Polarized fluorescence of polyatomic fragments produced through photodissociation of polyatomic molecules in the gas phase

A combined theoretical and experimental study is carried of the polarized e mission of polyatomic products produced through photodissociation of polyat omic molecules. A general approach, based on the formalism of dissociation kernels and orientational correlation functions, is developed to predict an isotropy of the fluorescence of photoproducts. We consider the most general case pf asymmetric top parent and product molecules. The rotational predis sociation effect is taken into account. Various kinds of photoreactions are studied: those when fragments after dissociation are in the electronically excited states and those when fragments are in the ground electronic state s so that additional laser pulse is necessary to excite their fluorescence. Particular attention is concentrated on those practically important extrem e cases, when predissociation times and lifetimes of the electronically exc ited stares of photoproducts are short or long as compared to the averaged period of free rotation. The steady state polarized fluorescence of radical s produced through dissociation of several disulfides into two identical ra dicals is measured. The results are interpreted in the framework of the fre e recoil model (FRM). In this model, photoproducts are assumed to experienc e no torque and fly apart freely, so that the only origin of the fragment r otation is rotation of the parent molecule. Predictions of the impulsive mo del (IM), in which fragments are supposed to suffer instantaneous torque du e to the rupture of the chemical bonds of the parent molecule, are demonstr ated to disagree strongly with our experimental data. This gives an additio nal confirmation of the validity of the FRM in describing dissociation of p olyatomic molecules into polyatomic fragments. The FRM can therefore be inv oked to estimate interrelation between the characteristic times, governing the processes of dissociation and emission, and the averaged period of free molecular rotation. Also, the FRM can be used for the determining orientat ions of the absorption and emission dipole moments in the reference frames of the parent and product molecules. (C) 1999 American Institute of Physics . [S0021-9606(99)04001-5].