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

TL;DR

This study investigates the ultrafast predissociation dynamics of the vibrationless level of the B state of CH3I using femtosecond-resolved imaging, revealing detailed fragment energy distributions, vibrational branching, and additional dissociation channels.

Contribution

It provides the first detailed time-resolved analysis of the predissociation process of CH3I's B state, including vibrational state-specific fragment distributions and newly observed dissociation pathways.

Findings

Fragment kinetic energy distributions vary with delay and vibrational excitation.

Significant differences in angular distributions and rise times of fragments.

Identification of two new dissociation channels near 9.2 eV and 15 eV.

Abstract

The predissociation dynamics of the vibrationless level of the first Rydberg state 6s (B 2E) 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 3pz 2A2" 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 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 extra 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.