Time-resolved predissociation of the vibrationless level of the B state of CH3I

Abstract

The predissociation dynamics of the vibrationless level of the first Rydberg 6s (B ¹E) state of CH₃I has been studied by femtosecond-resolved velocity map imaging of both the CH₃ and I photofragments. The kinetic energy distributions of the two fragments have been recorded as a function of the pump–probe delay, and as a function of excitation within the umbrella and stretching vibrational modes of the CH₃ fragment. These observations are made by using (2 + 1) Resonant Enhanced MultiPhoton Ionization (REMPI) via the state of CH₃ to detect specific vibrational levels of CH₃. The vibrational branching fractions of the CH₃ are recovered by using the individual vibrationally state-selected CH₃ distributions to fit the kinetic energy distribution obtained by using nonresonant multiphoton ionization of either the I or the CH₃ fragment. The angular distributions and rise times of the two fragments differ significantly. These observations can be rationalized through a consideration of the alignment of the CH₃ fragment and the effect of this alignment on its detection efficiency. Two additional dissociation channels are detected: one associated with Rydberg states near 9.2 eV that were observed previously in photoelectron studies, and one associated with photodissociation of the parent cation around 15 eV.