Modeling the coincident three-ion momentum imaging of diiodomethane photodissociation on reduced-dimensional potential energy surfaces

Abstract

We present an efficient theoretical model to simulate observables in the time-resolved coincident three-ion Coulomb explosion experiment of diiodomethane. The model employs two degrees of freedom to describe the C-I bond breaking and the $\text{CH}_2\text{I}$ rotation during photodissociation, and three degrees of freedom to describe the coincident $\text{CH}_2^{+} + \text{I}^{2+} + \text{I}^{2+}$ fragmentation during the subsequent Coulomb explosion. By solving the equations of motion, the photodissociation pathways are obtained on two-dimensional potential energy surfaces of the valence excited states of the neutral molecule, and the asymptotic momenta of the three ionic fragments are determined on the three-dimensional ground-state potential energy surface of the fivefold-charged cation. The photodissociation pathways are consistent with previous \textit{ab initio} molecular dynamics simulations and indicate a $\text{CH}_2\text{I}$ rotational period of approximately 340 fs. The theoretical time-resolved kinetic energy release and the correlation between the kinetic energy release and the angle between the two $\text{I}^{2+}$ momenta show good agreement with experimental signals in part, reflecting and confirming the static $\text{CH}_2\text{I}_2$ state and the $\text{CH}_2\text{I} + \text{I}$ dissociation channels.