Extracting the polarizability anisotropy from transient alignment of HBr

The fundamental response of a molecule to an applied laser field is governed in part by its anisotropic polarizability, $\Delta \alpha $, a quantity for which experimentally verifiable values are not always available in the literature. We show that comparison of the efficiency of transient molecular alignment of one diatomic species with known $\Delta \alpha $, to that of other diatomic species with unknown $\Delta \alpha $, can be an effective technique for measuring the unknown anisotropies. Specifically, we utilize identical 30 fsec, 800 nm laser pulses to transiently align N$_{2}$ and HBr. Using the accepted value of $\Delta \alpha $ for N$_{2}$, and comparing the time-dependent revival structures for each molecule with the predictions of a rigid rotor simulation, we extract values for the rotational temperatures for each molecule and determine $\Delta \alpha $ for HBr.