Strong-Field-Induced Coulomb Explosion Imaging of Tribromomethane

Abstract

The Coulomb explosion of tribromomethane (bromoform, CHBr₃) induced by 28 fs near-infrared laser pulses is investigated by three-dimensional coincidence ion momentum imaging. We focus on the fragmentation into three, four, and five ionic fragments measured in coincidence and present different ways of visualizing the three-dimensional momentum correlations. We show that the experimentally observed momentum correlations for 4- and 5-fold coincidences are well reproduced by classical Coulomb explosion simulations and contain information about the structure of the parent molecule that could be used to differentiate structural isomers formed, for example, in a pump-probe experiment. Our results thus provide a clear path toward visualizing structural dynamics in polyatomic molecules by strong-field-induced Coulomb explosion imaging.

Figure 1

The 3-, 4-, and 5-fold photoion coincidence plots of strong-field ionized CHBr₃ after subtraction of random coincidences, zoomed into the region of interest for the (a) Br⁺ + Br⁺ + CHBr⁺, (b) CH⁺ + Br⁺ + Br⁺ + Br⁺, and (c) H⁺ + C⁺ + Br⁺ + Br⁺ + Br⁺ channels. The full coincidence plots are shown in Figures S2, S5, and S7.

Figure 2

Newton plot of the ⁸¹Br⁺ + ⁸¹Br⁺ + CH⁸¹Br⁺ fragmentation channel measured in coincidence, showing the momentum correlation between the fragment ions. The momentum of one of the Br⁺ fragments is pointing to the right (red arrow), and the normalized momenta of the other Br⁺ and the CHBr⁺ fragment ions are plotted in the upper and lower half, respectively. The blue stars and semicircles are the momentum correlations obtained from a Coulomb explosion simulation for concerted and sequential breakup, respectively (see text).

Figure 3

Three-dimensional momentum correlation plot of the (a) four-body CH ⁺ + ⁸¹Br⁺ + ⁸¹Br⁺ + ⁸¹Br⁺ and (b) the five-body H⁺ + C⁺ + ⁸¹Br⁺ + ⁸¹Br⁺ + ⁸¹Br⁺ channel. The momentum vectors of each fragment in a 4- or 5-fold coincidence event are rotated such that the momentum of the reference ion, CH⁺ in (a) and C⁺ in (b), points along the x-axis, and the sum of the momentum vectors of the last two detected Br⁺ ions lies in the xy-plane with positive P_y values. The momenta of the remaining fragments, normalized to the magnitude of the momentum of the reference ion, are then plotted in this coordinate frame. (c, d) Coulomb explosion simulations for the equilibrium geometry of CHBr₃ (stars) compared to those for the equilibrium geometry of iso-CHBr₃ (circles). The two geometries are shown as insets. The reference ion and the xy-plane are defined as in panels a and c, and the plots are defined such that the momentum vector of the Br⁺ ion marked as “1” lies in the xy-plane. Plots where ions “2” or “3” lie in the xy-plane are shown in Figure S11.

Figure 4

Projections of the distribution of the three Br⁺ ions in the four-body channel shown in Figure 3a on the (a) yz-, (b) xz-, and (c) xy-planes. The blue stars indicate the normalized momenta obtained from the Coulomb explosion simulations for the equilibrium geometry of the CHBr₃ ground state (see the Methods section). Coulomb explosion simulations for the isomer geometry are shown in Figure S12.

Figure 5

Projections of the experimentally obtained momentum correlation of H⁺ and three ⁸¹Br⁺ ions in the five-body channel shown in Figure 3b on the (a) yz-, (b) xz-, and (c) xy-planes. The blue stars indicate the normalized momenta obtained from the Coulomb explosion simulations for the initial equilibrium geometry of the CHBr₃ ground state. Note that the distributions of the two Br⁺ ions with positive momenta along the y-directions overlap almost completely in (c) and that the two stars representing the simulated momenta of the two Br⁺ ions with positive y-momentum are in identical positions. The two rows below show the same projections obtained from the Coulomb explosion simulations of 10000 CHBr₃ (panels d–f) and iso-CHBr₃ (panels g–i) geometries sampling the Wigner distribution of the ground-state molecules at 60 K. Similar to Figure 3, the plots are defined such that the momentum vector of the Br⁺ ion marked as “1” lies in the xy-plane. Plots where ions “2” or “3” lie in the xy-plane are shown in Figure S13.