Knowledge Bank

Density matrix formalism in used to study resonance fluorescence processes of isolated gaseous molecules. The Wangsness-Bloch-Fano theory is applied to the Irreversible intramolecular relaxation process. In the present treatment, it is assumed that the rotational angular momentum J is a constant of motion in the zeroth order approximation. Thus intramolecular interactions can be classified as tensor operators with respect to J. The fluorescence decay patterns depend on excitation bandwidths and monitoring condition. Under narrow band excitation, the population (monopole), orientation (dipole) and alignment (quadrupole) of the excited state density matrix $\rho e$ can be prepared. For a sharply prepared angular momentum state under pulsed excitation, the population does not undergo radiation less decay and the resonance fluorescence intensity is a sum of three exponentials in general. For cw experiments, where lifetime limited lifetime or polarized fluorescence intensities are measured, the population can decay via an intramolecular relaxation channel providing the orientation component is initially prepared. Explicit J dependence of lifetimes are given for pulsed experiments. Under broad band excitation, quantum beats due to the coherence of $\rho e$ are derived from the Liouville equation. Most important of all, the population can decay under pulsed, broad-band excitation conditions. The scalar part of the non-adiabatic coupling becomes an additional contributor to the Intramolecular relaxation process in this last case.