Singlet and triplet potentials of the ground-state atom pair Rb + Cs studied by Fourier-transform spectroscopy

A comprehensive study of the X 1Σ+ and a 3Σ+ electronic states of the Rb and Cs atom pair in RbCs is presented. Abundant spectroscopic data for the 85Rb133Cs and 87Rb133Cs isotopologues were obtained from Fourier-transform spectra of laser-induced fluorescence (LIF) from the B 1Π and (4)1Σ+ states to the a 3Σ+ (4549 transitions) and X 1Σ+ (15 709 transitions) states. The X 1Σ+ state data were complemented by about 15 500 transitions obtained by Fellows et al.[J. Mol. Spectrosc. 197, 19 (1999)]. LIF progressions to the a 3Σ+ state range from va=0 to 37, reaching an outer turning point at 15.97 A. For the X 1Σ+ state, by analyzing LIF from the shelflike (4)1Σ+, the data are extended from vx=119 as observed in the above-cited paper to vx=129, extending the outer turning point from 10.7 to 17.55 A. From these data, potential energy curves were constructed simultaneously for both hyperfine coupled states X 1Σ+ and a 3Σ+ in a coupled-channels fitting routine. This allowed us to accurately model the potential close to the Rb(5s) + Cs(6s) atom-pair asymptote, which is required to simulate cold collision processes and to analyze the Feshbach resonances observed for 87Rb133Cs by Pilch et al.[Phys. Rev. A 79, 042718 (2009)]. Dissociation energies were determined for the X 1Σ+ state as De=3836.373(40) cm-1 and for the a 3Σ+ state as De=259.341(30) cm-1.