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 ͑REMPI͒ 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 3p y ,0 0 Rydberg state through a two-photon absorption. The (2ϩ2) and (2ϩ1) REMPI schemes may be distinguished depending on the impinging laser energy. If the (2ϩ2) 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 CH 2 ϩ 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 CH 2 ϩ . The second is a dissociative ionization, i.e., the CH 2 ϩ 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 (2ϩ1) 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.

Valence and Inner Electronic Excitation, Ionization, and Fragmentation of Perfluoropropionic Acid

“…An autoionization process offers an alternative but plausible explanation for this behavior. , In the present case, such a secondary process would lead to electron emission by the decay of a highly excited neutral system, followed by different processes giving COH + , C 2 F 4 + , and the parent species M + as distinctive events. This autoionization event would be initiated by the generation of a superexcited electronic state (PFPA**) with an internal excitation energy that has been known in other cases even to exceed the first ionization potential . Other conceivable photoevolution channels could occur via an adiabatic ionization mechanism followed by photofragmentation events.…”

“…This autoionization event would be initiated by the generation of a superexcited electronic state (PFPA ** ) with an internal excitation energy that has been known in other cases even to exceed the first ionization potential. 28 Other conceivable photoevolution channels could occur via an adiabatic ionization mechanism followed by photofragmentation events. Although the adiabatic ionization potential of PFPA has not yet been established, a lower value than the reported vertical IP would be expected as a result of the significant changes in geometry likely to occur between the fundamental (PFPA) and the excited ionic (PFPA + ) species.…”