Laser-induced Coulomb explosion of the LiI molecule and of its dimer

TL;DR

This work demonstrates that LiI monomer internuclear distributions can be recovered from the kinetic-energy distribution of Li^+ ions produced by laser-induced double ionization, using a high-level LiI^2+ ground-state potential to map E_kin to R. The center of the retrieved P(R) coincides with the Boltzmann-weighted theoretical distribution across the three populated vibrational states, but the width is broadened by about 52% due to internuclear motion during the pulse and R-dependent ionization, with excited-state channels found to be suppressed. The study also reveals a substantial LiI dimer content, using covariance analysis to identify multiple Coulomb-explosion channels up to six electrons removed, and assigns fragmentation pathways for the (LiI)2 dimer. Collectively, the results highlight the necessity of high-level ab initio potential energy curves to interpret Coulomb explosion data and open avenues for time-resolved imaging of vibrational dynamics in alkali halides and their dimers.

Abstract

A gas-phase sample consisting of lithium iodide, $\mathrm{LiI}$, molecules and their dimer $\mathrm{(LiI)}_2$, are Coulomb exploded by an intense 25 femtosecond laser pulse. In the case of $\mathrm{LiI}$, we focus on the double ionization that creates a pair of $\mathrm{Li}^+$ and $\mathrm{I}^+$ recoil ions. From the kinetic energy distribution of the $\mathrm{Li}^+$ ions, extracted using coincidence filtering, we determine the distribution of internuclear distances $P(R)$ via the ground state potential curve of $\mathrm{LiI}^{2+}$ obtained from an ab initio calculation that accounts for non-Coulombic effects. We find that the center of $P(R)$ is close to the expected internuclear separation based on the three vibrational states of $\mathrm{LiI}$ populated, whereas the width of $P(R)$ exceeds the theoretical value by $\sim$ 52 %. We discuss if fragmentation via excited $\mathrm{LiI}^{2+}$ potential curves affects the determination of $P(R)$. In the case of the dimer, $\mathrm{(LiI)}_2$, we observe kinetic energies and relative emission directions of $\mathrm{Li}^+$, $\mathrm{I}^+$, and $\mathrm{I}^{2+}$ recoil ions consistent with Coulomb explosion of the parallelogram-shaped dimer after removing up to six electrons by the laser pulse.

Figures (7)

• Figure 1: (a) Potential energy curves for the ground state X $^1\Sigma^+$ of LiI, ground state $^2\Pi$ of LiI+ schmidt_predissociation_2015 and for the lowest-lying electronic state of LiI^2+, $V_\text{QC}$. The Coulomb potential is shown as a dashed curve. The inset shows the square of the vibrational wave function for $v$ = 0, 1, and 2, and the red shape shows the Boltzmann weighted average of the three states. The vertical black arrows illustrate the multiphoton absorption causing sequential double ionzation of LiI. (b) Potential curves for the 10 states of LiI^2+ arising from the lowest-lying orbitals -- see text. Note there are two close-lying $^3\Sigma_1^-$ and $^3\Pi_0$ curves but only one term symbol is indicated in each case. The I+ terms are given to the right of the potential curves.
• Figure 2: (a1)--(c1) 2D momentum images for (a1) Li+, (b1) I+, and (c1) I^2+, scaled individually and with saturated colors for improved visual contrast. The white arrows in the bottom right corners show the polarization axis of the laser pulse. A linear colormap scale has been used for the plots. (a2)--(c2) Slices through the center of the reconstructed 3D momentum distributions, obtained by Abel inversion of the images in (a1)--(c1). (a3)--(c3) Radial momentum distributions $P(p_\text{r})$ in the detector plane. The peaks are labeled A--G, referring to different Coulomb fragmentation channels. The peaks associated with channel A and C are overlapping. (a4)--(d4) Kinetic energy distributions $P(E_\text{kin})$. The central positions of the peaks are annotated next to them. A zoomed-in view of the I+ peak associated with channel F is shown in the inset in (b4).
• Figure 3: (a) Covariance map of the radial momentum distributions for Li+ and I+. (b) Covariance map for the angular distributions of Li+ with 30 $<$$p_\text{r}(\text{Li}^+)$$<$ 95 $\text{amu}\cdot\text{km} / \text{s}$ and I+ with 30 $<$$p_\text{r}(\text{I}^+)$$<$ 95 $\text{amu}\cdot\text{km} / \text{s}$. The colormap scale is linear, and the colors in (b) have been saturated to increase the visual contrast.
• Figure 4: Coincidence-filtered Li+ (a) and I+ (b) ion images for channel A, plotted with a linear color scale. (c) Kinetic energy distribution of the Li+ ions in (a). (d) Distributions of internuclear distances for LiI, extracted from the kinetic energy distribution in (a) using the LiI^2+ potential (black, solid curve) or the Coulomb potential (black, dashed curve). The theoretical reference, $P_\text{theo}(R)$, is also shown (red curve).
• Figure 5: (a) Sketch of the parallelogram-shaped, C$_{2h}$ symmetric, equilibrium structure of (LiI)2 torring_structure_1996. (b)--(f): Covariance maps of the radial momentum distributions for Li+, I+, and I^2+, plotted with a linear color scale. The regions with significant signal are labeled from B to G.