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

Abstract

A combined theoretical and experimental study is carried of the polarized emission of polyatomic products produced through photodissociation of polyatomic molecules. A general approach, based on the formalism of dissociation kernels and orientational correlation functions, is developed to predict anisotropy of the fluorescence of photoproducts. We consider the most general case of asymmetric top parent and product molecules. The rotational predissociation 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 states so that additional laser pulse is necessary to excite their fluorescence. Particular attention is concentrated on those practically important extreme cases, when predissociation times and lifetimes of the electronically excited states of photoproducts are short or long as compared to the averaged period of free rotation. The steady state polarized fluorescence of radicals produced through dissociation of several disulfides into two identical radicals is measured. The results are interpreted in the framework of the free recoil model (FRM). In this model, photoproducts are assumed to experience no torque and fly apart freely, so that the only origin of the fragment rotation is rotation of the parent molecule. Predictions of the impulsive model (IM), in which fragments are supposed to suffer instantaneous torque due to the rupture of the chemical bonds of the parent molecule, are demonstrated to disagree strongly with our experimental data. This gives an additional confirmation of the validity of the FRM in describing dissociation of polyatomic molecules into polyatomic fragments. The FRM can therefore be invoked 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 orientations of the absorption and emission dipole moments in the reference frames of the parent and product molecules.