Ionization and dissociation mechanisms of ketene using resonance-enhanced multiphoton ionization mass spectrometer: (2+2) versus (2+1) schemes

Abstract

By using a resonance-enhanced multiphoton ionization (REMPI) technique, we have studied ionization and photodissociation mechanisms of ketene. Prior to ionization, the jet-cooled ketene is first excited at a wavelength 355 nm to a 3py,00 Rydberg state through a two-photon absorption. The (2+2) and (2+1) REMPI schemes may be distinguished depending on the impinging laser energy. If the (2+2) REMPI process dominates, the ketene ion is produced by the autoionization of a superexcited state, which lies in a rovibrationally excited Rydberg state. The autoionization then occurs due to energy transfer from nuclear to electronic degrees of freedom. The CH2+ is fragmented following two schemes. One is a consecutive process, i.e., the fragment ion is produced from the autoionized ketene. This conclusion is supported by a series of measurements of pulse field and pressure effects in this work. The factors of pulse field and collisions may enhance the autoionization rate significantly, imposing the same influence on the ketene ion and CH2+. The second is a dissociative ionization, i.e., the CH2+ ion is fragmented from the superexcited state in competition with the autoionization, as reported previously. These two schemes exclude the probability of ionic ladder or neutral ladder dissociation mechanisms. On the other hand, if the (2+1) REMPI scheme dominates under a low ionizing laser energy, the ketene ions are led by a direct photoionization. The increase of either pulse field intensity or interacting duration simply shortens the arrival time of the ketene ion on the detector. Finally, a time-resolved ketene ion spectrum is measured to characterize temporal behaviors for the autoionization and direct photodissociation. The relaxation lifetime for the autoionized ketene is found to be much faster than that for the directly photoionized source.