A comprehensive study of the $X\hspace{0.5em}{}^1\Sigma {}^{+}$ and $a\hspace{0.5em}{}^3\Sigma {}^{+}$ electronic states of the Rb and Cs atom pair in RbCs is presented. Abundant spectroscopic data for the ${}^{85}\mathrm{Rb}$${}^{133}\mathrm{Cs}$ and ${}^{87}\mathrm{Rb}$${}^{133}\mathrm{Cs}$ isotopologues were obtained from Fourier-transform spectra of laser-induced fluorescence (LIF) from the $B\hspace{0.5em}{}^1\Pi$ and $(4){}^1\Sigma {}^{+}$ states to the $a\hspace{0.5em}{}^3\Sigma {}^{+}$ (4549 transitions) and $X\hspace{0.5em}{}^1\Sigma {}^{+}$ (15 709 transitions) states. The $X\hspace{0.5em}{}^1\Sigma {}^{+}$ state data were complemented by about 15 500 transitions obtained by Fellows et al. [J. Mol. Spectrosc. 197, 19 (1999)]. LIF progressions to the $a\hspace{0.5em}{}^3\Sigma {}^{+}$ state range from $v_a=0$ to 37, reaching an outer turning point at 15.97 Å. For the $X\hspace{0.5em}{}^1\Sigma {}^{+}$ state, by analyzing LIF from the shelflike $(4){}^1\Sigma {}^{+}$, the data are extended from $v_x=119$ as observed in the above-cited paper to $v_x=129$, extending the outer turning point from 10.7 to 17.55 Å. From these data, potential energy curves were constructed simultaneously for both hyperfine coupled states $X\hspace{0.5em}{}^1\Sigma {}^{+}$ and $a\hspace{0.5em}{}^3\Sigma {}^{+}$ in a coupled-channels fitting routine. This allowed us to accurately model the potential close to the Rb(5$s$) $+$ Cs(6$s$) atom-pair asymptote, which is required to simulate cold collision processes and to analyze the Feshbach resonances observed for ${}^{87}\mathrm{Rb}$${}^{133}\mathrm{Cs}$ by Pilch et al. [Phys. Rev. A 79, 042718 (2009)]. Dissociation energies were determined for the $X\hspace{0.5em}{}^1\Sigma {}^{+}$ state as $D_e=3836.373(40)$ cm${}^{-1}$ and for the $a\hspace{0.5em}{}^3\Sigma {}^{+}$ state as $D_e=259.341(30)$ cm${}^{-1}$.

• Received 16 March 2011

DOI:https://doi.org/10.1103/PhysRevA.83.052519