Measurement of orientation and alignment moment relaxation by polarization spectroscopy: theory and experiment

A diagrammatic perturbation theory description of one-color polarization spectroscopy (PS) is developed which emphasizes the significance of orientation and alignment tensor moments of the rotational angular momentum, and their collisional evolution. The influences of Doppler motion, velocity-changing collisions, decay of population, orientation and alignment, and nuclear hyperfine depolarization on the calculated PS signal are discussed. Illustrative simulations are presented of the evolution of the PS signal as a function of pump-probe laser delay. These are generated by a Monte Carlo integration of the derived equations for the signal electric field over typical experimental pump and probe laser temporal profiles and velocity distributions for a commonly studied system, the OH A 2Sigma+ -X 2Pi (0,0) band. These predictions are compared with a preliminary set of results obtained in an experimental apparatus designed for one-color polarization spectroscopy using independent pump and probe lasers. Measurements are presented using linearly polarized pump light on the Q1(2.5) transition of the OH A 2Sigma+ -X 2Pi (0,0) band with He as the collision partner. The decay of the experimental PS pump-probe signal is discussed with reference to inelastic collisional population transfer rates in the literature. It is concluded that the collisional depolarization of rotational alignment is rapid, with a rate approximately twice that of population transfer. This is consistent with previous measurements in atmospheric pressure flames. PS is shown to be a viable novel spectroscopic method for determining rotational angular momentum orientation and alignment relaxation rates, which are valuable quantities because they are sensitive probes of the forces involved in inelastic collisions.