negligible population of those states. The predictions differ from those suggested by the Bethe high-energy asymptotic limit which favours dipole transitions, and assume significance in situations where excited states are important.

The one-electron model of the K-vacancy sharing process in heavy ion collisions is studied using a variation-perturbation scheme. The radial couplings are calculated analytically with the method of Macias and Riera. The validity of the Demkov-Meyerhof model is discussed and compared with numerical determinations of the transition probabilities.

High precision relative elastic differential cross sections d sigma /d Omega _e for electrons scattered from ammonia are presented over a wide range of momentum transfer K. The effect of the bond formation is seen qualitatively through comparison with an independent-atom model (IAM) calculation, and the validity range of this IAM is clearly defined. Recent calculations using CNDO type wavefunctions, CI computations, and ab initio calculations within the Borne approximation are also compared with experiment and found to constitute a qualitative improvement over the IAM approximation. Tavard's theorem is employed to estimate the exchange energy of the molecule.

A formal discussion of the distorted-wave expansion is presented for the case of arbitrary optical distorting potentials. The authors show that the use, without further modifications, of such potentials of order greater than n in a DW expansion limited to order n results in multiple counting. Several local approximations to the second-order off-diagonal potential are examined. These potentials, along with the partial-wave amplitudes to which they give rise, are compared with the results of an exactly equivalent potential treatment for the 1s to 2s excitation of atomic hydrogen at energies of 100 eV and above.

Modifications of the well known Bethe expression for the asymptotic behaviour of cross sections for electron-impact excitation of optically allowed transitions are derived. Attention is drawn to a simple relation between the coefficient c_j of the Bethe expression and the value of energy at which the polarisation of the impact radiation passes through zero. This relation is used to examine experimental data for the Lyman- alpha line of hydrogen, the 501.6 nm line of helium and the 313 nm line of the beryllium ion.

The authors report Glauber cross sections for the electron-impact optically-forbidden transition ¹S(1s,3s) from ¹S(1s)² in helium in the incident energy range 29.2 to 500 eV. The Glauber amplitude is calculated using an alternative procedure for the evaluation of the modified Lommel function instead of the usual series presentation which requires very high precision arithmetic for the intermediate values of the argument. The present results for both the differential and integral cross sections in the single-particle Glauber approximation (SPGA) and the full Glauber approximation (GA) are compared with experimental findings and with the predictions of recent theoretical calculations. They find that the GA predicts the shape of differential cross sections even at a few electron volts beyond threshold. At higher incident energies the GA yields cross sections in good agreement with experiment. At low energies the SPGA deviates strongly from the GA and fails to predict the angular distributions of scattered electrons.

The excitation by electron impact at 50 eV, of the 104.8 and 106.7 nm lines of Ar, have been studied separately using the electron-polarised-photon coincidence technique. Polarisation correlations have been measured for electron scattering angles of 0 and 5 degrees . From the measurements at 5 degrees the authors derived the parameters lambda (0.72+or-0.06, 0.60+or-0.05) and mod chi mod (0.99 rad+or-0.19, 0.0) for the 104.8 and 106.7 nm lines, respectively. Thus, contrary to the assumptions of Arriola et al. (see ibid., vol.8, p.1275 (1975)) significant differences may occur between the two lines. The value of _chi obtained for the 106.7 nm line implies total coherence at the energy and angle studied. The measurements at 0 degrees yielded 'threshold' polarisations of 0.41+or-0.13 (104.8 nm) and 0.94+or-0.14 (106.7 nm).