Formation of highly excited iodine atoms from multiphoton excitation of CH₃I

Mass resolved REMPI spectra, as well as CH₃⁺and I⁺ ion and photoelectron images, were recorded for two-photon resonant excitations of CH₃I via s, p and d Rydberg states (CH₃I**) in the excitation region of 55 700 to 70 000 cm^-1. Photoelectron (PE) and ion kinetic energy release spectra (KERs) were derived from the images. The data revealed that after the two-photon resonant excitation, an additional photon is absorbed to form one or more superexcited state(s) (CH₃I^#), followed by branching into three pathways. The major one is the dissociation of CH₃I^# to form excited Rydberg states of iodine atoms (I**) along with CH₃(X), a phenomenon not commonly observed in methyl halides. The second (minor) pathway involves autoionization of CH₃I^# towards CH₃I⁺(X), which absorbs another photon to form CH₃⁺ along with I/I* and the third one (minor) is CH₃I^# dissociation towards the ion pair, CH₃⁺ + I^-, prior to I^- electron ejection. Furthermore, one-photon non-resonant dissociation of CH₃I to form CH₃(X) and I/I* prior to three-photon ionization of the fragments is also detected.