Multiphoton ionization and fragmentation processes of methyl iodide clusters at 266 and 355 nm

Abstract

The multiphoton ionization (MPI) and dissociation dynamics of methyl iodide clusters at 266 and 355 nm are studied using a reflectron time-of-flight (RTOF) mass spectrometer. The excitation wavelengths were chosen to access the repulsive valence A state and the Rydberg C state. MPI mass spectrum at 266 nm consists mostly of fragment ions, CH 3 +, I +, and I 2 +. This is consistent with a ladder-switching mechanism in which rapid one-photon dissociation occurs from the repulsive potential energy surface followed by multiphoton ionization of neutral photofragments. The observation of considerable amount of parent ion and methyl-rich fragments (CH 3I) n CH 3 + ( n = 1–9) at 355 nm excitation is indicative of the ionic-ladder process in which the parent ions are produced directly by absorbing three photons and then dissociates through an additional one-photon absorption to produce fragment ions. These processes are further supported by the laser power dependence of product ions. The results are interpreted in terms of the electronic state diagram of the methyl iodide and effects resulting from intracluster chemical reactions.