Abstract
The modulation of the residual current in a Bayard-Alpert ion gauge (BAG) was studied in conditions where a positive ion current resulting from the electron bombardement of a gas layer adsorbed on the grid (the ESD-current) was a major component of the residual current. Measurements were carried out with BAG with an internal modulator and a closed grid in a UHV system. The collector current vs emission current curves were measured with the modulator at both the grid and collector potentials separately at a raised pressure of electronically desorbable gases. An analysis, according to Lavson's theory, was used to obtain the ESD-current as a function of the emission current. The modulation factor of the ESD-current computed from these results was found to be as high as 80% of the modulation factor of the gas-phase ion current. Since the modulation factor of the ESD-current is so close to that of the gas-phase ion current the modulated BAG cannot measure pressure reliably when the electronegative gases such as O 2 or CO are present in a vacuum system.
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