Differentiating Three-Dimensional Molecular Structures Using Laser-Induced Coulomb Explosion Imaging

Huynh Van Sa Lam, Anbu Selvam Venkatachalam, Surjendu Bhattacharyya, Keyu Chen, Kurtis Borne, Enliang Wang, Rebecca Boll, Till Jahnke, Vinod Kumarappan, Artem Rudenko, and Daniel Rolles

Phys. Rev. Lett. 132, 123201 – Published 19 March 2024

Abstract

Coulomb explosion imaging (CEI) with x-ray free electron lasers has recently been shown to be a powerful method for obtaining detailed structural information of gas-phase planar ring molecules [R. Boll et al., X-ray multiphoton-induced Coulomb explosion images complex single molecules, Nat. Phys. 18, 423 (2022).]. In this Letter, we investigate the potential of CEI driven by a tabletop laser and extend this approach to differentiating three-dimensional structures. We study the static CEI patterns of planar and nonplanar organic molecules that resemble the structures of typical products formed in ring-opening reactions. Our results reveal that each molecule exhibits a well-localized and distinctive pattern in three-dimensional fragment-ion momentum space. We find that these patterns yield direct information about the molecular structures and can be qualitatively reproduced using a classical Coulomb explosion simulation. Our findings suggest that laser-induced CEI can serve as a robust method for differentiating molecular structures of organic ring and chain molecules. As such, it holds great promise as a method for following ultrafast structural changes, e.g., during ring-opening reactions, by tracking the motion of individual atoms in pump-probe experiments.

Received 4 November 2023
Accepted 12 February 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.123201

Physics Subject Headings (PhySH)

Atomic & molecular processes in external fields Atomic & molecular structure Molecular dissociation Strong field ionization & excitation Ultrafast phenomena

Atomic, Molecular & Optical

Authors & Affiliations

Huynh Van Sa Lam ^1,*, Anbu Selvam Venkatachalam ¹, Surjendu Bhattacharyya ¹, Keyu Chen ¹, Kurtis Borne¹, Enliang Wang ¹, Rebecca Boll ², Till Jahnke², Vinod Kumarappan ¹, Artem Rudenko¹, and Daniel Rolles ^1,†

¹James R. Macdonald Laboratory, Kansas State University, Manhattan, Kansas 66506, USA
²European XFEL, 22869 Schenefeld, Germany

^*huynhlam@phys.ksu.edu
^†rolles@phys.ksu.edu

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Issue

Vol. 132, Iss. 12 — 22 March 2024

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Images

Figure 1
Top row: ball-and-stick models of the three molecules studied in this work: isoxazole, 3-chloro-1-propanol, and epichlorohydrin. Bottom row: schematic of a light-induced ring-opening reaction (modeled after the UV-induced ring opening of oxazole [32, 33]). Isoxazole, 3-chloro-1-propanol, and epichlorohydrin mimic the structures of the ring-closed, open-chain, and ring-chain products, respectively.
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Figure 2
Measured and simulated CEI patterns of isoxazole. The first column shows the ball-and-stick model views of isoxazole in three principal planes. The plots in the second and fourth columns are projections of the measured fragment-ion momenta (so-called Newton plots) from the $(H^{+}, C^{+}, N^{+}, O^{+})$ 4-fold coincidence channel onto these three principal planes. The first, second, and third rows are the $x y$ plane, $x z$ plane, and $y z$ plane, respectively. For each event plotted here, the coordinate frame is rotated such that the $O^{+}$ momentum points along the $x$ axis ( $p_{O_{y}} = p_{O_{z}} = 0$ ) and the $N^{+}$ momentum lies in the upper $x y$ plane $(p_{N_{y}} \geq 0, p_{N_{z}} = 0)$ . The momenta of $C^{+}$ and $H^{+}$ are plotted in this coordinate frame. No background was subtracted. All momenta are normalized to the $O^{+}$ momentum ( $| p_{O} | = 1$ , not shown). Newton plots of $H^{+}$ and $C^{+}$ show localized spots corresponding to three carbons and three hydrogens of isoxazole. For laser shots where more than one carbon or hydrogen ion was detected, multiple 4-fold coincidence events were created in the analysis by making all possible combinations of the detected $(H^{+}, C^{+}, N^{+}, O^{+})$ fragments. For the data visualization, the $C^{+}$ ion counts are multiplied by three since only one out of three carbon ions is plotted for each nitrogen and oxygen ion that is plotted. The third and fifth columns show the results of classical Coulomb explosion simulations for the neutral molecule in its equilibrium geometry.
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Figure 3
3D scatter plots showing the normalized measured momenta of individual ions from (a) isoxazole, (b) 3-chloro-1-propanol, and (c) epichlorohydrin from the (a) $(C^{+}, C^{+}, N^{+}, O^{+})$ , (b) $(C^{+}, C^{+}, O^{+}, {Cl}^{+})$ , and (c) $(C^{+}, C^{+}, O^{+}, {Cl}^{+})$ 4-fold coincidence channels. $C^{+}$ , $N^{+}$ and $O^{+}$ ions in 3D scatter plots are shown in purple, blue and red, respectively, to distinguish them from projections shown in gray scale. The position of each data point is determined by its normalized momentum, while its color corresponds to the density. In (a), the recoil frame is rotated such that the momentum of $O^{+}$ (so-called $x$ -reference ion) is set as the unit vector along the $x$ axis, and the momentum of $N^{+}$ (referred to as $x y$ -reference ion) is in the upper $x y$ plane. The momenta of other ions are plotted in this coordinate frame. The $x$ - and $x y$ -reference ions are ${Cl}^{+}$ and $O^{+}$ in (b), and ${Cl}^{+}$ and the first-detected $C^{+}$ in (c). The $x$ -reference ion is not plotted in any panel. We also do not plot the $x y$ -reference ion in the $p_{z} p_{x}$ and $p_{z} p_{y}$ projections since they are simply intense lines along $p_{z} = 0$ .
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Figure 4
Left: molecular views. The ball-and-stick model of 3-chloro-1-propanol is rotated such that the C-Cl bond is along the $x$ axis, the C-O bond points in the positive $y$ direction, and the two carbons near Cl and O lie in the $x z$ plane. For this given rotation, all three projections are shown (top to bottom). Note that in the $z y$ projection, one carbon is hidden behind the chlorine. Center: projections of the 3D momenta in Fig. 3 but not normalized (the momenta are in atomic unit). The momentum of ${Cl}^{+}$ (not plotted) is along the $x$ axis. The $x y$ -reference ion ( $O^{+}$ ) is plotted only in the top row (the $p_{x} p_{y}$ projection). Right: simulations of the projections in the middle panels using a classical Coulomb explosion model.
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