Table of contents

The second differences in Hartree-Fock SCF energies are found to be constant for isoelectronic series.The 1/Z perturbation treatment of the Hartree-Fock equations allows us to relate these differences to the numerical findings. A recurrence relationship for electronic total energies is obtained and from its application very good agreement with SCF data is found.

The three lowest singlet states of acetylene, 1 ¹A_u, 1 ¹B_u and 2 ¹A_u, are well described by a pi -electron model. The ratio of the coefficients of the main configurations assumes a simple analytical form. The behaviour of the ratio with respect to the CCH bending angle is illustrated. The origin of the difference in behaviour is elucidated.

The long standing discrepancy between theoretical and experimental interpretations of the nature of the A ² Pi state in the BeF is re-examined. Calculated and experimental Lambda -doubling constants are now in good agreement.

The energy spectrum of electrons resulting from the multiphoton ionisation of xenon by a frequency-doubled Nd-YAG laser is analysed. It is shown that for intensities of 10¹¹ W cm^-2, processes involving up to four continuum-continuum transitions coexist with the lowest-order ionisation process, with comparable probabilities.

Calculations are reported of the differential cross section of the laser-assisted X-ray photoeffect from K shells showing that drastic changes occur in the shape of the ejected-electron angular distribution when laser photons are exchanged during the ionisation. This process appears well suited to the study of laser effects and multiphoton exchanges in laser-assisted atomic collisions.

Linear and circular polarisation measurements have been carried out for photons from the 6³P₁-6¹S₀ transition in mercury, which were detected in delayed coincidence with electrons having excited the 6³P₁ state. These measurements have been used to determine values for the parameters lambda , chi and the spin-orbit phase angles Delta and epsilon for the 6¹S₀ to 6³P₁ to 6¹S₀ excitation/de-excitation process in mercury.

The polarisation of 2154 nm photons emitted from Hg atoms which are excited by electrons scattered in the forward direction has been measured for collision energies between 5.5 and 15 eV. From the results the relative contribution of exchange excitation could be determined. Exchange is found to be the dominant excitation mechanism from threshold up to 8 eV collision energy. The results are also expressed by the alignment parameter lambda and good agreement is found with a previous measurement using incident polarised electrons. The results are explained in terms of intermediate coupling.

Electron-molecule collision processes involving vibro-rotational and, possibly, electronic excitations are analysed in a fixed-nuclei picture, and the influence of the nuclear motion isolated for the first time in the total scattering T matrix. Corrections due to the nuclear relaxation are in the form of an exponential operator, involving both space and energy differentiations of a fixed-nuclei electronic T matrix. The theory suggests that this amplitude should be evaluated off-shell, and the correct 'electronic' energies of the total system, in the entrance and exit channels, respectively. It is finally shown that, under the hypothesis of a short-lived resonance, Herzenberg's boomerang model follows naturally from the present theory.

The polarisation of the hydroxyl fragment radiation arising from highly rotational excited OH radicals produced in the electron-impact dissociation of H₂O has been measured for incident energies ranging from just above threshold (9.3 eV) up to 500 eV. Whereas the OH fluorescence is essentially unpolarised for energies greater than about 150 eV, a significantly non-zero polarisation occurs when the impact energy approaches threshold; by monitoring either pure P-branch fluorescence or a mixture of P- and predominantly Q-branch radiation extrapolated threshold polarisations of +7 and -9%, respectively, are obtained. The sign of the polarisation at threshold implies that rapidly rotating (abnormal) hydroxyl radicals arise from a parent H₂O state of symmetry A₁.

The role of the atomic structure in inverse bremsstrahlung for high incident electron energy is explored. For the electron-hydrogen system it is seen that the contribution due to the atomic structure increases uniformly with frequency and that at excimer laser frequencies the atomic structure may produce a substantial effect.

The authors show that additional contributions, which depend on the spectral width of the driving laser in weak-field resonance fluorescence, have nothing to do with the boson character of the photon. Simple calculations show that these different contributions can be associated with different models of the density matrix of the laser pump, but not with the indistinguishability of light quanta as has been argued in recent publications.

The energy function of a perturbed quantum system is derived directly as a quotient of the form N( lambda )/D( lambda ). The Taylor series coefficients of N( lambda ) and D( lambda ) are found to satisfy certain relations, but there remain many unspecified degrees of freedom which may be freely exploited. Certain choices of coefficients lead to well known Pade and lesser known Levin approximants, but the general formalism includes many other rational function forms. In particular, the present procedure allows information other than the Taylor series coefficients to be included naturally, and it is shown to lead to low-order approximants of high accuracy.

Second-order Lambda -doubling constants p and q are calculated for SH to an accuracy of about 1% for p₀ and about 4% for q₀ without any need to incorporate effects of core polarisation. The third-order constants, p_J and q_J are very sensitive to the quality of the potential curve for the excited state but comparison of ab initio values with experimentally derived parameters shows reasonable agreement.

