Laser-Induced Alignment and Orientation of Quantum-State-Selected Large Molecules

A strong inhomogeneous static electric field is used to spatially disperse a supersonic beam of polar molecules, according to their quantum state. We show that the molecules residing in the lowest-lying rotational states can be selected and used as targets for further experiments. As an illustration, we demonstrate an unprecedented degree of laser-induced one-dimensional alignment $(⟨{cos}^2{\theta }_{2\mathrm{D}}⟩=0.97)$ and strong orientation of state-selected iodobenzene molecules. This method should enable experiments on pure samples of polar molecules in their rotational ground state, offering new opportunities in molecular science.

Figure 1

Scheme of the experimental setup. In the inset, a cut through the deflector is shown, and a contour plot of the electric field strength is given.

Figure 2

The vertical profile of the molecular beam measured by recording the laser-induced ${\mathrm{I}}^{+}$ signal (see text). The experimental data are shown by (black) squares (deflector off), (red) triangles (5 kV), and (blue) circles (10 kV), together with the corresponding simulated profiles. In the inset the energies of the lowest quantum states of iodobenzene are shown as a function of the electric field strength. The shaded area represents the range of field strengths present in the detector for a voltage of 10 kV. The value of the dipole moment of iodobenzene [22] is given below the molecular model.

Figure 3

${\mathrm{I}}^{+}$ ion images, recorded when the probe pulse Coulomb explodes the iodobenzene molecules, illustrating alignment and orientation. The polarization state of the YAG and the probe pulse with respect to the detector plane and the static field is shown schematically at the left. Images labeled “no deflection” are recorded at lens position $y=0.0\mathrm{mm}$, “deflection” at $y=1.0\mathrm{mm}$, and “depletion” at $y=-0.9\mathrm{mm}$, the latter two with the deflector at 10 kV (see Fig. 2). The value of $\beta$ is shown for each image in row B and C. The intensity of the YAG pulse is $8\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$ and $5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ for the probe pulse. $E_{\mathrm{stat}}=595\mathrm{V}/\mathrm{cm}$.

Figure 4

Orientation, represented by $\mathrm{N}({\mathrm{I}}^{+}{)}_{\mathrm{up}}/\mathrm{N}({\mathrm{I}}^{+}{)}_{\mathrm{total}}$, as a function of $\beta$; see text for details.