Preflashover mass spectrometry

Abstract

A bakeable time‐of‐flight mass spectrometer has been combined with a novel probe assembly in order to directly investigate the ion species formed prior to and during the flashover of an ultrahigh‐vacuum diode. Species are identified by their known kinetic energy and the time correlation between their arrival at a detector and a high‐voltage pulse applied to the cathode of the diode. By varying the ratio of the pulse amplitude to a positive bias applied to the anode, the extent of ion production in the gap can be probed. Observations at 300 °K and 10⁻⁹ Torr indicate the presence of both highly charged metal anode ions and adsorbed residual gas prior to flashover. No detected species were formed in the gap, in contrast to what was expected from the results of Davies and Biondi. It is suggested that the absence of such ions may be related to the magnitude of the macroscopic field in the gap, since preliminary observations at flashover show a noticeable change in the nature and abundance of ion species. Electron‐induced desorption appears to be the primary preflashover production mechanism, although field‐enhanced desorption may also be significant.