Laser-induced adiabatic alignment of molecules dissolved in helium nanodroplets

A moderately intense, nonresonant, linearly polarized nanosecond laser pulse is used to induce one-dimensional (1D) adiabatic alignment of 1,4 diiodobenzene (C${}_6$H${}_4$I${}_2$), iodobenzene (C${}_6$H${}_5$I), and methyliodide (CH${}_3$I) molecules dissolved in helium nanodroplets. The alignment sharpens as the laser intensity is increased, similar to the behavior in gas phase. For diiodobenzene the highest degree of alignment, $\langle {\cos }^2{\theta }_{2\mathrm{D}}\rangle =0.90$, is essentially identical to the value obtained for isolated molecules in a supersonic beam, whereas the highest degrees of alignment for iodobenzene and for methyliodide in helium droplets fall below the values obtained for isolated molecules. We believe this is due to the deviation from axial recoil in the Coulomb explosion process that probes the alignment of the molecules in the He droplets.

Figure 1

Schematic of the key elements in the experiment. The electrostatic plates projecting the ions onto the imaging detector are not shown. The polarization state of the YAG and the probe laser pulses are shown in the coordinate system and indicated by the pulses.

Figure 2

False color ion images from Coulomb explosion of C${}_6$H${}_4$I${}_2$ [rows (a) and (b)], C${}_6$H${}_5$I molecules [rows (c) and (d)], and CH${}_3$I molecules [rows (e) and (f)] recorded with the probe and YAG pulse polarizations as indicated. The color scale at the side indicates the relative number of ions. The circles indicate the radial ranges used for calculating $\langle {\cos }^2{\theta }_{2\mathrm{D}}\rangle$. $I_{\mathrm{probe}}=1.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ [24]. The velocity space in the gas phase and droplet experiment is the same for each molecule, but different for C${}_6$H${}_4$I${}_2$ as compared to the others.

Figure 3

$\langle {\cos }^2{\theta }_{2\mathrm{D}}\rangle$ recorded at different intensities of the YAG pulse for (a) diiodobenzene, (b) iodobenzene, and (c) methyliodide. Results for isolated molecules are shown by green squares and blue diamonds and results for molecules in He droplets by red circles.