Table of contents

It is shown that semi-classical theory provides an adequate description of the dynamical Stark effect recently observed in sodium vapour. Theory predicts consistently both optical nutation and a three-peaked Stark spectrum. Similar results are expected from the quantized theory.

The collision of positrons with sodium atoms is treated in the polarized orbital method. Results of calculations for the elastic and diffusion cross sections Z_eff, and the angular correlation function are given.

Reports the observation of resonances in the differential excitation function of the state 3p⁵4s (¹P) of argon in the autoionization region (24-32 eV). The measurement was carried out in the forward direction by means of a high-resolution electron spectrometer with a static gas target. More than 12 features are detected and compared with the observations performed by photon detection, transmission and electroionization spectroscopies. The interpretations of these features are discussed.

Cross sections for electron-mercury scattering at incident electron energies from 0.1 to 1.4 eV have been calculated by numerical solution of the Dirac equation including exchange and a polarization potential but not coupling to inelastic channels. A resonance has been found in the p_1/2 partial wave at about 0.2 eV which appears to correspond to the (6s²6p_1/2)²P_1/2 ground state of Hg. This is probably the resonance observed experimentally by Burrow and Michejda (1975) at 0.63 eV.

The rotational analysis of a sub-band ³ Pi ₀-³ Sigma of a P₂ radical's new transition is carried out. The kind of transition is determined: the upper state is the already observed b ³ Pi _g state (c³ Pi _u-b³ Pi _g transition), the lower state is a new state a³ Sigma _u⁺. The transition is analogous to the first positive system of N₂. Constants for the two states are calculated.

The possible use of level-crossing spectroscopy to study properties of excited states generated by gas excitation of fast ionic beams has been investigated. With a 20-400 keV Be⁺ beam excited by means of helium gas, radiative lifetimes of ²D and ²F levels in Be II have been determined together with the fine-structure separation of the 5f ²F level in Be II and the hyperfine coupling constant for the 2s3d ¹D level in Be I and the 2p ²p level in Be II.

The states were studied by selective detection of the fluorescence generated in atoms of the vapour by stepwise excitation, using as sources a conventional caesium lamp for the first step from the ground state and a pulsed, tunable laser for the second. The techniques of delayed coincidence counting were applied to measure the time distribution of intensity of the fluorescent light. The observed modulation, representing in each case a superposition of the hyperfine intervals showed a characteristic period which was interpreted as the reciprocal of the magnetic interaction constant a for the states in question. The relative simplicity of the modulation patterns is a consequence of the negligibly small quadrupole interaction in ¹³³Cs. With the polarizer oriented at cos^-1 3^-1/2 to the analyzer the modulation was suppressed, leaving a simple exponential decay from which the lifetime of the population tau , could be determined.

Radio-frequency power shifts in the 2 ²S_1/2-2 ²P_1/2 microwave resonance in atomic hydrogen have been measured and compared with the results of a numerical integration for the two- and three-level situations. Effects due to the initial phase of the RF field, the finite size of the atomic beam and its trajectory have been included in the numerical integration.

The electron impact excitation of hydrogen in the presence of an electromagnetic field has been formulated within the framework of the Glauber approximation. For a low frequency EM field the cross section formula takes a simple analytic form analogous to the one discussed by Kroll and Watson (1973) for the case of potential scattering. Numerical results are given for 1s to 2p excitation and results are compared with those obtained by the first Born approximation.

Using the form factor approximation and nonrelativistic Coulomb wavefunctions, cross section profiles for Compton scattering of gamma rays are calculated for all subshells of the atom. Spherical coordinate are used in order to facilitate a straightforward application of self-consistent wavefunctions. Relativistic energy and momentum relations are applied. Numerical data are presented, for E_gamma =279 keV and the K-, L- and M-subshells of copper and lead. Comparisons are made with cross section profiles obtained by replacing the final-state Coulomb wavefunction by a plane wave. Reasonable agreement between the two methods of calculation has been found for the L- and M-shells of copper for energies close to the centre of the profile. Larger differences occur for the K-shell of copper and the K-, L- and M-shells of lead even close to the centres of the profiles. Secondary maxima occur which are related to the nodes in the bound-state wavefunctions.

The angular resolution of a differential scattering apparatus of the common type is investigated. The primary and scattered beam are each assumed to be non-parallel and collimated by two arbitrary rectangular apertures. A function eta ( theta , Theta ) describing the acceptance of the apparatus for particles scattered at laboratory angle theta is derived, which connects the directly measurable angular distribution dN/d Theta with the differential cross section sigma ( theta ). The two cases of a gas cell target and a locally defined target such as an atomic or molecular beam are both treated.

Analytic approximations for the evaluation of matrix elements in free-free absorption are examined and assessed by comparison with accurate results. It is shown that satisfactory approximations can be obtained for s to or from p and p to or from d transitions and these are then employed to evaluate the free-free absorption coefficient for the negative helium ion. The results are expected to be accurate to better than 20%.

The R-matrix method is used to solve equations describing the scattering of electrons by helium atoms for incident energies near the n=2 thresholds. The 1¹S,2³S,2¹S,2³P and 2¹P target eigenstates which are included in the expansion of the total wavefunction are represented by configuration-interaction wavefunctions. In addition ¹P and ¹D pseudostates are retained in some of the calculations. Elastic and inelastic scattering cross sections are calculated and are compared with experiment and with other theories wherever possible. Particular attention is paid to resonances and cusps which dominate the cross sections in this energy region.

