Table of contents

The adiabatic and distortion approximations to a two-state treatment in the impact-parameter formulation of heavy-particle collisions are used to calculate the 2s and 2p single-excitation cross sections describing collisions between two ground-state hydrogen atoms with impact energy E in the range 360 ev <or= E <or= 100 kev. Comparison with results of an exact two-state treatment shows that the adiabatic approximation accounts remarkably well for the effect of back-coupling, which is neglected in the distortion approximation and which assumes increasing importance as the impact energy is lowered.

An impact-parameter treatment is carried out for the process H(1s) + He(1s²) -> H(2s or 2p) + He(1s²), with incident hydrogen atom energies E in the range 1 kev <or= E <or= 100 kev. Full account is taken of distortion, back-coupling, rotational coupling and the virtual transition sequence 1s l harp over r 2p l harp over r 2s. The calculated inelastic cross sections and the percentage polarization of the emitted Lyman-α radiation show satisfactory agreement with the recent experimental results of Ankudinov et al. and of Dose et al. The effect of simultaneous excitation of both the target and the projectile atoms is discussed.

Absolute excitation cross sections for the emission of the first positive bands of nitrogen under electron impact have been obtained. The maximum total cross section of the B ³capital Pi_g level was found to be 65·9 × 10^-18 cm². From measurements made on the cascade contribution from the D ³Σ_u level an approximate maximum cross section for excitation of this level of 3 × 10^-18 cm² was obtained. Other cascade features were observed and discussed. The results are compared with those of other workers.

The classical impulse approximation is used to calculate the cross section for electron loss from fast projectile atoms. The total loss cross section is the sum of an inelastic contribution which arises from collisions in which the target system is excited or ionized, and an elastic contribution which arises from collisions in which the state of the target system is unchanged. The inelastic contribution may be attributed to electron-electron scattering and its calculation presents no difficulty. The position with regard to the elastic contribution, which is due to electron scattering in the static field of the target, is different. It is inferred from the uncertainty principle that the application of classical mechanics may here lead to serious error, at least for light target systems. Such error is indeed found to occur. However, a classical description of the scattering is remarkably successful in the case of heavy target systems: for example, the derived total loss cross sections in H-Ar collisions do not differ from the corresponding measured cross sections by more than 25% through the impact energy range, 10 kev to 1000 kev, studied.

Calculations are also carried out with the target system replaced by a structureless model. Good agreement is obtained with the results of recent measurements which Gilbody et al. (1968) made in the range 60 to 450 kev on loss from metastable helium atoms passing through molecular hydrogen.

The general theory of fine structure transitions in atom-atom collisions is developed. It is shown that there is considerable simplification by first considering the LS coupling scheme and then transforming to the jj coupling scheme. The theory is then applied to collisions between oxygen atoms where it is assumed that the scattering is dominated by the first-order quadrupole-quadrupole term in the interaction potential. Results are presented for O(³P)-O(³P) collisions for energies below 0·12 ev. The total cross section is found to vary from 492 πa₀² at threshold to 128 πa₀² at 0·12 ev.

The cross section for the charge-exchange reaction H⁺ +H^- -> H+H has been measured, using an inclined-beams technique, over an energy range from 0·25 to 10 kev (in the frame of reference in which the target H^- ion is at rest). A proton beam collides with an H^- beam at an intersection angle of 20° and hydrogen atoms formed from proton charge exchange in the above reaction are detected. This technique provides a relative collision energy much lower than the energy of either beam and ensures separation together with efficient detection of the reaction products in each beam after the collision. The measured cross section, which falls from about 2·1 × 10^-14 cm² at 0·25 kev to 2·5 × 10^-15 cm² at 10 kev, is compared with the theory derived by Bates and Lewis in 1955 and three points of discrepancy are identified. At high energies the calculated cross section decreases as E^-1/2, whereas the experimental energy dependence is closer to E^-1. The experimental cross section is between two and three times larger than the calculation and it shows structure in the region of 1 kev.

The cross sections for quenching of orthopositronium by hydrogen atom, lithium atom and electron impact are calculated, using the Born approximation, as a function of relative incident velocity. An attempt is made to analyse the experimental results on positron annihilation in dilute solutions of lithium in liquid ammonia.

The classical equations of motion for binary collision between a low-energy argon ion and a gold atom have been solved computationally. Data are presented which relate the scattering angle of the incident ion in terms of the impact parameter for argon ion energies between 10 ev and 10 kev. The assumed interatomic potential between argon and gold is of the Born-Mayer form and is evaluated as the best approximation to experimental and theoretical data. For comparison, the computations were repeated using more recent but strictly theoretically derived potentials.

A detailed comparison has been made between the binary-encounter and Bethe theories for ionization of the (2p, 0) hydrogen atom by a fast incident charged particle. For this purpose we have mathematically reformulated the binary-encounter theory to treat the non-isotropic velocity distribution in the initial state of the atom considered. The pronounced dip found before in the Bethe theory is reproduced in the binary-encounter theory and is shown to be caused solely by the angular part of the initial-state wave function. Good agreement between the theories is found for energy transfers > 1 ryd. Applications to other initial states and other atoms are discussed.

A frequency-dependent dipole shielding factor β_infinity(ω) is defined as the coefficient of proportionality between the electric-dipole induced field and a uniform external electric field, at the nucleus, when the external field varies sinusoidally with time and with circular frequency ω. From oscillator sum rules it is shown that β_infinity(ω) = N/Z₀ + ω²α(ω)/Z₀ where N is the number of electrons, Z₀ is the nuclear charge and α(ω) is the dynamic dipole polarizability. Numerical calculations of β_infinity(ω) are reported for He, Be, Ne and several ions using coupled Hartree-Fock theory. It is found that calculations of β_infinity(ω) provide a severe test of the accuracy of zero- and first-order frequency-dependent wave functions.