Rotational alignment in inelastic collisions

Abstract

Using recently developed expressions for fully state‐selected atom–molecule rotationally inelastic cross sections, we determine the degree of rotational alignment under l‐dominant (weak coupling) and potential dominant (strong coupling) conditions. In the former limit, expected to be appropriate whenever strong long‐range forces are present, the alignment effects are contained in form factors which are products of Wigner 3j symbols. The largest cross section is predicted to correspond to the case where both the initial and final rotational angular momentum vectors lie perpendicular to the initial relative velocity vector. Also, in the l‐dominant limit the degree of alignment of the inelastically scattered molecules is predicted to be independent of the initial degree of rotational alignment. Under extreme potential dominant conditions, which we simulate with a statistical description of the inelastic T matrix, the final m_j‐state distribution will be uniform. In molecular beam experiments where fluorescence emission is used to monitor the occurrence of rotationally inelastic collisions, the predicted l‐dominant alignment effects should be easily detectable by observing the linear polarization of the absorbed and emitted radiation.