Novel analytical techniques based on an enhanced electron attachment process. First year progress report, 1996--1997

Abstract

'Organic Mass Spectrometry Group, Chemical and Analytical Sciences Division, ORNL The objective of this research program is to develop new analytical techniques based on the large cross sections associated with dissociative electron attachment to highly-excited molecular states. Such highly-excited states can be populated via laser irradiation or in glow discharges via excitation transfer from high-lying, metastable states of rare gases. In one part of the research program, the analyte molecules are populated via laser excitation, and the resulting negative ions are mass analyzed using a time-of-flight mass spectrometer. In the other research project, a gas-discharge-based novel plasma mixing scheme is used to excite the molecules. In the early studies, basic studies on these two schemes will be conducted in order to clarify the electron attachment mechanisms involved. During the past year the authors have conducted extensive measurements using this scheme. In these experiments, a gas jet is intercepted by a laser beam that ionizes and excites the gas to produce attaching electrons and the excited molecules for attachment. Both positive and negative ion measurements were conducted in order to clarify the electron attachment mechanisms involved in molecules laser-excited to energies above their ionization potentials (IPs). In these experiments, a molecular gas jet is intercepted by an excimer laser pulse, which excites the molecules to energies above their IPs. This leads to the ionization of some, but not all molecules. Contrary to the generally-accepted notion that the ionization is the only possible outcome, the studies have shown that highly-excited neutral states (superexcited states) are also produced, and that such states can live for long times of the order of microseconds. In a long-lived superexcited molecule, the excited electron is in a high-Rydberg orbital, and the excess energy is stored in the vibrational/rotational modes of the positively-charged core. A paper describing these results has been accepted for publication in Chemical Physical letters.'