SummaryThe energy ratio and attachment coefficient for electrons in water vapour have been determined in the range 20 < Elp < 60 V cm-l torr-l. The results for the attachment coefficient are in general agreement with other recent determina· tions, but those for the energy ratio differ significantly from the results of Bailey and Duncanson. The use of these new data, in conjunction with values of the drift velocity determined by Pack, Voshall, and Phelps, is shown to remove the serious discrepancy which previously existed between the results of single-collision and swarm experiments.
r. INTRODUCTIONWhen electrons having energies of the order of 5 e V pass through water vapour at a pressure of a few torr, an appreciable fraction of the electrons form negative ions by electron attachment. According to Laidler (1954), Craggs and McDowell (1955), and others, the dominant attachment process for energies up to about lO e V is the following resonance capture process:(1)Laidler states that the water molecule is first raised to a repulsive 2Al or 2A2 state of H 2 0-which at once dissociates into H -(IS) and OH(27T). The appearance potential is of the order of 5·5 e V, and the cross section is a maximum for electrons having energies of about 6·5 eV.The curve showing the variation with electron energy of the rate of production of H -ions from H 2 0 shows a second, smaller peak which has a maximum value at an energy of about 8·5 eV (Mann, Hustrulid, and Tate 1940; Buchel'nikova 1959). For electrons of energy greater than about 7·5 eV, 0-ions may also be produced, the reaction being (2) It is widely believed (see, for example, Cottin 1959) that the H-ions formed as above are rapidly converted to negative hydroxyl ions by the following process:At gas pressures of the order of 1 torr, Branscomb and Smith (1955) and Muschlitz and Bailey (1956) observed that the OH-ions so formed were considerably more abundant than either H -or 0-ions.