Coulomb explosion imaging of small polyatomic molecules with ultrashort x-ray pulses

Open Access

Coulomb explosion imaging of small polyatomic molecules with ultrashort x-ray pulses

X. Li et al.

Phys. Rev. Research 4, 013029 – Published 13 January 2022

Abstract

Ultrashort x-ray pulses from free-electron lasers can efficiently charge up and trigger the full fragmentation of molecules. By coincident detection of up to five ions resulting from rapid Coulomb explosion of highly charged iodomethane, we show that the full three-dimensional equilibrium geometry of this prototypical polyatomic system can be determined from the measured ion momenta with the help of a charge buildup model. Supported by simulations of how the ion momenta would reflect specific changes in molecular bond lengths and angles, we demonstrate that Coulomb-explosion imaging with ultrashort x-ray pulses is a promising technique for recording movies of multidimensional nuclear wave packets, including hydrogen motions.

Received 18 January 2021
Revised 22 September 2021
Accepted 23 September 2021

DOI:https://doi.org/10.1103/PhysRevResearch.4.013029

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Atomic & molecular processes in external fields Atomic & molecular structure Light-matter interaction Single- and few-photon ionization & excitation Ultrafast phenomena X-ray beams & optics

Atomic, Molecular & Optical

Authors & Affiliations

Click to Expand

Article Text

Click to Expand

References

Click to Expand

Issue

Vol. 4, Iss. 1 — January - March 2022

Subject Areas

Reuse & Permissions

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Fragmentation of ${CH}_{3} I$ molecules after ionization by 2-keV, 0.8-mJ x-ray pulses. (a) Charge state distribution of carbon and iodine ions recorded in coincidence. (b) Average number $k$ of hydrogen ions produced from the fragmentation of a ${CH}_{3} I$ molecule for carbon and iodine ion pairs [ $I^{m +}, C^{n +}$ ] with $m \leq 10$ . $k$ is calculated by momentum conservation of the measured momenta (shown in Appendix pp3, Fig. 5), assuming that neutral H do not receive any momentum. The dashed line is an exponential fit to guide the eye.
Reuse & Permissions
Figure 2
Molecular-frame momentum distributions of the ions in the fragmentation channel [ $I^{9 +}$ (purple), $C^{3 +}$ (cyan), $3 \times H^{+}$ (red, green and blue)]. The coordinate frame is chosen such that the $I^{9 +}$ momentum lies along the $x$ axis, and the $z$ axis is defined such that the momentum sum of the carbon ion and the red hydrogen ion lies in the $x - z$ plane with $z > 0$ . Also shown are the projections on the three perpendicular planes.
Reuse & Permissions
Figure 3
Comparison between the experimental and simulated molecular-frame momenta of the ions in the fragmentation channel [ $I^{9 +}$ (purple), $C^{3 +}$ (cyan), $3 \times H^{+}$ (red, green and blue)]. The spheres are the centroids of the experimental distributions in Fig. 2 after outlier removal, with the three hydrogen spheres evenly distributed along the golden circle. The momentum directions are shown by the steel-blue arrows. Tetrahedrons and cubes are the simulated momenta based on the instantaneous charge-up model and the charge buildup model, respectively. The three hydrogen ion centroids are evenly distributed around the golden circle which is centered around and perpendicular to the $x$ axis. Also shown are the projections on the three perpendicular planes.
Reuse & Permissions
Figure 4
Simulated change of molecular-frame momentum of the fragmentation channel [ $I^{9 +}$ (purple), $C^{3 +}$ (cyan), $3 \times H^{+}$ (red, green, and blue)] as a function of the changes in the C–I bond length and the I–C–H angle. (a) Simulated molecular-frame momentum for five different initial C–I bond lengths changed relative to the equilibrium value $l_{C − I}^{0}$ by $\pm 25 %$ in steps of $12.5 %$ [C–I “stretch” mode ( $v_{3}$ )]. The smaller the percentage change, the larger and more transparent are the data symbols. (b) Percentage change of the fragment average momentum as a function of the C–I bond length percentage change, relative to the equilibrium. (c) Simulated molecular-frame momentum for five different initial I–C–H angles changed relative to the equilibrium value $∠_{I - C - H}^{0}$ by $\pm 25 %$ in steps of $12.5 %$ [ ${CH}_{3}$ “umbrella” mode ( $v_{2}$ )]. The larger the percentage change, the larger and more transparent are the data symbols. (d) Percentage change of the fragment average momentum as a function of the I–C–H angle percentage change, relative to the equilibrium.
Reuse & Permissions
Figure 5
Average momenta along the iodine ion momentum direction ( $x$ axis) of iodine (top), carbon (middle), and hydrogen (bottom) ions in different fragmentation channels, which are used to calculate the number of hydrogen ions in the fragmentation as shown in Fig. 1.
Reuse & Permissions
Figure 6
Molecular-frame momentum distributions and the corresponding centroids of the ions in three different fragmentation channels: [ $I^{6 +}, C^{2 +}, 3 \times H^{+}$ ] [panels (a) and (d)], [ $I^{7 +}, C^{3 +}, 3 \times H^{+}$ ] [panels (b) and (e)], and [ $I^{10 +}, C^{3 +}, 3 \times H^{+}$ ] [panels (c) and (f)]. The same procedures as the ones to produce Fig. 2 are applied for plotting the momentum distributions (a), (b), and (c) on the left column. For the corresponding centroids plotted in panels (d), (e), and (f), circles are the experimental data. Triangles are the simulated momenta based on the instantaneous charge-up model. Open circles are the simulated momenta using the charge buildup model. The parameters of the model are obtained by optimization procedures explained in the main text and summarized in Table 1.
Reuse & Permissions

Physical Review Research