CsAr, CsXe, and RbXe B 2 Σ 1/2 + Interatomic Potentials Determined from Absorption Spectra and Calculations of Franck-Condon Factors for Free-Free Optical Transitions of Atomic Collision Pairs.

Interatomic potentials for the B 2 Σ 1/2 + states of CsAr, CsXe, and RbXe have been determined through comparisons of experimental B ← X absorption spectra for alkali vapor-rare gas mixtures with calculations of the Franck-Condon factors (FCFs) associated with free-free transitions of thermal atomic pairs. Simulations of optical transitions of alkali-rare gas atomic pairs between the thermal and vibrational continua of the X 2 Σ 1/2 + and B 2 Σ 1/2 + states of the molecule, responsible for the blue satellites of the Cs and Rb D 2 resonance lines in a rare gas background, require the incorporation of ground-state J values above ∼400 into the FCF calculations and proper normalization of the free-particle wave functions. Absorption spectra computed on the basis of several X and B state interatomic potentials available in the literature were found to be sensitive to the height of the B 2 Σ 1/2 + state barrier, as well as the X 2 Σ 1/2 + state repulsive wall contour and the location of the van der Waals minimum. Other spectral simulations entailed iterative modifications to a selected B 2 Σ 1/2 + interatomic potential, again coupled with comparison to experimental B ← X spectra. Comparisons of calculated spectra with experiment yield a CsXe B 2 Σ 1/2 + potential, for example, exhibiting a barrier height of 76 cm -1 at 5.2 Å and yet is nearly flat at smaller values of internuclear separation ( R ). The latter contrasts with previous theoretical calculations of V B ( R ) in the vicinity of the barrier maximum. For the CsAr molecule, the B 2 Σ 1/2 + barrier height was found to be 221 cm -1 , which is within 3% of the value determined from pseudopotential calculations incorporating the spin-orbit effect. Reproducing Cs-rare gas experimental absorption spectra also requires the existence of a broad, shallow potential well lying beyond the B 2 Σ 1/2 + barrier that, for CsAr, has a dissociation energy ( D e ∼ 24 cm -1 ) a factor of 3 larger than values predicted by theory. Similar results are obtained for the RbXe and CsXe complexes.