Impulsive Laser Induced Alignment of Molecules Dissolved in Helium Nanodroplets

We show that a 450 fs nonresonant, moderately intense, linearly polarized laser pulse can induce field-free molecular axis alignment of methyliodide (${\mathrm{CH}}_3\mathrm{I}$) molecules dissolved in a helium nanodroplet. Time-resolved measurements reveal rotational dynamics much slower than that of isolated molecules and absence of the sharp transient alignment recurrences characteristic of gas phase molecules. Our results presage a range of new opportunities for exploring both molecular dynamics in a dissipative environment and the properties of He nanodroplets.

Figure 1

(a) Schematic of the key elements in the experiment. The electrostatic plates projecting the ${\mathrm{I}}^{+}$ ions onto the imaging detector are not shown. The polarization state of the kick and the probe laser pulses are shown in the coordinate system and indicated by the pulses. (b) ${\mathrm{I}}^{+}$ ion images recorded for isolated ${\mathrm{CH}}_3\mathrm{I}$ molecules with the probe pulse only, $t=0.3\mathrm{ps}$ and $t=33.6\mathrm{ps}$ with the polarization of the kick pulse indicated. (c) ${\mathrm{I}}^{+}$ or ${\mathrm{IHe}}^{+}$ ion images recorded for ${\mathrm{CH}}_3\mathrm{I}$ molecules in He droplets with the probe pulse only and $t=20\mathrm{ps}$. The dark red spot originates from ionization of background gas in the target region. It does not contribute to our determination of $⟨{cos}^2{\theta }_{2\mathrm{D}}⟩$ which is based only on the ion signal in the area between the yellow circles. The velocity ranges depicted in all images in (b) and (c) are identical and the color scale for each image is normalized to the peak intensity outside the central dark red spot ${\mathrm{I}}_{\mathrm{kick}}=1.2\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 2

The degree of alignment, represented by $⟨{cos}^2{\theta }_{2\mathrm{D}}⟩$ determined from ${\mathrm{I}}^{+}$ images, of isolated ${\mathrm{CH}}_3\mathrm{I}$ molecules (black squares) and of ${\mathrm{CH}}_3\mathrm{I}$ molecules in He droplets [gray (red) squares] as a function of time after the peak of the kick pulse (centered at $t=0$). The parameters of the kick and the probe pulse are identical for the two data series. ${\mathrm{I}}_{\mathrm{kick}}=1.2\times 13\mathrm{W}/{\mathrm{cm}}^2$. The inset expands on the first 150 ps.

Figure 3

$⟨{cos}^2{\theta }_{2\mathrm{D}}⟩$ as a function of time, recorded at different intensities of the kick pulse and for both isolated ${\mathrm{CH}}_3\mathrm{I}$ molecules (a) and ${\mathrm{CH}}_3\mathrm{I}$ molecules in He droplets (b).

Figure 4

$⟨{cos}^2{\theta }_{2\mathrm{D}}⟩$ as a function of time, recorded for ${\mathrm{CH}}_3\mathrm{I}$ (red squares), ${\mathrm{CS}}_2$ (black circles), and ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{I}$ (green triangles) in He droplets. ${\mathrm{I}}_{\mathrm{kick}}=1.2\times 13\mathrm{W}/{\mathrm{cm}}^2$.