Molecular Level-Crossing Spectroscopy

Classical and quantum formulations are presented for the effects of level crossings on the angular distribution of molecular resonance fluorescence. It is shown that a study of the ``molecular Hanle effect,'' or its electric-field analog, may be used to determine the product of the radiative lifetime ρv′J′ with the magnetic moment μm, or the electric dipole moment μe, respectively, for the (v′, J′) excited state. The theory is developed for diatomic or polyatomic molecules characterized by symmetric-top wavefunctions. Order-of-magnitude estimates are performed, and the observation of zero-field molecular level crossings appears to be quite feasible.