Quasi-Trapping of Low-Energy Electrons in Water Vapour

Abstract

THE interaction of low-energy (less than 1 eV.) electrons with water vapour has been examined by means of a swarm method^1–3 in which water vapour at pressure f₁p was added to ethylene, carbon dioxide and methane, at pressures f₂p. It was found that when f₁p ranged from 1 to 16 mm. mercury, and f₂p was of the order of 100–200 mm. mercury, no electron capture occurred for an E/P [V. cm.⁻¹ (mm. mercury)⁻¹] ranging from 0.01 to 4.0. (An exception must be made for the case of water–carbon dioxide where a very small capture cross-section was observed and may be due to the formation of carbonic acid.) However, an interesting and significant increase in the electron transport time was observed and is illustrated in Fig. 1 for mixtures of water and ethylene. In Fig. 1, w⁻¹ is the transport time per cm. of travel in the field direction, and it is observed that: (1) w⁻¹ increases linearly with the ratio f₁/f₂; (2) w⁻¹ is independent of the total pressure p for the range 100–400 mm. mercury. These results suggest that electrons make collisions of long duration with water and that the lifetime of the ‘quasi-trap’ [H₂O]⁻ is limited by collision with ethylene for the pressure-ranges already quoted.