Table of contents

The Glauber approximation has been used to study the elastic scattering of electrons from lithium atoms. The effect of core electrons has been ignored by adopting a frozen core approximation. The total cross section for elastic scattering is compared with other theoretical calculations and the experimental data.

It is shown that the differential cross section for the elastic scattering of electrons by helium atoms in the forward direction, calculated using the third order approximation is in satisfactory accordance with the values obtained by Bransden and McDowell employing a dispersion relation, and with their extrapolations of experimental data to zero angle.

Quantum defects for He I and Li II obtained from an extrapolation of phase shifts for elastic scattering of electrons by hydrogenic positive ions are compared with those obtained from z expansion perturbation theory by Blanchard.

It is argued that the translation factor in the basis functions of the perturbed stationary state approximation must, in the separated atoms limit, involve v.r rather than Rr where v is the relative velocity of the nuclei, r is the position vector of the active electron, and R is the distance between the nuclei. The adoption of the incorrect translation factor leads to the appearance of spurious terms, increasing indefinitely with R, in the transition matrix elements. This has an insignificant effect on capture in slow H⁺-H(1s) collisions.

Expressions are derived for the rotational energy levels of ⁴I states of diatomic molecules taking into account the centrifugal distortions of the spin-spin and of the spin-rotation interactions in addition to the perturbations of neighbouring terms transferred by the spin-orbit interaction. The new expressions are applied to the ⁴ Sigma state of the SnH molecule and show better agreement with the experimental data than the earlier formulas.

The third virial coefficient of polar gases has been calculated on a preaveraged 12-6-6 potential model. The nonadditive corrections due to dipole, dispersion and repulsion three body interactions have also been computed. It has been found that while the dipole and dispersion nonadditive corrections are always positive, the repulsion nonadditive correction is negative. The net effect of these corrections is sizeable in the temperature range of our interest. The theoretical results have been compared with the experimental data of ammonia and methyl fluoride. The agreement between theory and experiment is reasonable in view of the large uncertainties in the experimental data. The overall agreement between experimental values and those obtained on the preaveraged potential for polar gases is comparable to that of the Lennard-Jones (12-6) model for nonpolar gases.

A double modulation microwave spectrograph complete with phase sensitive detection and frequency standard has been set up. Widths of the OCS J1-2 line in the ground vibrational state at v=24325.92 MHz as broadened by six different non-polar perturbers, C₆H₆, CCl₄, CS₂, C₇H₁₆, C₆H₁₂ and C₆H₁₄, have been measured. The linewidths have been calculated theoretically on the basis of the theory of Anderson and of Murphy and Boggs and compared with the measured values.

From the data on the line strengths, the band strengths of the 1-0 and 2-0 vibration-rotation bands of H⁷⁹Br molecule have been respectively calculated to be 10.65+or-0.22 and 0.174+or-0.008 cm^-2 atm^-1 at 294 K. From the vibrational matrix elements involving the Morse and anharmonic oscillator wave functions, two sets of values for the dipole moment coefficients M₁ and M₂ have been calculated. They are M₁=0.316 debye AA^-1, M₂=0.518 debye AA^-2 or M₁=0.344 debye AA^-1, M₂=-0.096 debye AA^-2. These values are compared with the values reduced from the low-resolution data.

New bands with open but complex rotational structure have been obtained in the regions 4700-4800, 5030-5110 and 5210-5500 AA, in absorption behind reflected shock waves through titanium powder in hydrogen-argon mixtures. The isotope shift shows that they are all due to (0,0) transitions. The bands are probably slightly predissociated.

A beam attenuation technique, developed previously in this laboratory, has been used to investigate the formation of metastable helium atoms by electron capture of 10-200 keV He⁺ ions in hydrogen, helium, neon, argon, krypton and nitrogen. The fractions of metastable atoms formed in both thin and thick targets have been determined and used to obtain separate cross sections for the formation of ground state and metastable atoms. The equilibration of the metastable component of the beam in thick targets has also been described quantitatively in terms of the relevant charge changing collisions.

The method of steepest descents is used in the evaluation of certain matrix elements found in the impact parameter treatment of atomic scattering. The final form of the matrix element leads to very efficient numerical calculations when certain of the parameters become large and avoids the rapid oscillations encountered in the more straightforward treatment.

Investigation of the large angle scattering behaviour of the narrow wave packet approximation is made. First an expression is derived for the scattering amplitude in the narrow wave packet approximation when the potential is arbitrary. Then the same expression is derived for the case when a potential is spherically symmetric. In many aspects the approximation behaves as Jeffrey's approximation. Since the narrow wave packet approximation breaks down in the vicinity of the singularity of a potential, the validity of the approximation for the case when the potential is short range and has a singularity near the origin is investigated.

The Glauber approximation has been applied to the 1s-2s and the 1s-2p excitations of hydrogen atom by proton impact. The total cross sections for the processes have been calculated for incident proton energies ranging from 1 keV to 1 MeV. These have been compared with the corresponding results of other theoretical workers. Comparison has also been made with the available experimental results. At incident energy values of 20 and 30 keV, the results for the total cross sections for the 1s-2p excitation coincide with the experimental values.

An equivalence is established between the alignment tensor introduced by Sandle and Williams to describe the polarization properties of resonance radiation and the reduced density matrix for the ensemble of excited state atoms which emits the resonance radiation.

Investigates the levels of the 2p⁵3p configuration of neon by studying the polarization of the light emitted by a dc neon discharge. The pressure broadening of the Hanle signals was studied and cross sections were obtained for the destruction of alignment for collisions with both helium and neon ground state atoms. The measurements have been made at 315 K and 85 K. The cross sections for the 2p₂ and 2p₆ levels (Paschen notation) change considerably with temperature, while the cross sections for other levels are almost independent of temperature.

The impact theory is used to calculate the cross sections for the destruction of alignment of excited neon atoms in the levels of the 2p⁵3p configuration by collisions with ground state helium or neon atoms. The interatomic force is assumed to be due to the electrostatic dipole-dipole interaction. The results are compared with experimental values. There is good numerical agreement for depolarization by neon but for helium perturbers the theoretical values are smaller by a factor of two than the experimental values. The detailed comparison of the experimental and theoretical results suggests that for collisions with helium and neon atoms the repulsive part of the potential is important and that the deviation of the colliding particles from straight line paths should also be taken into account in the theory.