Influence of ion induced secondary electron emission on the stability of ionisation vacuum gauges

Surface modification of different materials exposed to an environment typical for hot cathode ionisation vacuum gauges was investigated. Such environment has been generated in a specially designed setup which simulates conditions in a Bayard-Alpert ionisation vacuum gauge. Characterisations by means of X-ray photoelectron spectroscopy and work function study have been performed before and after the sample exposure to Ar and H₂ gas discharges. The majority of studied materials, such as molybdenum, gold or stainless steel, are considered to be of interest as ion collectors. In addition, experiments with copper and graphite revealed the major processes that are taking place during the exposure to an ionisation vacuum gauge environment. Since the stability of ionisation vacuum gauges can be affected by the change of secondary electron emission properties of ion collectors during the operation, the corresponding electron yield induced by bombardment of low energy Ar⁺ ions was measured. Results show that exposure to ionisation vacuum gauge environment contributes to development of hydrocarbon layers, independently from the collector material or the dominant species in the gas discharge. Therefore, ion induced secondary electron yield of clean materials will be changing during the gauge operation and eventually reach saturation regardless of the collector material. The results also show that the hydrocarbon layer cannot be desorbed by standard vacuum baking procedures, whilst ion bombardment will even increase the rate of the hydrocarbon layer formation.