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.

Figures (6)

• Figure 1: Molecular diagram of $\text{CH}_2\text{I}_2$ in $C_s$ symmetry. $\textbf{r}_i(i=1\dots4)$ denote the Jacobi coordinates employed to model the photodissociation and Coulomb explosion processes.
• Figure 2: Potential energies of the $\text{CH}_2\text{I}_2$ molecule as functions of $r_1$ and $\alpha$ in Jacobi coordinates for the lowest 17 electronic states. These states can be grouped according to their corresponding dissociation thresholds, as indicated in the figure. The black dot marks the equilibrium geometry, which also corresponds to the Franck-Condon point upon photoexcitation.
• Figure 3: Two-body and three-body interaction potentials for the ground-state potential energy surface of the $\text{CH}_2\text{I}_2^{5+}$ cation. (a) The two-body interaction potentials $V_{AB}$ (blue) and $V_{BC}$ (red), with the pure Coulomb contribution subtracted, shown as a function of the pair distance $R$. The dots represent the original ab initio energies, and the curves correspond to the fitted potential function. (b) The three-body interaction potential $V_{ABC}$ as a function of $R_{AB}$ and $R_{BC}$, with $R_{AC}$ fixed at 3 Å. (c) $V_{ABC}$ as a function of $R_{AB}$ and $R_{AC}$, with $R_{BC}$ fixed at 3.6 Å. Note that only physically allowed $V_{ABC}$ values--i.e. those for which $R_{AB}$, $R_{AC}$, and $R_{BC}$ satisfy the triangle inequality--are shown in panels (b) and (c).
• Figure 4: The I--I (a) and C--I (b) pair distances and the I--C--I bond angle (c) as functions of reaction time along the photodissociation pathways for the $2\Gamma_1$ (blue) and $6\Gamma_2$ (black) states, which lead to the $\text{CH}_2\text{I} + \text{I}$ and $\text{CH}_2\text{I} + \text{I}^*$ products, respectively. Results from a representative AIMD trajectory (red, reproduced from Ref. liu2020) are also shown for comparison. The AIMD results are reproduced with permissions from Phys. Rev. X 10, 021016 (2020). Copyright 2020 American Physical Society.
• Figure 5: Experimental KER signals at different pump-probe delays (reproduced from Ref. anbu2025) compared with theoretical predictions. The experimental KER is the sum of the kinetic energies of the three ionic fragments, $\text{CH}_2^{+}$, $\text{I}^{2+}$, and $\text{I}^{2+}$, measured in coincidence. The curves represent the theoretical results. Photodissociation is modeled using the $2\Gamma_1$ (blue) and $6\Gamma_2$ (black) states that lead to $\text{CH}_2\text{I}+\text{I}$ and $\text{CH}_2\text{I}+\text{I}^*$ dissociation channels, respectively. The subsequent three-fragment Coulomb explosion is modeled using the ground-state PES of the $\text{CH}_2\text{I}_2^{5+}$ cation (solid curves) and, for comparison, a pure Coulomb potential (dashed curves). The experimental data are reproduced with permissions from J. Chem. Phys. 163, 164308 (2025). Copyright 2025 American Institute of Physics.