Locating Electron Swarms in Hydrogen by Far Ultra-Violet Signals

An electron swarm is released from a pulsed gas discharge and passes through a hydrogen-filled chamber in which a uniform electric field is maintained whereby some electrons excite and ionize gas molecules by collisions. From observations of the ultra-violet signals released by the truncated electron swarm, one can derive the drift velocity of electrons, the diffusion coefficient and the degree of radiation quenching. The drift velocity is found from time resolved oscillograms of the intensity of the near and far ultra-violet light, emitted from two different points in the chamber. Measurements are presented for gas pressures between 2 and 90 mmHg in reduced fields X/p between 14 and 43 V/cm per unit gas density with a pulse length of the order 10-7 s. The electron drift velocities agree well with those previously observed below X/p = 20 but for larger values of X/p some difference is noted. From the analysis of the light pulses an effective value of the electron diffusion coefficient is estimated and found to be larger than the value expected from classical theory. Histograms of the light pulses indicate spatial anisotropy of the electron density at low pressure. At higher gas pressures the near and far ultra-violet radiation suffers considerable quenching which is most likely associated with dissociation of the molecule. A rough experimental estimate of the quenching cross-section of the C-state gives 6.3 x 10-17 cm2, and for the a-state 2.2 x 10-16 cm2, which are smaller than earlier values.