Table of contents

The interaction between some 1s2p4l⁴L terms and the corresponding 1s2snl'/ epsilon l'⁴L series has been studied in Be II. The 1s2p4f⁴G and 1s2p4d⁴P⁰ terms are found to be distributed over a large number of 1s2snl' states, the ⁴G distribution even extending well into the continuum. This makes it unlikely that these terms can be associated with isolated lines in the Be II spectrum. It is also found that which of the ⁴F⁰ eigenstates has the largest 1s2p4d contribution depends sensitively on the interaction with the series.

An MCDF-EAL calculation on the 1s2s2p², 1s²2s2p and 1s2p³ states in the beryllium isoelectronic sequence has been used to predict fine structures and transition wavelengths for atomic numbers in the range 4<Z<20. The authors report improved agreement for the ⁵P-⁵S transition wavelengths with experiment in C III, N IV and O V by comparison with previous MCDF-OL calculations.

Computation of hyperfine splitting in the 1s2s (2³S₁) state of two-electron systems ^6,7Li⁺ has been carried out. The ratio of the splitting intervals in such a state deviates from the Lande rule. This deviation is caused by an additional shift delta of the triplet level corresponding to the quantum number F equal to I, I being the nuclear spin. It is shown that the main contribution to the shift is given by the second-order correction in the hyperfine interaction. Values calculated in the present paper, delta (⁷Li⁺2³S₁)=371.5 kHz and delta (⁶Li⁺2³S₁)=28.4 kHz, are in satisfactory agreement with experimental ones, delta (⁷Li⁺2³S₁)=366 (29) kHz (Kotz et al.), delta (⁶Li⁺2³S₁)=13 (37) kHz (Neumann et al. (1981)). They are also to be compared with previous theoretical results, given in the work of Kotz et al. (1981), delta (⁷Li⁺2³S₁)=511 kHz and delta (⁶Li⁺2³S₁)=39 kHz.

Points out that angular momentum exchange in a post collision interaction (PCI) between a slow photoelectron and a fast Auger electron should lead to observable structure in the PCI-broadened Auger line. A semiclassical formula is derived for the spectral shape of the Auger line for the case of angular momentum exchange, and it is shown that the predicted shape can account for some unexplained features of PCI-broadened Auger lines recently observed for the process of electron impact ionisation of the Ar L shell.

Detached electron energy spectra yielded by the collision of H^- with He, H₂ and N₂ in the collision energy range from 10 to 750 eV have been obtained. Excellent agreement with previous experimental and theoretical studies of He was found. The evolution with energy of charge exchange to the N₂^-2 Pi _g shape resonance was determined. No charge exchange to the ² Sigma _u⁺ resonance was observed in the case of H₂.

For pt.I see J. Chem. Phys., vol.77, p.3571 (1982). The recently proposed perturbative treatment of core-valence correlation effects is extended to valence correlated wavefunctions. As expected, the valence correlation slightly diminishes the core-valence correlation for the ground state. This effect is illustrated for the atomic electron affinity of K. The core-valence correlation plays a very important role in the spectroscopy of K₂ and KH, where it increases the lowest X¹ Sigma ⁺ to A¹ Sigma ⁺ transition energies by 2300 and 1100 cm^-1, respectively.

The many-body perturbation theory is used for relativistic calculations of energy levels and of fine-structure (FS) intervals for the 6s, 7s, 8s, 6p, 7p, 5d, 6d and 4f states of Cs. The second-order correlation corrections in the residual Coulomb interaction are calculated taking into account all intermediate configurations of a discrete and continuous spectrum with orbital angular momentum of each electron l<or=5. The wavefunctions and energies in the zeroth approximation are obtained by the relativistic Hartree-Fock (RHF) method. The correlation corrections substantially improve the agreement between the calculated and experimental values. The calculated values of the FS intervals are the following (in cm^-1): 556 (6p), 185 (7p), 63 (5d), 48 (6d), and -0.180 (4f). The experimental data are: 554; 181; 97.6; 42.9; -0.1813, respectively. The mechanisms of fine splitting in Cs are discussed.

