Probing the structure of cyclic hydrocarbon molecules with X-ray-induced Coulomb explosion imaging

TL;DR

This work demonstrates that X-ray-induced Coulomb explosion imaging (CEI) can distinguish hydrocarbon isomers with formula C7H8 without requiring marker atoms. By combining COLTRIMS-based coincidence measurements at an XFEL with classical 3N-body Coulomb explosion simulations, the authors map momentum-space patterns to specific carbon and hydrogen sites, using angular correlations to define effective molecular frames. The study shows distinct, isomer-specific signatures in Newton plots and molecular-frame angular distributions across toluene, cycloheptatriene, and 1,6-heptadiyne, with hydrogen imaging providing additional discriminatory power. These findings advance time-resolved tracking of ultrafast nuclear motion in hydrocarbons and highlight the potential of gating strategies and higher-repetition-rate sources to enhance structural reconstruction from CEI data.

Abstract

Coulomb explosion imaging (CEI) is a powerful experimental technique that maps a molecule's geometric structure onto the momenta of ionic molecular fragments produced by rapid multiple ionization. Here, we apply CEI induced by pulses from an X-ray free-electron laser in order to image and distinguish complex hydrocarbon isomers with the chemical formula C7H8: toluene, cycloheptatriene, and 1,6-heptadiyne. The measured fragment-ion momentum distributions show discernible differences between the three isomers and provide signatures of specific carbon and hydrogen sites in the molecule. In contrast to previous work, we demonstrate that distinct 'marker atoms' are not strictly required for constructing a meaningful molecular frame of reference for the interpretation of the momentum-space data. Our work paves the way for tracking the ultrafast motion of nuclei during isomerization reactions in pure hydrocarbons.

Figures (7)

• Figure 1: Molecular geometries and Newton plots: A) toluene, B) cycloheptatriene, C) 1,6-heptadiyne. D)-F) Experimental Coulomb explosion Newton plots showing the momentum distribution (in atomic units, a.u.) of three C^2+ ions detected in coincidence. The molecular frame is defined by orienting the emission direction of one C^2+ ion, which is randomly chosen among the three detected C^2+ ions, along the positive $p_x$-direction and the emission direction of another C^2+ ion, also randomly chosen from the three ions detected in coincidence, in the upper half of the $(p_x/p_y)$-plane. G)-I) Simulated Newton plots for C^2+, built by adding the simulated momentum distributions in all the recoil frames obtained from all possible combinations of two C^2+ ions, as explained in sections \ref{['sec:sim']} and \ref{['sec:sim_results']}. In the simulations, a charge of +2 is placed on each carbon atom and +1 on each hydrogen atom.
• Figure 2: Simulated three-dimensional Newton plots of toluene, plotted for different combinations of reference ions, corresponding to different carbon sites in the molecule. Carbon i denotes the ion momentum that defines the x-axis, while carbon j is that which lies within the (x,y)-plane, with the numbering of the carbon atoms and the color coding of the scatter plot symbols referring to the ball-and-stick model of the molecule shown at the center of the figure. The momenta of the two reference ions are not shown in this representation, and projections of the momentum distributions on the three Cartesian planes are shown in light blue to aid with the three-dimensional visualization.
• Figure 3: Data filtering on angular correlations of the reference ions employed for defining the recoil frame of the toluene Newton plots. A) Simulated (red) and experimental (blue) distribution of the relative emission angle of the two ions $i$ and $j$ used to define the recoil frame. B) Matrix showing the average $\langle\cos(\theta_{i,j})\rangle$ in the simulated datasets for different combinations of carbon ions. This matrix acts as a guide for which gating region corresponds to which reference ions defining the Newton plot. Darker blue colors correspond to $\langle\cos(\theta_{i,j})\rangle$ values closer to $-1$, darker red colors correspond to values closer to $+1$. C) Toluene Newton plot without filtering. D), E) Toluene Newton plots with filtering on the relative angular distribution for the angular range indicated above each panel.
• Figure 4: Data filtering on angular correlations of the reference ions employed for defining the recoil frame of the cycloheptatriene Newton plots, similar to Fig. \ref{['fgr:toluene_angular_distribution']}, which shows the same for toluene. A) Simulated (red) and experimental (blue) distribution of the relative emission angle of the two ions $i$ and $j$ used to define the recoil frame. B) Matrix showing the average $\langle\cos(\theta_{i,j})\rangle$ in the simulated datasets for different combinations of carbon ions. C) and D) Simulated Newton plots filtered on the peaks in panel A) corresponding to alternating and neighboring carbons, respectively, selected by angular gating on the range of $\langle\cos(\theta_{i,j})\rangle=(-0.83, -0.3)$ and $(0.1, 0.65)$, respectively. E) and F) Corresponding experimental results for the same angular gating as in panels C) and D).
• Figure 5: Molecular-frame angular emission distributions in spherical coordinates for cycloheptatriene. The plots show the emission directions of ions (in the same molecular frame of reference as before) as a function of the polar ($\theta$) and azimuthal ($\phi$) angles for certain ranges of the relative angle $\alpha_{ij}$, similar to Fig. \ref{['fgr:cyclo_angular_distribution']}. Simulated data are shown in the top row, panels A)-C), measured data in the middle row, panels D)-F). G) Definition of the spherical coordinates. H) Simulated Newton plot for a recoil frame defined by only $i=1$ and $j=3$. H) Simulated molecular-frame angular distribution for this idealized recoil-frame definition.