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

By using a resonance-enhanced multiphoton ionization (REMPI) technique, we have studied ionization and photodissociation mechanisms of ketene. Prior t o 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 (2+2) a nd (2+1) REMPI schemes may be distinguished depending on the impinging lase r energy. If the (2+2) REMPI process dominates, the ketene ion is produced by the autoionization of a superexcited state, which lies in a rovibrationa lly excited Rydberg state. The autoionization then occurs due to energy tra nsfer from nuclear to electronic degrees of freedom. The CH2+ is fragmented following two schemes. One is a consecutive process, i.e., the fragment io n 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 ra te significantly, imposing the same influence on the ketene ion and CH2+. T he second is a dissociative ionization, i.e., the CH2+ ion is fragmented fr om the superexcited state in competition with the autoionization, as report ed previously. These two schemes exclude the probability of ionic ladder or neutral ladder dissociation mechanisms. On the other hand, if the (2+1) RE MPI scheme dominates under a low ionizing laser energy, the ketene ions are led by a direct photoionization. The increase of either pulse field intens ity or interacting duration simply shortens the arrival time of the ketene ion on the detector. Finally, a time-resolved ketene ion spectrum is measur ed to characterize temporal behaviors for the autoionization and direct pho todissociation. The relaxation lifetime for the autoionized ketene is found to be much faster than that for the directly photoionized source. (C) 2001 American Institute of Physics.