Ionization and dissociation mechanism of superexcited ketene using time-of-flight mass spectrometer

Abstract

The interrelation between ionization and fragmentation of ketene is studied. A jet-cooled ketene is first excited to a 3p Rydberg state prior to ionization with the same laser irradiation; the resultant parent and fragment ions are then detected by a time-of-flight (TOF) mass spectrometer. From the dependence measurements of laser power, ketene pressure, and intensity and duration of a pulsed electric field, we find that the ketene ions are obtained predominantly by an autoionization process from a superexcited state. The superexcited ketene lies in the energy level of ∼5.6 eV above the first ionization potential by absorbing four photons energy. The CH2+ ion is fragmented by a dissociative ionization of the superexcited state in competition with the autoionization. The factors of pulse field and collisions may enhance the autoionization rate significantly, leading to an opposite effect on the branching ratio of ketene ion and CH2+ fragment. In the time-resolved ketene ion measurement, we may differentiate two types of ion sources. The first type, resulting from autoionization, relaxes in the 440–550 ns range, depending on various Rydberg states excited; the second, resulting from direct photoionization, is ten times slower. The consequence may be caused by a distinctly different temporal behavior between the first excited and the ground state ketene ion, as evidenced previously in photoelectron experiments. Finally, we examined an arrival time distribution of the CH2+ fragment at the TOF detector as a function of polarization direction of the photolysis laser. The CH2+ mass spectra for both parallel and perpendicular positions appear to be identical when the 3p Rydberg state is excited. The independence of molecular alignment indicates that the dissociation lifetime of the superexcited ketene is longer than its rotational period, which is estimated to be >20 ps.