Measurement of Appearance Potentials of Ions Produced by Electron Impact, using a Mass Spectrometer

Abstract

THE mass spectrometer is often used for the measurement of the appearance potentials of ions produced by electron bombardment of gaseous molecules. For this purpose, two gases are admitted to the instrument simultaneously; one is the gas under investigation, while the other is a ‘standard gas’ (usually argon or krypton), the ionization potential of which is accurately known from spectroscopic data. Measurements are made of the ion beam intensities as a function of electron energy, both for the ion the appearance potential of which is required and for that for which it is known. These measurements are made over a range of a few volts above the potentials at which the beams are first detected. Several rather arbitrary methods have been used for deducing the difference between the two potentials¹.