Alignment and Imaging of the ${\mathrm{CS}}_2$ Dimer Inside Helium Nanodroplets

The carbon disulphide (${\mathrm{CS}}_2$) dimer is formed inside He nanodroplets and identified using fs laser-induced Coulomb explosion, by observing the ${\mathrm{CS}}_2^{+}$ ion recoil velocity. It is then shown that a 160 ps moderately intense laser pulse can align the dimer in advantageous spatial orientations which allow us to determine the cross-shaped structure of the dimer by analysis of the correlations between the emission angles of the nascent ${\mathrm{CS}}_2^{+}$ and ${\mathrm{S}}^{+}$ ions, following the explosion process. Our method will enable fs time-resolved structural imaging of weakly bound molecular complexes during conformational isomerization, including formation of exciplexes.

Figure 1

Schematic of the key elements in the experiment. In the case depicted here, the alignment (probe) laser pulse is linearly polarized along the $Z$ axis ($X$ axis). The electrostatic plates of the VMI spectrometer projecting the ${\mathrm{CS}}_2^{+}$ or ${\mathrm{S}}^{+}$ ions onto the imaging detector are not shown. The inset shows a sketch of the gas-phase dimer structure.

Figure 2

(a1)–(d1) ${\mathrm{CS}}_2^{+}$ ion images and (a2)–(d2) corresponding angular covariance maps created from ions outside the yellow circles. The polarization state of the probe (alignment) pulse is given in the lower left (right) corner of each ion image. Data in rows (a),(c),(d) [(b)] were recorded for the dimer-[monomer-]doping condition. (a3)–(d3) illustrate the spatial alignment of the monomer or dimer derived from the ion images and covariance maps (see text), curved arrows indicate axes about which there is free rotation.

Figure 3

(a1)–(b1) ${\mathrm{S}}^{+}$ ion images and (b1)–(b2) corresponding angular covariance maps created from ions outside the yellow circles. The polarization state of the probe (alignment) pulse is given in the lower left (right) corner of each ion image. Image (a) [(b)] was recorded under the monomer-[dimer]-doping condition. Here, the central region is removed due to background contaminants occurring at $m/z=32\mathrm{Da}$.

Figure 4

(a1)–(a3): Detector view of the dimer for three rotation angles, $\alpha$, around the alignment polarization axis ($Z$ axis). (b1)–(b3): The velocities of the corresponding ${\mathrm{S}}^{+}$ ions in the detector ($YZ$) plane.