Reduced equations of motion for collision-induced intersystem crossing

A model is proposed for collision-induced intersystem crossing in 'intermediate case' and small molecules. The collisions are assumed to cause dephasing (T2) among the zero order singlet and triplet molecular states. The combined effect of the intramolecular spin-orbit coupling (μ) and the collisional dephasing, results in the experimentally observable relaxation of populations (T1). The basic assumption of the present model is that the duration of a collision tausub(c) is short compared to the intramolecular coupling (μtausub(c)<<1). Reduced equations of motion for the molecular density matrix are derived and conditions for observing nonexponential relaxations are discussed. The model demonstrates the equivalence of T1 and T2 processes, depending on the choice of a basis set. (Auth.)