Determining the stereo configuration of carbonyl sulfide dimers using Coulomb-explosion imaging

A four-body coincident measurement of carbonyl sulfide dimers is conducted using a femtosecond laser-induced Coulomb explosion. Sequential and concerted processes that occur during the Coulomb explosion are distinguished. Four isomers of carbonyl sulfide dimer are resolved, and their structures are retrieved from the measured coincident momenta of the fragment ions generated in the concerted process, with the help of a classical trajectory simulation. As a result of our study, we can resolve the isomers of a weakly bound molecular cluster and determine their stereo configuration by using the Coulomb explosion. There is also the potential in femtosecond time-resolved imaging for structural evolution of molecular complexes in dissociation.

Figure 1

(a) Schematic of the experimental setup. (${\mathrm{b}}_1$)–(${\mathrm{b}}_3$) Geometries of polar, cross-shaped, and nonpolar isomers.

Figure 2

Kinetic energy correlation maps for ${\mathrm{CO}}^{+}$ and ${\mathrm{S}}^{+}$, (${\mathrm{a}}_1$)–(${\mathrm{a}}_3$) for ${\mathrm{CO}}^{+}$ vs ${\mathrm{S}}^{+}$, (${\mathrm{b}}_1$)–(${\mathrm{b}}_3$) for ${\mathrm{CO}}^{+}$ vs ${\mathrm{CO}}^{+}$, and (${\mathrm{c}}_1$)–(${\mathrm{c}}_3$) for ${\mathrm{S}}^{+}$ vs ${\mathrm{S}}^{+}$. Parts (${\mathrm{a}}_1$)–(${\mathrm{c}}_1$) are the experimental results; (${\mathrm{a}}_2$)–(${\mathrm{c}}_2$) and (${\mathrm{a}}_3$)–(${\mathrm{c}}_3$) are the results of concerted and sequential fragmentation CCESs for the nonpolar S-in isomer, respectively.

Figure 3

(a) Correlation map distribution of ${\theta }_{{\text{CO}}^{+}-{\text{CO}}^{+}}$ and $\beta$ for the concerted fragmentation events, with the experiments in a color map and the simulations in a contour map. Distribution of integrated angle (b) dihedral $\beta$ and (c) ${\theta }_{{\text{CO}}^{+}-{\text{CO}}^{+}}$ for experiments (black stars) and simulations (colored dashed lines, green, red, and blue for polar, cross-shaped, and nonpolar isomer, respectively), along with the best fitting of the experiments according to simulations in black dashed lines. (d) The schematic of the relative angles between momenta of fragments.

Figure 4

(a) Coordinates of ${\left(\mathrm{OCS}\right)}_2$. S is yellow, C is red, and O is blue. $R$ is the distance of the center of mass (CM) of the two molecules, ${\theta }_1$ and ${\theta }_2$ are the angles between the molecular axis and the axis that connects the CM of the OCS moieties of the dimer, and $\phi$ is the azimuth angle around the axis that connects the CM of the OCS moieties of the dimer. (b) Reconstructed angle distributions of $\phi$ for nonpolar and polar events. (c),(d) The correlation maps of reconstructed ${\theta }_1$ and ${\theta }_2$ for (c) nonpolar and (d) polar events.

Figure 5

(a)–(d) The reconstructed geometries of the isomers and the corresponding predicted geometries shown as ball-and-stick models, (a) for nonpolar S-in, (b) for nonpolar O-in, (c) for polar, and (d) for cross-shaped isomers. Blue and yellow dots represent the CM of ${\mathrm{CO}}^{+}$ and ${\mathrm{S}}^{+}$, respectively. Coordinates are rotated to the molecular frame of reference, where the CM of the dimer defines the origin, the direction along intermolecular bond $R$ defines the $x$-axis, and the bisector of $\phi$ defines the x-y plane. All other coordinates are oriented relative to these definitions. For better visibility, the cross-shaped structure (d) is rotated ${45}^{\circ }$ clockwise around the $x$-axis to match the visual angle of the calculated structure coordinates.

Figure 6

(a),(c) Density plot of ${\mathrm{CO}}^{+}+{\mathrm{S}}^{+}+{\mathrm{CO}}^{+}+{\mathrm{S}}^{+}$ events as a function of ${\mathrm{KER}}_{\mathrm{CO},\mathrm{S}}$ and ${\theta }_{12}$. (c),(d) Correlation map of the ${\theta }_{12}$ from the two pairs of ${\mathrm{CO}}^{+}$ and ${\mathrm{S}}^{+}$. Diagrams (a),(b) of whole events mixed with concerted and sequential events, and (c),(d) of concerted events after disentanglement. (e) Relative momenta in the four-body sequential fragmentation. Black and red arrows represent the first and second breakup steps, respectively, with the angle ${\theta }_{12}$ in between.

Figure 7

Simulated kinetic energy correlations between the momenta of four ions generated from the concerted channel for three possible structures, (${\mathrm{a}}_1$)–(${\mathrm{d}}_1$) nonpolar S-in, (${\mathrm{a}}_2$)–(${\mathrm{d}}_2$) polar, and (${\mathrm{a}}_3$)–(${\mathrm{d}}_3$) cross-shaped. Subscripts 1 and 2 of the ions represent the parent OCS molecule from which the ions are generated.

Figure 8

Simulated kinetic energy correlation of the four ions generated from the sequential channel for the three possible structures, (${\mathrm{a}}_1$)–(${\mathrm{d}}_1$) nonpolar S-in, (${\mathrm{a}}_2$)–(${\mathrm{d}}_2$) polar, and (${\mathrm{c}}_3$)–(${\mathrm{c}}_3$) cross-shaped isomers.

Figure 9

(a)–(d) The correlation maps of the reconstructed and initial coordinates for $R, \phi , {\theta }_1$, and ${\theta }_2$. (e)–(h) The correlation map of ${\theta }_1$ and ${\theta }_2$ of (e),(g) initial geometries and (f),(h) their corresponding reconstructed geometries.