The argon 2p photoelectron spectrum has been measured using Al K alpha radiation. A complex satellite spectrum within 35 eV of the main photoelectron lines is reported with an intensity of 9.6% relative to the main 2p lines. The spectrum has been interpreted with the aid of a theoretical study using correlated final-ion-state wavefunctions calculated with a configuration interaction technique. The major contributions to the satellite spectrum come from 'shake-up'-type processes which have their origin in relaxation effects and involve 3p to np excitations.

The authors present quantum mechanical calculations of Stark broadening parameters for the Li I 2s²S-2p²P line. Semiclassical calculations have also been performed and both have been compared with other available results. A detailed analysis of Stark broadening parameters as functions of the impact electron angular momentum quantum number and of the temperature is carried out. The influence of the polarisation potential and of the completeness of the set of energy levels on the Stark broadening parameters is investigated. The low temperature behaviour of the half width is discussed too.

A new B² Pi _3/2-X² Sigma ⁺ transition has been observed in emission in the blue region. The constants derived from a rotational analysis are given. The X² Sigma ⁺ state and the A² Pi state, the lower state of a previously analysed C² Delta -A² Pi transition, are assumed to form a unique perturber pair.

Individual rotational levels in the B³ Pi 0⁺u state of ⁷⁹Br₂ and ⁸¹Br₂ were excited by an acousto-optically chopped, single-mode Ar⁺ laser. The intensity of the fluorescent light, measured as a function of time following the laser pulses, was found to exhibit Zeeman quantum beats. A study of the beat profiles has permitted the determination of the rotational Lande factors of the levels involved.

The radial second-order differential equations of the theory of collisions between atomic systems plus the boundary conditions at infinity are transformed to an equivalent system of first-order differential equations by introducing for each channel the Wronskian of the wavefunction and the Jost solution of the uncoupled equation (incoming and outgoing Jost solutions for open and closed channels, respectively). In the asymptotic region the Wronskian gives directly the value of the corresponding S-matrix element. By using the first-order Magnus-Fer approximation an approximate analytical expression for the S-matrix elements of the multichannel problem is obtained; the formula is also particularised to the two-channel problem.

A new type of multicoincidence technique, that simultaneously handles 64 channels is used to carry out a detailed investigation of the He (n=2) excitation in He-He collisions. Processes leading to formation of metastables are studied using recoil-scattered-particle coincidence measurements whereas He(2¹P) excitation processes are investigated by photon-scattered-particle coincidences. The magnetic sublevel population for the He(2¹P) excitation is determined from the angular correlation between the photon and the scattered particle. The data are discussed in the framework of the independent-electron quasi-molecular model. One-electron excitation is consistent with the model whereas, for two-electron excitation, effects are found which cannot be accounted for within the independent-electron picture.

The polarisation of the 492 (1s4d¹D to 1s2p¹P) and 668 nm (1s3d¹D to 1s2p¹P) fluorescence lines of He I impact excited by 10-35 keV Ne⁺ ions has been investigated. Using magnetic depolarisation techniques, the contribution of cascade processes to the polarisation fraction could be separated. For both spectral lines the authors find a strong but approximately energy-independent cascade contribution to the polarisation fraction. Therefore the significant energy dependence of the total polarisation fraction reflects mainly an energy dependence of the direct excitation process. This result confirms the lack of correlation between emission cross sections and polarisation data of He I lines previously observed by Andersen et al. (1977) for Ne⁺-He collisions.

Using level-crossing techniques, the authors measured relative excitation cross sections sigma _m of Zeeman sublevels of the 1snd ¹D level (n=3,4) in He I excited by 10-35 keV Ne⁺ ion impact. The results are free of influences of cascades. A pronounced energy dependence has been observed. Even in the 25-35 keV energy range, where the polarisation fraction of the observed fluorescence light is approximately energy independent, the relative values of sigma _m vary strongly with energy.

The influence of charge exchange to a continuum state on the shape of autoionising resonances observed in the ionisation of atoms by fast protons has been investigated. The authors have used a simple method for taking account of the final state interaction between the scattered proton and the ejected electron, which employs the first iteration in solving the Faddeev equations. The quantitative results obtained for the autoionising states of the helium atom qualitatively explain the available experimental data.

For pt.I see ibid., vol.13, p.1601 (1980). The alignment of ions with an inner-shell vacancy resulting from electron capture in ion-atom collisions is treated theoretically. The general expressions for the alignment are obtained in the Oppenheimer-Brinkman-Kramers and Born approximations. Simple analytical formulae for the alignment are derived within the framework of the hydrogenic model. The dependence of the degree of alignment on the incident particle velocity and the ratio of charges of the colliding particles is analysed. The alignment due to electron capture is shown to differ essentially from the alignment due to direct Coulomb ionisation. The sensitivity of the degree of alignment to the form of captured-electron wavefunctions and to the type of perturbing interaction is investigated. The experimental data on the angular distribution of Auger electrons in the ionisation of the 2p_3/2subshell of Mg by protons and He⁺ ions are reanalysed taking into account the electron capture mechanism.

