Ultrafast decay of anisotropy due to electronic decoherence in systems with twofold or threefold degeneracy

The time-dependent anisotropy rg(t) – measured by exciting degenerate levels in an ensemble of stationary molecules of sufficiently high symmetry and probing a transition from the g-fold degenerate levels (g=2 or 3) to a non-degenerate level – falls (within a few hundred femtoseconds) to an asymptotic value significantly lower than rg(0). The loss of electronic coherence in such systems is examined afresh, and the anisotropy is shown to follow a double or single exponential decay depending on whether the random variables of interest (population and phase) are statistically independent or not; some errors in previous derivations are also corrected.