Imaging transient molecular configurations in UV-excited diiodomethane

TL;DR

This study uses time-resolved Coulomb explosion imaging to observe ultrafast transient molecular configurations in UV-excited diiodomethane, revealing new reaction pathways and short-lived species within 100 femtoseconds.

Contribution

It introduces a method to identify and characterize transient molecular geometries in diiodomethane following UV excitation, highlighting a previously unobserved reaction pathway.

Findings

Transient species resemble iso-CH2I-I geometries

Transient configurations form within ~100 fs

Decay within ~100 fs after formation

Abstract

Femtosecond structural dynamics of diiodomethane ($\mathrm{CH_2I_2}$) triggered by ultraviolet (UV) photoabsorption at 290 nm and 330 nm are studied using time-resolved coincident Coulomb explosion imaging driven by a near-infrared probe pulse. We map the dominant single-photon process, the cleavage of the carbon-iodine bond producing rotationally excited $\mathrm{CH_2I}$ radical, identify the contributions of the three-body ($\mathrm{CH_2} + \mathrm{I} + \mathrm{I}$) dissociation and molecular iodine formation channels, which are primarily driven by the absorption of more than one UV photon, and demonstrate the existence of a weak reaction pathway involving the formation of short-lived transient species resembling iso-$\mathrm{CH_2I{-}I}$ geometries with a slightly shorter I-I separation compared to the ground-state $\mathrm{CH_2I_2}$. These transient molecular configurations, which can be separated from the other channels by applying a set of conditions on the correlated momenta of three ionic fragments, are formed within approximately 100 fs after the initial photoexcitation and decay within the next 100 fs.