Ab initio study of highly charged ion-induced Coulomb explosion imaging

TL;DR

This work addresses how ion-induced Coulomb explosion imaging (CEI) can reliably reconstruct molecular structures under varying impact geometries. It employs ab initio TDDFT with Ehrenfest dynamics to model a high-energy C5+ projectile impacting pyridazine, systematically comparing orthogonal and in-plane trajectories. Key findings show that avoiding direct atomic collisions yields the most faithful structural representations, while planar and direct-hit trajectories cause greater ionization and broader fragment-momentum distributions, introducing noise and distortions in reconstructed geometries. The results clarify the intrinsic limitations of ion-induced CEI and offer guidance for interpreting experimental data and designing more robust structure-recovery approaches.

Abstract

We present a theoretical investigation of ion-induced Coulomb explosion imaging (CEI) of pyridazine molecules driven by energetic C$^{5+}$ projectiles, using time-dependent density-functional theory (TDDFT) with Ehrenfest nuclear dynamics. By systematically varying the projectile's impact point and orientation relative to the molecular plane, we compare orthogonal and in-plane trajectories and quantify their effects on fragment momenta, electron-density response, and atom-resolved ionization. Newton plots and time-resolved density snapshots show that trajectories avoiding direct atomic collisions yield the most faithful structural reconstructions, whereas direct impacts impart large, highly localized momenta that distort the recovered geometry. Planar trajectories generate substantially greater ionization and broader momentum distributions than orthogonal ones due to deeper traversal through the molecular electron cloud. Quantitative analysis of electron removal at 10~fs confirms that projectile proximity and orientation strongly modulate both local and global ionization. These findings clarify how impact geometry governs the fidelity of ion-induced CEI structural recovery and help explain the variability and noise observed in experimental CEI measurements. More broadly, the results highlight both the strengths and the intrinsic limitations of ion-induced CEI and identify key considerations for interpreting experiments.

Figures (7)

• Figure 1: Molecular structure and atom labels for pyridazine (C4H4N2), oriented in the $xy$-plane. The six red "$\otimes$" symbols indicate the impact points where the ion approaches orthogonally to the molecular plane. The purple arrows along the sides of the molecule denote impact points for which the projectile travels within the molecular plane.
• Figure 2: Newton plots showing the normalized $x$- and $y$-components of the ion momenta for pyridazine fragments. Each panel corresponds to a different IP where the projectile strikes the molecule orthogonally to its plane. The subcaption of each panel indicates the corresponding IP. Carbon, hydrogen, and nitrogen atoms are colored gray, red, and blue, respectively.
• Figure 3: Snapshots of the interaction between the C$^{5+}$ ion (black) and pyridazine for the case where the projectile strikes the C2 atom orthogonal to the molecular plane. Two viewing perspectives are shown: the $xy$-plane (top row) and the $yz$-plane (bottom row). Columns correspond to increasing simulation time. Electron density isosurfaces at values of 0.5, 0.1, 0.01, and 0.001 are rendered in purple.
• Figure 4: Newton plots showing the normalized $x$- and $y$-components of the ion momenta for pyridazine fragments. Each panel corresponds to a different IP where the projectile travels within the molecular XY plane, striking the molecule in-plane. The subcaption for each panel specifies the corresponding IP.
• Figure 5: Snapshots of the interaction between the C$^{5+}$ ion and pyridazine when the projectile strikes the C2--C3 bond within the molecular XY plane. Electron density isosurfaces at values of 0.5, 0.1, 0.01, and 0.001 are shown in purple.