By adopting a frozen-core approximation with the two valence electrons treated in suitably chosen model potentials, the correlations of doubly-excited states of Li^- and Be are studied in hyperspherical coordinates in the adiabatic approximation. The characteristics of correlation patterns for these states are found to be similar to those for the corresponding states of H^- and He except for the gradual loss of the strong angular correlation for Li^- and Be at large hyper-radius. Adiabatic potential curves and eigenenergies for the doubly-excited states lying below the n=2 and n=3 limits of the one-electron ions are calculated and compared with results from the configuration interaction method using the same model potential to assess the validity of the adiabatic approximation. It is concluded that for certain (L, S, pi ) states the adiabatic approximation is quite adequate but for others, e.g. ¹P⁰, couplings between neighbouring channels are important.

The 'flash pyrolysis' technique-i.e. evaporation of a finely divided sample by a strong flux of light from a high-energy flash discharge-has been used to obtain the absorption spectrum of Te I in the range 1370-1990 AA. Wavelengths and estimated intensities are given for 196 lines, most of which are new and are classified. A revision and extension of the earlier analysis of Te I has added 51 new odd-parity levels to the 32 previously known levels. Triplet and singlet terms of the 5s5p⁵, 5s²5p³(²D)6s and 5s²5p³(²P)6s configurations have been located. In addition to the absorption lines from the 5s²5p⁴³P₂ ground state, series have been observed from the excited levels ³P₁, ³P₀ and ¹D₂. The unresolved series 5s²5p³(⁴S)ns and 5s²5p³(⁴S)nd, observed in emission by Ruedy, have been revised and extended with the assignment of J values to most of the observed levels. Examples of configuration mixing and perturbations are revealed in the analysis.

Using classical mechanics the author analyses the effect of a magnetic field on a hydrogen atom. The field is assumed weak enough for both classical and quantal degenerate perturbation theory to be valid. An approximate Hamiltonian describing the mean motion is found and is shown to have several unusual features. In particular it has a double minimum with phase space comprising regions of both rotational and librational motion. This double minimum produces a degeneracy in the lower levels of the quantal spectrum for sufficiently large principal quantum number. A comparison between the predictions of the classical model, presented here, and purely quantal calculations is made and agreement obtained.

The author has studied the effect of an adiabatically and near-adiabatically increasing field intensity on the resonance fluorescence spectrum of the two-level atom. There is an asymmetry in the spectrum for off-resonance excitation and that this is due to the fact that the field turn-on repopulates the dressed state which has been depopulated by spontaneous emission. Also found are the conditions under which the temporal shape of the field turn-on may be found in a simple way. Corrections to this shape due to atomic recoil are discussed, and found to be small under typical experimental conditions.

Radiative lifetimes have been determined for 25 odd-parity levels of neutral Ir using the technique of laser-excited fluorescence from sputtered metal vapour. The lifetimes are combined with branching ratio measurements to derive absolute oscillator strengths for 27 transitions in Ir I. The new lifetime values are a factor of 2.5 higher for two out of three of the only previously reported lifetimes in Ir, while the oscillator strengths are systematically smaller, by factors of up to seven, than the arc emission results of Corliss and Bozman (1962).

A semiclassical analysis of the differential cross section for elastic scattering at low energies is presented. Three energy regimes are distinguished: for high energies one has ordinary rainbow scattering. In a transition region above the critical orbiting energy the energy dependence of the differential cross section at fixed scattering angle shows an infinite sequence of rainbows. Below the critical energy, broad classical orbiting resonances are present, which are due to the constructive interference of an infinite number of classical trajectories and which must be distinguished from the contribution of the sharp quantum tunnelling resonances. Numerical results are presented for H⁺+He collisions.

