Time-dependent density functional theory study of strong-field laser-induced coulomb explosion of the HCl dimer

TL;DR

This work tackles how laser-driven Coulomb explosion of the HCl dimer unfolds through distinct fragmentation channels. It employs real-time TDDFT on a real-space grid to propagate electrons and nuclei with explicit ionization, sampling ensemble-averaged laser–molecule orientations. A key finding is that early-time ionization strongly biases channel branching, with higher ionization correlating with near-simultaneous four-body breakup and lower ionization favoring sequential or three-body pathways; fragment charges, especially on HCl2, further differentiate channels. Long-time observables, including the KER and emission-angle distributions, reflect these charge-dependent dynamics and agree qualitatively with experimental trends, providing a unified interpretation that can guide future investigations of Coulomb explosion in similar systems.

Abstract

We present a channel-resolved interpretation of laser-driven Coulomb explosion of the HCl dimer from an ensemble of trajectories. Three dominant outcomes are identified: a minor three-body channel and two four-body channels (sequential and near-simultaneous dissociation of both molecules). The key result is that pathway selection is strongly correlated with the degree of ionization during the laser interaction, which is in turn strongly modulated by laser-molecule orientation. Higher early-time ionization predisposes the system toward near-simultaneous four-body breakup, whereas lower ionization favors sequential and three-body fragmentation; for low-ionization cases, a fragment-resolved charge metric further differentiates three-body and sequential behavior. These charge-dependent trends consistently map onto experimentally accessible observables: the simultaneous mechanism dominates the high-energy tail of the kinetic energy release (KER) spectrum and populate distinct regions of the emission-angle distributions, while sequential events concentrate at lower KER. Overall, early-time charge evolution provides a unifying explanation for channel branching and for the channel-resolved fragmentation signatures.

Figures (24)

• Figure 1: Ground-state geometry and electron-density distribution of the HCl dimer. The Cartesian coordinates are given in angstrom (Å). Four linearly spaced electron-density isosurfaces are shown. The isosurface values shown are 0.10, 0.567, 1.033, and 1.50.
• Figure 2: Electric field (unit: V/Å) versus time (unit: fs) for the laser applied in the simulations.
• Figure 3: Branching ratio and average total ionization(at 25fs, when the laser field has essentially vanished) for each of the three dissociation channels obtained from 96 TDDFT trajectories. The average total ionization across all trajectories is 4.06 e.
• Figure 4: Violin plot of the total ionization at $t=13$ fs grouped by dissociation channel. The mean total ionization (mean $\pm$ standard deviation) is $3.07 \pm 0.02~e$ for 3-body events, $3.14 \pm 0.09~e$ for 4-body sequential events, and $3.91 \pm 0.62~e$ for 4-body simultaneous events.
• Figure 5: Zoomed-in view of the total ionization distributions for 3-body and 4-body sequential events, shown with a reduced $y$-axis range to resolve the low-ionization region.