Cross sections, differential in angle, were measured for electrons elastically scattered from argon atoms in the energy range from 20 to 800 eV and included scattering angles from 12 degrees to 153 degrees . By measuring all of the physical quantities involved and calibrating the efficiency of the counting system, absolute cross sections were obtained that are independent of any other experiment. A description of the experimental apparatus and procedure as well as a discussion of the errors involved is given. Also a comparison with previous results is made. Within error bars, the present results agree with the measurements of Bromberg (1974) and Jansen et al. (1974) in the limited regions of overlap.

The collisions of thermal energy electrons which cause de-excitation of excited states of some metal ions are studied experimentally using flowing afterglow techniques. For all levels whose calculated radiative lifetimes exceed 100 ns, the effects of these collisions are apparent at electron densities of order 10¹³ cm^-3 as a destruction rate competitive with radiative decay. Large rate constants of order 10^-5cm³s^-1 are deduced. The theoretical basis of the temperature variation of collision rates is examined and it is shown that deexcitation rates are much less sensitive to electron temperatures than are the corresponding excitation rates. The results of this study have implications for the future development of metal-ion lasers because susceptibility of collisional depopulation of the upper level is reflected in the saturation behaviour of the laser transition.

Close coupling calculations of doubly excited autoionization states of three low-lying ¹P⁰ series in neutral neon are reported and compared with the results of photoionization experiments. Certain anomalies appearing in both the experiments and the calculations are discussed.

A lg-magnitude phase representation is used to effect the efficient approximation of multi-centre integrals through polynomial interpolation. When the multi-centre integrals are evaluated by numerical quadrature, computation costs can be reduced by an order of magnitude or more.

A model for the mechanism of charge exchange is proposed. This model motivates the triple-centre expansion and predicts which basis states are needed to adequately model a charge-changing reaction.

The utility of the adiabatic and the diabatic representations in molecular eigenfunction expansions for He⁺-He charge exchange scattering has been considered. The charge transfer probability and cross section have been calculated in both representations over a wide energy range in the uncoupled approximation. Comparison with experiments above 1 keV shows good agreement with the uncoupled diabatic calculations, but a few features appear which are expected to be due to coupling between the two crossing ² Sigma _g states of the quasi-molecule.

Absolute cross sections, differential in angle and energy of the scattered particles, are obtained for single charge transfer and elastic scattering of He²⁺ from He for incident ion energies 200 eV to 600 eV and lab angles 2.5 degrees to 20 degrees . No ground state He⁺ are found and oscillations due to double charge transfer are observed in the elastic channel. These data are interpreted in terms of ¹ Sigma _g⁺ and ¹ Sigma _u⁺ potentials in the incident channel and curve crossings of this ¹ Sigma _g⁺ potential with higher states.

From the thermal beam-gas cell absorption measurements van der Waals constants for the Cs-He, Ar, and Rb-He, Ar, Kr and Xe interactions of C₆= 34+or-4, 284_-14⁺⁴², 30+or-3, 252_-13⁺⁴⁰, 399_-10⁺⁶⁰ and 681_-34⁺¹⁰²*10^-60 erg cm⁶ were obtained, in good agreement with quasi-theoretical values.

Two types of spectra obtained with low energy bombarding electrons are presented which have an energy resolution of 40 meV (FWHM). The energy loss spectrum obtained in the forward direction (10 degrees ) with E_i=25 eV electrons shows a series of unresolved peaks that agree well with the Rydberg states D,D',D",... and B,B',B",... and especially with photoionization data. Complementing this, the threshold excitation spectrum of H₂ shows a simpler structure which agrees well with the following series of triplet states c³ Pi _u, e³ Sigma _u⁺, g³ Sigma _g⁺, d³ Pi _u, k³ Pi _u and n³ Pi _u. It was observed that most of these states have a very large and sharp excitation cross section at threshold which has its origin in several series of resonances lying within 30 meV of these triplets. The angular behaviour of the threshold excitation spectra is discussed and absolute differential cross sections are reported for the c³ Pi _u (v=1) states as a typical example.

For pt.VI, see abstr.A19902 of 1974. Inclined beams of electrons and H₂⁺ ions have been used in an experiment to measure cross sections for the reaction e+H₂⁺ to H⁺+H^-. The cross section was found to fall from 4.9*10^-18 cm² at 0.4 eV to 1.16*10^-18 cm² at 4.96 eV and systematic errors of the measurement were assessed to be less than +or-8%. An ion source was used which produced H₂⁺ with the vibrational population measured by von Busch and Dunn (1972). The results are discussed and compared with theoretical predictions by Dubrovskii and Ob'edkov (1967).

An expression for the inelastic collision cross section sigma (lm to l'm'), for rotational-translational energy exchange has been obtained using the distorted wave approximation. Values of sigma (0 to 2) have been calculated for systems pH₂-He, , pH₂-Ne, pH₂-Ar, oD₂-He, oD₂-Ne, and oD₂-Ar. Using the averaged sigma (0 to 2) values, rotational collisional numbers, Z_rot, have been obtained as a function of temperature. The results have been compared with the experimental values, and are found to be in good agreement. It is found that the attainment of rotational equilibrium is easier in D₂ systems than in H₂ systems. The theory predicts the existence of a maximum in the Z_rot against temperature curve in the low temperature region. The possibility of a minimum in the high temperature region is discussed.