Doppler line shape of atomic fluorescence excited by molecular photodissociation

A semiclassical treatment of the photodissociation of a diatomic molecule is developed. It is shown that the angular distribution of products will often be peaked parallel or perpendicular to the direction of the incident light beam. The form of the anisotropy is usually determined just by the orientation of the electronic transition dipole moment within the molecule and the polarization of the exciting light. From the angular distribution, the Doppler line shape of fluorescence emitted by an excited fragment atom is derived by averaging the geometrical factors over the translational velocity distribution of the parent molecules and the distributions in magnitude and angle of the recoil velocity of the excited atoms. A comparison is made with dissociative electron impact processes which show similar features. The photodissociation of NaI is treated in detail, and the factors influencing the fluorescence width are evaluated for possible optical maser systems in which the supply of excited atoms is generated by molecular dissociation.