SummaryMeasurements have boon made of the drift velooity W and the ratio of diffusion ooefficient to mobility D/,.. for low energy electrons in deuterium at 77°K, and of D /,.. in hydrogen at 77 and 293°K. Tabulated data are given for D /,.. within the range 2x 10-20 .;; E/N .;; 1·2x 10-16 V cm2 and for W within the range 8x 10-20 .;; E/N .;; 9·5x 10-17 V cm2• The low temperature results show a considerably reduced scatter when oompared with previously published data and are of an aocuracy comparable with the results of recent measurements made at room temperature. The data obtained complement those of Lowke and of McIntosh thus completing a set from which a more acourate determination of low energy elastic and inelastic cross sections can be made.
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.
Recent publications have shown that electron impact collisions are more important than Penning collisions in populating the upper level of the 441.6 nm transition of He-Cd+ lasers operated as positive column discharges. The authors present evidence that this is not the case in hollow cathode discharges.
A computer simulated electron swarm at E/P293 = 1·0 V cm -1 tore 1 in a model gas has been used to examine the validity of a recent theory of electron drift and diffusion. The computed results are in agreement with well-established theories for the electron energy distribution function, drift velocity and transverse diffusion coefficient, and confirm that, for a constant momentum transfer cross section, the longitudinal diffusion coefficient is approximately half the transverse coefficient. However, significant differences have been found between the computed swarm and the predictions of the theory of Huxley (1972). In particular, over the time scale considered, the electron swarm is not symmetric about its centroid but is spatially anisotropic in such a way that it could appropriately be described as 'pear shaped'.
A simple hollow cathode He-Cd+ laser which emits at least 100 mW multimode power at each of 441.6 and 533.7/537.8 nm from a 4 mm bore, 60 cm active length is described. Emphasis is given to the features needed to achieve discharge stability in this as well as other inert gas-metal vapour systems. Measurements showed that the axial extent of the negative glow associated with a single anode almost doubled when cadmium was introduced into a helium discharge. Although this suggests the possibility of some design simplification, it was found that little improvement could be achieved in practive because of the onset of discharge arcing when the current carried by a single anode reached 250 mA.
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