Effect of collisions on the orientational relaxation of photofragments

Abstract

Effect of collisions is investigated on the photofragment anisotropy decay. We restrict ourselves to studying linear fragments, and no rotational predissociation is assumed. Photoproducts are produced with a nonequilibrium rotational distribution, basically due to the applied torque and the parent molecule rotation. A kinetic equation, describing rotational relaxation of linear fragments under nonequilibrium conditions, is derived and solved for angular momentum correlation functions (CFs), rotational energy CFs, and also orientational CFs (OCFs). The characteristic decay times for the angular momentum and rotational energy CFs are shown to be insensitive to the mechanism of the photofragmentation. On the contrary, OCF of the second rank, that completely determines anisotropy of the photoproduct emission, is demonstrated to be very sensitive to peculiarities of the dissociation process and collision dynamics. This is confirmed by comparison of the calculated photoproduct anisotropies with experimental [M. Volk, S. Gnanakaran, E. Gooding, Y. Kholodenko, N. Pugliano, R.M. Hochstrasser, J. Phys. Chem. A 101 (1997) 638] and simulated [I. Benjamin, U. Banin, S. Ruhman, J. Chem. Phys. 98 (1993) 8337] ones. The fragment ensemble is proved to retain some memory about its initial nonequilibrium distribution. This pertains not only to the short time behavior for the anisotropy, but also for its long time decay. Therefore, the study of the polarization response of the ensemble of photoproducts allows one to know about features of the photofragmentation dynamics, and also to get information about peculiarities of collisions of fragments with buffer medium species.