Field Ionization from H2 Layers

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 $\text{F\hspace{0.17em}≅\hspace{0.17em}1}\mathrm{V}\mathrm{/\AA }$ ; 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 $\text{F\hspace{0.17em}≤\hspace{0.17em}1.1}\mathrm{V}\mathrm{/\AA }$ , but that H₃⁺ and H⁺ are formed at higher fields. Plots of lni vs $\text{1\hspace{0.17em}/\hspace{0.17em}F}$ 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.