Nondissociative Electron Capture in Complex Molecules and Negative-Ion Lifetimes

The formation and decay of parent negative ions have been studied with electron-swarm and electron-beam techniques. The rate of attachment at thermal energies under electron-swarm conditions for SF6 and C6H5NO2 was determined to be 1.24×109 and 2.1×107 sec−1·torr−1, respectively. Autoionization lifetimes for SF6−, C6H5NO2−, and (CH3CO)2− are reported to be 25, 40, and 12 μsec, respectively. The absolute rate of electron attachment in SF6 was found to be independent of the temperature (298°≤T≤418°K), and is thus consistent with an electron capture cross section for SF6 which varies inversely with the speed of the electron. The attachment rates and lifetimes were combined through the principle of detailed balance to calculate the ratio of density of states of the negative ion to that of the neutral plus electron. A simple theoretical treatment of long-lived negative ions is presented, and the possibility of estimating electron affinities from measurements of attachment cross sections and negative-ion lifetimes is pointed out.