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Measurements of electron drift velocities have been made in 1·160% and 2·892% hydrogen-neon mixtures at 294 K and values of EI N from O· 12 to 1·7 Td. The measurements are highly sensitive to the region of the threshold of the v = 0 --+ 1 vibrational excitation cross section for hydrogen and have enabled more definitive tests of proposed cross sections to be made than was possible using drift velocity data for H 2 -He and H 2 -Ar mixtures. The theoretical v = 0 --+ 1 vibrational excitation cross section of Morrison et aL (1987) is shown to be incompatible with the present measurements. A new set of hydrogen cross sections has been derived from the available electron swarm measurements in pure hydrogen and hydrogen mixtures.

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Rotational excitation of nitrogen by low-energy electron impact has posed an unsolved problem for more than three decades. Early analysis of the results of swarm experiments in nitrogen found that the data could be matched remarkably well by assuming that the energy dependences of the Δj = 2 cross sections from threshold to a few tenths of an eV are given by a simple formula based on the Born approximation. Moreover, the quadrupole moment (the only adjustable parameter in the formula) which gave the best fit to the data was commensurate with existing experimental values. This finding posed an enigma, since the quadrupole Born expression is known to incorrectly represent the interaction potential for scattering except within a few meV of threshold. We have analysed new swarm data, taken in a dilute mixure of nitrogen in neon, using theoretical rotational and momentum transfer cross sections based on a solution of the Schrödinger equation using static, exchange, and polarisation potentials. This work explains the long-standing enigma and provides the basis for a subsequent analysis in which theoretical vibrational excitation cross sections are also investigated using the new swarm data for the mixture.

A joint experimental-theoretical attack on low-energy e-H2 scattering is described. The cross sections calculated from a highly converged numerical solution of the nonrelativistic Schrodinger equation, using a parameter-free interaction potential, are first compared with results from swarm experiments, and are later used to improve the accuracy of the swarm analysis at energies above the first vibrational threshold. To provide further perspective, the theoretical results are compared with a variety of other experimental data. The theoretical results for the momentum-transfer and rotational-excitation cross sections are in excellent agreement with the results from swarm experiments, but there is an unresolved and significant difference in the threshold behaviour of the vibrational-excitation cross sections. Both the theoretical and experimental approaches are subjected to close scrutiny in an attempt to uncover possible sources of error that could explain this difference. The failure to locate likely sources points to the need for further independent theoretical and experimental work to resolve a problem that has serious implications.