SummaryThe results of electron drift and diffusion measurements in parahydrogen have been analysed to determine the cross sections for momentum transfer and for rotational and vibrational excitation. The limited number of possible excitation processes in parahydrogen and the wide separation of the thresholds for these processes make it possible to determine uniquely the J = 0 ->-2 rotational cross section from threshold to 0·3 eV. In addition, the momentum transfer cross section has been determined for energies less than 2 eV and it is shown that, near threshold, a vibrational cross section compatible with the data must lie within relatively narrow limits. The problems of uniqueness and accuracy inherent in the swarm method of cross section analysis are discussed. The present results are compared with other recent theoretical and experimental determinations; the agreement with the most recent calculations of Henry and Lane is excellent.
The J = 1 --+ 3 rotational cross section for H2 has been derived from an analysis of electron transport coefficients; A new technique is described for calculating the energy distribution functions taking into account superelastic collisions, since these must be included for an analysis of low energy transport data in D2. Unique rotational cross sections cannot be obtained for this gas from the experimental data available, but two sets of cross sections have been derived which are compatible with the existing data and are also in accord with recent theoretical calculations. Evidence is also presented to show that there is probably a small difference between the momentum transfer cross sections in H2 and D 2.
Measurements of the angular and energy distributions of electrons ejected from helium atoms by protons with energies between 20 and 100 keV are presented in tabular and graphical form. The electron energy range is between 5 and 100 eV and the angular range is between 0 and 100°. The distributions have been converted to double differential cross sections by normalisation against other published data. An analysis of the accuracy of the results is presented.AINSE fellow 1982-84; now with TRIUMF (Tri-University Meson Facility), Vancouver, Canada.
The v =0 1 vibrational cross section for H& derived from swarm experiments is shown to be a good approximation to the sum of the cross sections for vibrational transitions that take place with and without simultaneous rotational excitation of the molecule. Experiments are described that prove that this summed cross section does not depend on whether the 82 molecules are in the J'= 0 or the J'=1 rotational state, in agreement with a recent theoretical calculation. Reasons are given for the disagreement between the two published swarm-derived vibrational cross sections. However, the discrepancies between the threshold behavior of the cross sections determined by the swarm Ineasurements, beam experiments, and theory have not yet been explained.
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