Collision-induced coherent S to D excitation is investigated. A polarisation analysis of the subsequent D to P transition by the photon-particle coincidence method is not sufficient to completely determine the excitation amplitudes for the D state. Two distinct solutions are obtained. The electron distributions associated with these two are mirror images of one another in the plane determined by the normal to the scattering plane and the direction of maximum intensity of linearly polarised light, as measured from the direction of the normal. The Li(2s to 3d) excitation induced by collisions with helium is studied both theoretically and experimentally in the keV energy range. The measurements confirm the existence of a theoretically predicted low-energy structure in the circular polarisation. The results are interpreted by generalising concepts previously developed for S to P excitation in quasi-one-electron systems.

The spectra of Ne K X-rays excited by beams of 4.7 MeV He ions, 16.7 MeV C ions, 32.4 MeV Mg ions and 54.0 MeV Ar ions have been measured with a curved crystal spectrometer. A detailed analysis of the multiplet line structure contributing to the K_alpha satellites and hypersatellites was performed and comparisons with the experimental spectrum excited by C ions yielded quite good agreement with the theoretical predictions. The correspondence of the ratios of the hypersatellite to satellite relative intensity ratios to those predicted by the ratios of the average theoretical fluorescence yields was investigated. The dependence of the average L-vacancy fraction for Ne on the ratio of the projectile velocity to average charge was extended beyond the region previously explored and large deviations from the original linear function were found.

The total cross sections for resonant capture of electrons from atomic hydrogen by protons is calculated for energies from 0.1 to 50 keV and compared with the measured cross sections and other theoretical calculations. The impact parameter Schrodinger equation is solved using an expansion in a two-state orthonormal basis constructed with travelling 1s atomic orbitals in which the static charge seen by each orbital is allowed to vary with the internuclear distance.

The high-energy higher-order Born (HHOB) approximation is applied to the elastic scattering of electrons from the excited 2s state of atomic hydrogen. Results of calculations at intermediate energies are reported along with the recent data.

A method is proposed for calculating the cross sections of the collision processes of elastic scattering, vibrational excitation, dissociation and dissociative attachment in electron scattering with an initially vibrationally excited nitrogen molecule. The method is based on the treatment of the collision as a three-body process, using Faddeev equations. The results are compared with available experimental results and other calculations, giving satisfactory agreement on the average.

The alternative definitions of the local complex potential for nuclear motion in short-lived electronic states are discussed. It is shown that different definitions may yield very different potentials in the case of broad resonances. Within an exactly solvable model the electron scattering cross sections calculated in the various local approximations are compared with one another and with the exact results. It is found that there is no generally preferable local potential. The local potential obtained by a Breit-Wigner analysis of the phaseshift gives superior results in the elastic channel, whereas the local potential determined from the pole of the S matrix yields better results in the vibrationally inelastic channels. As a by-product of the analysis, it is found that wildly differing local complex potentials may yield very similar electron scattering cross sections. This casts serious doubts on the uniqueness and reliability of empirical local complex potentials obtained by fitting experimental scattering cross sections.

The energy and angular dependences of the elastic and inelastic scattering of electrons from methane have been measured in the range of collision energies from 0.1 to 1.8 eV. Of special interest are the shapes of the excitation functions of the four possible vibrational modes of the molecule, because the elastic cross section shows a pronounced Ramsauer-Townsend minimum and this is compatible with the existence of a short-lived compound state of the electron-molecule system being detectable very close to the thresholds of the inelastic channels. The measurements show structures right at the thresholds, which have half widths of 0.1-0.3 eV and seem to be predominantly in the s and f-waves of the scattered electrons. In addition, a cusp structure has been found in the nu ₂, nu ₄ channel. The transmission functions of both the electron gun and the electron optics of the detector have been carefully controlled and therefore the peak shapes and half widths of the threshold structures should be more accurate than the results of Rohr (1980) who first reported the existence of the threshold peaks in methane at a 60 degrees scattering angle.

The hydrogen-deuterium excitation transfer process is investigated experimentally for the case of the n=3 levels. Furthermore, a numerical simulation is provided for testing the method of analysis. The corresponding cross section is evaluated and compared with theoretically calculated values. Results of the numerical simulation are discussed.