Depolarization of luminescence of polyatomic molecules in the gas phase as a method of determining the efficiency of collisional transfer of angular momentum

The theory of collisional depolarization of luminescence of extended polyatomic molecules in rarefiedgases is considered. The interrelation between the frequency of collisions, the relaxation time of the angular momentum, and the cross section of the luminescence depolarization is established, and the dependence of these parameters on the efficiency of an abrupt change in the angular momentum is calculated. The use of the theory of collisions of solids in the Enskog approximation made it possible to take into account the effect of the shape and mass of colliding molecules on the degree of depolarization. It is established that, in terms of this theory, there exists a limiting efficiency of an abrupt change in the angular momentum, which, however, does not attain the value proposed in the model of strong collisions (Jdiffusion). The dependence of the depolarization of luminescence of 1,4-di-(2-5-p-tolyloxazolyl) benzene molecules on the concentration of a buffer gas (argon) is measured. It is found that about five collisions with Ar atoms are required for randomization of the angular momentum of these molecules.