A theory for the resonance light scattering in a molecular system interacting with the heat bath is developed, taking into account the multilevel structure of the molecular system. An expression for the cross section of the resonance light scattering process is derived by using the Liouville operator method in the Markoff approximation; the cross section consists of three terms: resonance Raman scattering, resonance fluorescence, and their mixing. The mixing term vanishes in the limit of the zero vibrational dephasing constant. Analytical expressions for the resonance Raman scattering and resonance fluorescence cross sections are derived in the displaced harmonic oscillator model. Model calculations of the vibrational band structure in the resonance light scattering cross section are performed by using the expressions derived. Differences of the band intensity distribution in the resonance fluorescence from that in the resonance Raman scattering are illustrated.
The problem of light scattering from a monochromatic laser beam by an ensemble of atmospheric atoms or molecules in thermal equilibrium under ambient wind and temperature is revisited with two emphases. First, the essence of and difference between laser Cabannes scattering (CS) and laser-induced resonance fluorescence (LIF), which permit atmospheric temperature and wind measurements, are simply presented. A classical model is shown to yield the same angular distribution in scattered power for both, a factor of two larger Doppler shift for the former (CS), in the backward direction with the same line-of-sight (LOS) wind, and a dramatic 17 order larger backscattering cross section for the latter (LIF). Second, the LIF process is developed from first principles, and we present the relations between absorption cross section, differential scattering cross sections, and LIF emission spectrum for atoms with hyperfine structure. By doing this, we (1) equate the decoherence rate among the mixed excited substates to half of the Einstein coefficient to comply with energy conservation, and (2) present a clear physical insight into the origin of the Hanle effect, whose quantitative evaluation for these atoms requires quantum mechanical treatment. We then derive LIF emission rates for each of the eight and 10 pathways associated with the D1 and D2 transitions, and deduce the temperature and LOS wind dependence in differential backscattering cross sections of atmospheric sodium and potassium atoms. Using these cross sections allows retrieval of atmospheric parameters in the mesopause region (80–110 km in altitude) by a metal resonance lidar.
The de-excitation or quenching cross section of thallium for the
63P1 state of mercury has been obtained over a range of temperatures of 800 to 900\ifmmode^\circ\else\textdegree\fi{}C. Comparison of the resonance fluorescence from a mercury-thallium vapor mixture with the resonance fluorescence from a pure mercury vapor at the same number density, as determined by line absorption, allows the calculation of the de-excitation cross section. The present data together with Garrett's value of the lifetime of the
63P1 state of mercury and existing data on the vapor pressures of mercury and thallium were used to obtain values of the de-excitation cross section. Values of the cross section range from 108\ifmmode\pm\else\textpm\fi{}41
A˚2 at 800\ifmmode^\circ\else\textdegree\fi{}C to 80\ifmmode\pm\else\textpm\fi{}19
A˚2 at 900\ifmmode^\circ\else\textdegree\fi{}C. The corresponding quenching-rate coefficients varied from (5.1\ifmmode\pm\else\textpm\fi{}1.9)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$
cm3/sec at 800\ifmmode^\circ\else\textdegree\fi{}C to (4.0\ifmmode\pm\else\textpm\fi{}0.9)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$
cm3/sec at 900\ifmmode^\circ\else\textdegree\fi{}C.