Intensity rovibronic transitions

The ability to detect discrete rovibronic spectral features attributed to transitions of two distinct conformers of the ground-state Rg XY complexes and to monitor changing populations as the expansion conditions are manipulated offered an opportunity to evaluate the concept of a thermodynamic equilibrium between the conformers within a supersonic expansion. Since continued changes in the relative intensities of the T-shaped and linear features was observed up to at least Z = 41 [41], the populations of the conformers of the He - lCl and He Br2 complexes are not kinetically trapped within a narrow region close to the nozzle orifice. We implemented a simple thermodynamic model that uses the ratios of the peak intensities of the conformer bands with changing temperature in the expansion to obtain experimental estimates of the relative binding energies of these complexes [39, 41]. 

As detailed in Section 2, we have derived and programmed the expression for line strengths of individual rotation-vibration transitions of XY3 molecules the line strengths depend on the vibronic transition moments entering into equation (70). With the theory of Section 2, we can simulate rotation-vibration absorption spectra of XY3 molecules. In computing the transition wavenumbers, line strengths, and intensities we use rovibronic wavefunctions generated as described in Ref. [1]. 

At sufficiently high pump intensity, population inversion may be achieved between the level t>o in Si and higher rovibronic levels Vk in So, which have a negligible population at room temperature, due to the small Boltzmann factor exp[-E(vk)/kT]. As soon as the gain on the transition uo(Si) -> Vk(So) exceeds the total losses, laser oscillation starts. The lower level U)t(So), which now becomes populated by stimulated emission, is depleted very rapidly by collisions with the solvent molecules. The whole pumping cycle can therefore be described by a four-level system. 

Relative intensities of the different rotational lines in a rovibrational or rovibronic spectrum are determined only partly by the transition moment integral, which is most sensitive to the value of / when / is small. The variation in intensities more strongly reflects the number of molecules in each rotational level, which depends on the degeneracy of the level, the symmetry of the molecule, and the temperature. The temperature-dependence of the relative intensities is no longer a function of the properties of individual molecules, however, and so it is discussed instead in Thermodynamics, Kinetics, and Statistical Mechanics, Section 6.3. 

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