The impact parameter dependence of the capture of a target L-subshell electron by a light projectile is calculated within the semiclassical impulse approximation (SCIA). The anisotropy of the resulting target 2 p vacancies is strongly dependent on impact parameter and projectile energy. As an example, (p, Ar) and (p, Ne) collisions are studied and the capture cross sections are compared with experimental data.

Electron-hydrogen and electron-helium elastic scattering problems have been studied from an asymptotic analysis of the off-shell amplitudes. The resulting differential and total cross sections are observed to agree nicely with the experimental results and with the most elaborate theoretical calculations, such as the many-body theory employing the Hartree-Fock Green's function for electron-helium scattering and the pseudostate calculation of Fon, Burke and Kingston (1978) for electron-hydrogen scattering, over wide energy ranges.

A modified electron-photon coincidence experiment on the excitation of the 3¹D state of helium is suggested. The idea is to measure the scattered electrons, which have excited the 3¹D state, in coincidence with 2¹P to 1¹S photons resulting from the 3¹D to 2¹P to 1¹S cascade. Due to their large energy the latter can be detected with higher efficiency than the directly emitted 3¹D to 2¹P photons. A theory is developed to show that the proposed experiment yields exactly the same information on the 3¹D excitation as a conventional coincidence experiment between scattered electrons and 3¹D to 2¹P photons. Preliminary experiments have been performed, in which the 3¹D excitation is measured simultaneously with the 3¹S and 3¹P excitation. The coincidence signal due to the 3¹S excitation, with its isotropic radiation field is used to calibrate the 3¹P and 3¹D measurements.

Utilising a crossed electron-beam-molecular-beam scattering geometry, relative values of differential electron scattering cross sections for caesium chloride at 5 and 20 eV electron impact energies and at scattering angles between 10 and 120 degrees have been measured. These relative cross sections have been normalised to the cross section at 15 degrees scattering angle calculated by the hybrid S-matrix technique. In the angular range between 0 and 10 degrees and between 120 and 180 degrees extrapolations have been made to obtain integral and momentum transfer cross sections. An energy-loss spectrum is also presented which gives various spectral features lying between the 4 and 10 eV regions in CsCl.

Calculations are reported of total and differential cross sections for elastic scattering and excitation of the 1¹S to 2¹S and 1¹S to 2¹P transitions in helium by positrons with energies between 60 and 400 eV. Results are given in three-state close-coupling, Born, Glauber, eikonal Born series, optical model and a distorted-wave Born model. They are compared with electron impact results. Excitation cross sections are summed and an estimate of the ionisation cross section derived from the total cross section measurements of Twomey et al. Cross sections for 2¹S to 2¹P and coincidence parameters for 1¹S to 2¹P transitions at 80 eV are briefly discussed.

The rate coefficients and product ion distributions for the reactions of H₂O⁺ with Xe, NO, O₂, H₂S, NH₃, COS, H₂, CH₄ and CO have been determined at 295+or-1K using a selected-ion flow-tube (SIFT) apparatus. Reaction was generally seen to proceed by electron transfer where this was energetically possible. Additional channels of H and H^- transfer were observed with CH₄. Reaction with H₂ was by H transfer and with CO by molecular rearrangement. The majority of the reactions observed had rate coefficients significantly smaller than the classical collision limit. No evidence for the presence of excited N₂O⁺ was found.

Reactions of HN₂O⁺ have been investigated in flow tubes at room temperature. For the reactions of HN₂O⁺ with both CH₄and NO the observations may be described accurately in terms of two components of the reactant ions distinguishable by different reactivities. The ratio of the two components is approximately 5:1. The HN₂O⁺ ions were produced from the reaction of N₂O⁺(X² Pi ) with H₂.

A new flow-drift apparatus is described in which gas phase ion mobilities and reaction rate coefficients may be determined over an ion energy range extending from thermal energy at 300K to several eV. Rate coefficients for the reactions of Ne⁺ with H₂S, SO₂, CO₂, NH₃, N₂ and CO have been obtained as a function of ion-neutral relative kinetic energy E_CM. Measurements of the mobility of Ne⁺ in helium and of the He⁺+N₂ reaction rate coefficient are compared with existing experimental data. The six neon ion reaction rate coefficient are compared with existing experimental data. The six neon ion reactions studied exhibit three distinctive forms of energy dependence; large rate coefficients weakly dependent on E_cm (SO₂, H₂S); rate coefficients showing pronounced minima as E_CM is varied (CO₂, NH₃), and rate coefficients increasing by several orders of magnitude as E_CM is increased (N₂, CO). These energy dependences are discussed in terms of simple models for the interactions.