Spectroscopy and photodissociation of size-selected (CH 3I) n+ cluster ions

Abstract

The resonant photodissociation of (CH 3I) n + cluster ions ( n = 1–5) was studied in a crossed molecular-electron-laser beam apparatus. These results were complemented by a study of the ionization/dissociation as a function of variable energy electron ionization (EI). The fragmentation that first appears as the EI energy is raised is loss of CH 3. This channel is particularly strong for n = 3. The lower energy dissociation of neutral I, on the other hand, is essentially quenched for cluster sizes of n > 2. Intermolecular dissociation (evaporation) of CH 3I was evident, but was not extensive, based on a minor sensitivity of the cluster ion distribution to EI energy. The photodissociation of (CH 3I) n + resulted in different dissociation behavior than for EI. Resonant excitation of monomer CH 3I + to the predissociative A electronic state gave only I + CH 3 +. A state excitation of the cluster ions, on the other hand, resulted only in intermolecular dissociation. The photoexcitation cross-sections for the A 2E 1/2 ← X 2E 1/2 transition are presented as a function of cluster size n. A two-photon dissociation channel is also identified and the crosssections and dissociation products are reported. Finally, the frequency resolved photofragment excitation spectra of CH 3I + and (CH 3I) 2 + are compared.