Dominance of electron-stimulated desorption neutral species in ultra-high vacuum pressure measurements

Abstract

An energy-dispersive residual gas analyzer (EDRGA) has been developed which enables us to separate the gas-phase ions from the electron-stimulated desorption (ESD) ions at each mass number. With this EDRGA, neutral ESD fragments from the grid, normally indistinguishable from the “gas phase” signals, were evaluated for the first time. Namely, the mass spectrum from the gas phase ions was compared with one synthesized from the fragmentation patterns of the existing parent molecules in the gas phase and normalized by partial pressure. It was clear that the fraction of neutral ESD fragments in the total ESD signal depends on the mass and the history of the evacuation, i.e. with or without baking. Neutral species dominate the ESD in UHV vacuums after bakeout, which suggests that even energy-dispersive total pressure gauges are unable to eliminate ESD errors.