CsAr, CsXe, and RbXe B²Σ_1/2⁺ Interatomic Potentials Determined from Absorption Spectra and Calculations of...

Interatomic potentials for the B²Σ_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 X²Σ_1/2⁺ and B²Σ_1/2⁺ states of the molecule, responsible for the blue satellites of the Cs and Rb D₂ 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 B²Σ_1/2⁺ state barrier, as well as the X²Σ_1/2⁺ state repulsive wall contour and the location of the van der Waals minimum. Other spectral simulations entailed iterative modifications to a selected B²Σ_1/2⁺ interatomic potential, again coupled with comparison to experimental B ← X spectra. Comparisons of calculated spectra with experiment yield a CsXe B²Σ_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 B²Σ_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 B²Σ_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.