Appearance Potentials of Positive and Negative Ions by Mass Spectrometry

Abstract

Appearance potentials (AP) of ions were obtained by the retarding potential difference (RPD) technique using a time-of-flight mass spectrometer. Confirmatory measurements were made using a 180° magnetic instrument. Information about processes such as ion-pair formation, electronic excitation of ions, rearrangement of ions, autoionization and metastable transitions is inferred from the structure of the ionization efficiency (IE) curves of Xe, H2, CH4, CD4, C2H4, C2H6, C3H8, and C4H10 over about a 4 eV range. The AP's in Xe at 12.02, 12.35, 12.75, and 14.0 correspond in a qualitative way to autoionization levels. The structure observed in parent and fragment IE curves for hydrocarbons can, in some systems, be correlated with known processes. Structure due to vibrational excitation of ions was not resolved. The IE curves for polyatomic ions indicate that many ions formed by electron impact are formed in electronically excited states, and that appearance potentials of fragment ions often correlate with upward breaks in the parent IE curves. Parent ion intensities did not decrease with increasing electron energy even at large degradation ion intensities. An AP at about 0.7 eV above onset was observed in all the IE curves of R+ from RH and shown to arise from R++H— formation. The ion-pair process is as probable as the ground-state ion—neutral reaction in the systems investigated. The structure in the fragment IE curves can explain some of the difficulties encountered in measuring appearance potentials. An example of interference by ion—molecule reactions is also reported. Appearance potential data obtained by RPD and by conventional techniques are compared.