CsAr, CsXe, and RbXe BΣ 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 BΣ 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 XΣ and BΣ states of the molecule, responsible for the blue satellites of the Cs and Rb D resonance lines in a rare gas background, require the incorporation of ground-state 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 BΣ state barrier, as well as the XΣ state repulsive wall contour and the location of the van der Waals minimum. Other spectral simulations entailed iterative modifications to a selected BΣ interatomic potential, again coupled with comparison to experimental B ← X spectra. Comparisons of calculated spectra with experiment yield a CsXe BΣ potential, for example, exhibiting a barrier height of 76 cm at 5.2 Å and yet is nearly flat at smaller values of internuclear separation (). The latter contrasts with previous theoretical calculations of () in the vicinity of the barrier maximum. For the CsAr molecule, the BΣ barrier height was found to be 221 cm, 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 BΣ barrier that, for CsAr, has a dissociation energy ( ∼ 24 cm) a factor of 3 larger than values predicted by theory. Similar results are obtained for the RbXe and CsXe complexes.