The dissociation dynamics of state selected metastable aniline ions by single and multiphoton ionization

The lifetimes with respect to dissociation of state selected aniline ions have been investigated by photoion–photoelectron coincidence. Lifetimes between 0.3 and 100 μs have been found when the aniline ion is prepared in energies ranging from 13.6 to 12.6 eV, respectively. These lifetimes are in good agreement with those of a recent study by Proch, Rider, and Zare, who observed metastable aniline ions in a UV multiphoton ionization (MPI) study in which three photons were required to reach the metastable ion energy range. By contrast, our visible MPI study of aniline shows no evidence for long-lived ions. This paradox is resolved by postulating that under the UV MPI conditions, a radiationless transition to the ground electronic state allows the 13 eV ions time to dissociate slowly, while under the much higher photon intensities needed in the visible MPI study, rapid up pumping of the excited ions prevents radiationless transitions. A lower limit for the radiationless transition rate of 2×108 s−1 is derived.