Depolarization of optically prepared molecules by two randomly oriented spins

Abstract

A general expression is developed for the time development of the alignment of an ensemble of isolated molecules having two random internal spins when the ensemble is optically pumped by absorption of a linearly polarized light pulse. This derivation assumes that the internal spin structure (fine structure/hyperfine structure) is coherently excited and is not resolved. As an example the pulsed infrared excitation of hydrogen fluoride is considered, in particular the vibration-rotation transition HF(ν″ = 0, N″) → HF(ν = 1, N). The presence of the nuclear spins I H = 1/2 and I F = 1/2 is found to cause a significant reduction in the degree of alignment of the low rotational levels but the extent of depolarization diminishes rapidly with increasing N.