Observation of vibrationally excited ions in Franck—Condon gaps by threshold photoelectron spectroscopy

Abstract

Photoionization and threshold photoelectron spectra have been recorded of NO, H 2S, CS 2, COS, CH 3I, CH 2Br 2, C 3H 4 (allene) and C 6H 6 (benzene) from their respective ionization onsets to 14 eV. The results indicate that autoionization produces, in part, threshold electrons accompanied by the formation of highly vibrationally excited ions. Many of these ionic states are within the Franck—Condon gap region where the probability for forming excited molecular ions by direct ionization is vanishingly small. The vibrational enhancement is attributed to resonant autoionization, a process accompanied by the production of low-energy electrons.