  It has recently been shown that relaxation of the rotational energy of hot
non-equlibrium photofragments (i) slows down significantly with the increase of
their initial rotational temperature and (ii) differs dramatically from the
relaxation of the equilibrium rotational energy correlation function,
manifesting thereby breakdown of the linear response description [Science 311,
1907 (2006)]. We demonstrate that this phenomenon may be caused by the angular
momentum dependence of rotational friction. We have developed the generalized
Fokker-Planck equation whose rotational friction depends upon angular momentum
algebraically. The calculated rotational correlation functions correspond well
to their counterparts obtained via molecular dynamics simulations in a broad
range of initial non-equilibrium conditions. It is suggested that the angular
momentum dependence of friction should be taken into account while describing
rotational relaxation far from equilibrium.
