Time-resolved photoionization mass spectrometry. Unimolecular fragmentation of toluene ions near threshold.

Abstract

A new technique, time resolved photoionization mass spectrometry, allows direct observation of the fragmentation of excited ions. The loss of a hydrogen atom from toluene ions is studied using photoionization wavelengths from 1053 to 1026 Å. Nonlinear least-squares fitting of the decay curves of the molecular ion show that only three discrete states are involved in the fragmentation. One state has a fragmentation rate which is fast (k≫2×104 s−1) on the time scale of the experiment while the other two have rates of 1.3±0.4 and 1.9±0.4 s−1. The present results are consistent with earlier electron impact studies. A careful analysis of previous metastable ion lifetime measurements shows that they do not constitute evidence for a continuous distribution of lifetimes as had earlier been claimed. Evidence presently available indicates that the internal energy distribution of ions formed by electron or photon impact is best described in terms of a relatively small number of discrete states rather than a continuous distribution.