Time-resolved imaging of excitation, ionization and dissociation of iodomethane by femtosecond 400 nm pulses.

The iodomethane (CH₃I) molecule has been widely used as a prototypical model system for ultrafast photochemistry. Its dynamics upon single-photon ultraviolet excitation in the first (A) and second (B) absorption bands as well as strong-field-driven ionization and dissociation in the near-infrared range have been extensively studied. Here, we attempt to bridge the gap between these two regimes and study the CH₃I dynamics triggered by the interaction with 400 nm laser pulses. We map these dynamics by strong-field ionization and Coulomb explosion induced by an intense 800 nm probe pulse. For moderate intensities of the 400 nm pump, the most prominent processes reflected in the time-dependent spectra are vibrational excitation in the ground electronic state of the neutral CH₃I and its two-photon excitation within the B-band, which results in a predissociation with ~1.5 ps lifetime, similar to one-photon absorption at ~200 nm. At higher 400 nm intensities, the dynamics are dominated by the ionization processes, including dissociative ionization of singly and doubly charged cationic states. In this regime, signatures of vibrational excitation in both CH₃I and CH₃I⁺ can be clearly observed.