Field-Free Three Dimensional Molecular Axis Alignment

We investigate strategies for field-free three dimensional molecular axis alignment using strong nonresonant laser fields under experimentally realistic conditions. Using the polarizabilites and rotational constants of an asymmetric top rotor molecule (ethene, ${\mathrm{C}}_2{\mathrm{H}}_4$), we consider three different methods for axis alignment of a Boltzmann distribution of rotors at 4 K. Specifically, we compare the use of impulsive kick laser pulses having both linear and elliptical polarization to the use of elliptically polarized switched laser pulses. We show that an enhanced degree of field-free three dimensional alignment of ground vibronic state molecules obtains from the use of two orthogonally polarized, time-separated laser pulses.

Figure 1

Alignment of ethene at a rotational temperature of 4 K with a linearly polarized kick pulse with ${\tau }_{\mathrm{rise}}={\tau }_{\mathrm{fall}}=600\mathrm{fs}$, $t_0=0.0\mathrm{ps}$, and ${\mathcal{E}}_0=2.746\times {10}^9$ (corresponding to ${10}^{12}\mathrm{W}/{\mathrm{cm}}^2$) polarized along the LF $Z$ axis. The pulse envelope is indicated below the lower plot.

Figure 2

Alignment of ethene at a rotational temperature of 4 K with two linearly polarized kick laser pulses propagating along the LF $Y$ axis. The first laser pulse at $t_0=0.0\mathrm{ps}$ is linearly polarized along $Z$ with ${\tau }_{\mathrm{rise}}={\tau }_{\mathrm{fall}}=600\mathrm{fs}$. The second laser pulse at $t_0=8.3\mathrm{ps}$ is linearly polarized along $X$ with ${\tau }_{\mathrm{rise}}={\tau }_{\mathrm{fall}}=250\mathrm{fs}$. Both laser pulses have a peak electric field of ${\mathcal{E}}_0=2.746\times {10}^9\mathrm{V}/\mathrm{m}$. The pulse envelopes are indicated below the lower plot.

Figure 3

Alignment of ethene at a rotational temperature of 4 K with an elliptically polarized switched laser pulse propagating along the LF $Y$ axis with ${\tau }_{\mathrm{rise}}=30\mathrm{ps}$, ${\tau }_{\mathrm{fall}}=50\mathrm{fs}$, and $t_0=0\mathrm{ps}$. A peak laser field of ${\mathcal{E}}_0=2.972\times {10}^9\mathrm{V}/\mathrm{m}$ was used with the major axis of the polarization ellipse along the $Z$ axis and a phase difference of $\pi /8$ between the $Z$ and $X$ components (i.e., $e_Z=0.924$ and $e_X=-0.383i$). The pulse envelope is indicated below the lower plot.

Figure 4

Alignment of ethene at a rotational temperature of 4 K with an elliptically polarized kick pulse propagating along the LF $Y$ axis with ${\tau }_{\mathrm{rise}}={\tau }_{\mathrm{fall}}=600\mathrm{fs}$ and $t_0=0.0\mathrm{ps}$. The polarization vector components and peak electric field were chosen to be the same as for Fig. 3. The pulse envelope is indicated below the lower plot.