The Measurement and Interpretation of Onset Energies

Abstract

The measurement and interpretation of ionization energies and fragment ion appearance energies are discussed in the light of new experimental methods, including multiphoton ionization. The ionization energy of very cold molecules can now be measured with a precision of 0.1 meV. In order to use these data to determine equally accurate heats of formation, it will be necessary to know the vibrational frequencies of the molecule and ion. Fragment ion onsets cannot yet be measured with such high accuracy. In addition, the use of appearance energies in determining fragment ion and neutral heats of formation require a number of assumptions. These ideas are reviewed here. The problems associated with the interpretation of fragment ion onsets are illustrated for the case of methyl loss from the ethylbenzene ion.