Experimental and theoretical investigations of the field-free alignment of the nonrigid methanol molecule are reported. The molecule is subject to a $140-\mathrm{TW}/{\mathrm{cm}}^2$-intensity laser pulse with a 100-fs duration. The experimental signal displays a constant permanent alignment and a fast decaying transient alignment consistent with a prolatelike molecule with $(B+C)/2$ on the order of $0.808{\mathrm{cm}}^{-1}$. The theoretical model takes into account the large-amplitude internal rotation of the methyl group with respect to the hydroxyl group. In the case of a continuous-wave laser field, a rotational alignment close to that of a rigid molecule is predicted. Torsional alignment also occurs even though there is no explicit dependence of the polarizability tensor on the angle of internal rotation. In the case of a strong short laser pulse, the theoretical approach shows that permanent and transient rotational alignment take place. The latter displays an exponential-like decay due to the high density of rotation-torsion levels. Torsional alignment also occurs and depends on the temperature. The theoretical model allows us to reproduce the experimental signal provided one component of the polarizability tensor is adjusted and dissipation effects due to molecular collisions are taken into account.

• Received 4 July 2019

DOI:https://doi.org/10.1103/PhysRevA.100.043425