SummaryMeasurements of the drift velocity, the ratio of diffusion coefficient to mobility, and the "magnetic drift velocity" for electrons in helium have been made at 293°K in the range 1· 8 X 10-19 < E/N < 3 X 10-17 V cm 2 • From an analysis of the drift velocity data, an energy-dependent momentum transfer cross section has been derived for which an error of less than ± 2 % is claimed over the central portion of the energy range. The cross section agrees with the theoretical cross section of La Bahn and Callaway to within 2% over the whole energy range. The agreement with the cross section derived by a number of procedures from the total elastic scattering cross section measured by Golden and Bandel is less satisfactory. The drift data are sufficiently accurate to enable a search to be made for the effects of fine structure in the cross section at low energy. The results do not support the existence of such structure.

SummaryThe use of data from swarm experiments for electron energies approaching those corresponding to thermal equilibrium demands results of greater precision than has hitherto been available. In order to examine the possibility of producing such data, the swarm method for detennining WID has been extensively examined over a range of values of the parameter Elp where the agreement between the results of recent investigations is not good. A number of factors influencing the accuracy of measurements of this type are discussed. The results for hydrogen which are presented are considered to be subject to an error of less than 1 %. I. INTRODUCTIONThe results of swarm methods for determining the ratio WID of electron drift velocity to diffusion coefficient have found application in a number of recent papers in which collision phenomena between low energy electrons and gas molecules have been discussed (Gerjouy and Stein 1955;Huxley 1956Huxley , 1959Shkarofsky, Bachynski, and Johnston 1961;Frost and Phelps 1961). For some of these applications, more especially those dealing with collision phenomena for electrons with mean energy of several electron-volts, the accuracy of existing data is sufficiently good. Not only are the results of a number of investigations substantially in agreement for this energy range but the degree of accuracy to which they have been obtained is adequate for most purposes. On the other hand, for those applications where the difference in energy between the electrons and gas molecules is important, small errors in the determination of WID become significant as this difference approaches zero. Unfortunately it is in this energy range that the measurements become more difficult and the agreement between the results from various laboratories is not good.In this paper an account is given of a systematic investigation of the swarln method for determining WID using an apparatus which enables the dimensions of the diffusion chamber to be quickly and simply varied. A number of possible sources of error has been investigated both theoretically and experimentally, as a result of which it has been possible to determine the experimental procedures which lead to results of maximum accuracy. The results which have been obtained where these procedures were followed show good agreement over a wide range of values of the experimental parameters and it is considered that the values of WID in hydrogen from O'1

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.

SummaryValues of Townsend's energy factor kl for electrons in dry, carbon dioxide free air have been determined as a function of the parameter E Ip for 0 . 2 < E Ip < 40 at a temperature of 293°K. The results are first compared with those of other workers and are then utilized in a recomparison of the cross sections for electron attachment deduced from swarm and beam-type experiments.

SummaryExperimental results are given for the ratio W xl W. of transverse to longitudinal drift velocity for electron swarms in nitrogen moving in crossed electric and magnetic fields. The results, obtained by Huxley's method, cover the range 0·04 < Elp < 8·0V cm-1 torr-1 at 293°K. The apparatus and experimental procedures which have been developed permit accurate measurements to be made so that significant tests of the method have been possible over wide ranges of the experimental parameters. Information concerning the variation of the momentum transfer cross section with electron energy, and concerning the energy distribution function, can be obtained by comparing a quantity WM = (EIB) (WxIW.) with the true drift velocity W. The results of this comparison are discussed in relation to recent theoretical analyses.