Appearance potential mass spectrometry: Discrimination of dissociative ionization products

Appearance potential mass spectrometry (APMS) has recently gained importanc e for detection and quantitative measurements of reactive radical species i n plasmas using line-of-sight sampling of radicals. In this work, we have i nvestigated the assumption that the extraction efficiency of ions produced by direct ionization of radicals, and ions produced by dissociative ionizat ion of the parent molecule used as the reference signal, are equal in the i onizer of the mass spectrometer. We find that the dissociative ionization p roducts are extracted with much lower efficiency (2-50 times smaller for th e cases studied) than the direct ionization products. This is expected due to the excess kinetic energy of the dissociatively ionized products as a re sult of the Franck-Condon effect. Use of this procedure will thus lead to a n overestimation of the radical number density by a factor of 2-50, dependi ng on the nature of the parent and the daughter ion. We recommend an altern ate procedure for APMS that utilizes an inert gas direct ionization signal as a reference for calibration of the radical signal to calculate the radic al number density. The biggest uncertainty in the radical number density es timation with the recommended procedure is expected to be the uncertainty i n the published cross sections. (C) 2000 American Vacuum Society. [S0734-21 01(00)08602-4].