A method is presented for measuring appearance potentials with an analytical mass spectrometer. Slight modifications of the ion source and a unique method of interpreting ionization efficiency curves produce values which are generally accurate to within 0.5 per cent. Electron energy in the bombarding beam is partially controlled by a grid. The differential ion current due to a small change in ionizing potential is recorded and the interpretation produces efficiency curves which approach the form which could be obtained with mono-energetic electrons. Conventional use of the instrument is not affected by the modification. Bond-strengths determined from data by this method are in good agreement with published data.

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