Field Ionization from H2 Layers

Abstract

Field ionization from hydrogen layers adsorbed at 4.2°K on tungsten emitters has been studied in sealed-off tubes and by means of a mass spectrometer which could probe small regions of the tip. At 4.2°K and 2 × 10−7 torr, ion currents of ∼10−9 A could be obtained at F ≅ 1 V/Å; the low field indicates that the dwell time of H2 in the ionization region is much longer than in gas-phase field ionization, while the high current obtained at this pressure shows that supply occurs by diffusion of H2 from the emitter shank. The mass spectrometric experiments indicate that H2+ predominates at F ≤ 1.1 V/Å, but that H3+ and H+ are formed at higher fields. Plots of lni vs 1 / F indicate that at low fields molecular tunneling controls the over-all rate of field ionization, while at higher fields supply of H2 is rate limiting.