Rotational laser spectroscopy of the $X^1{\Sigma }_g^{+}\to B^1{\Pi }_u$ transition of ${\text{Rb}}_2$ molecule in a supersonic beam: As good as it gets

Abstract

High-resolution laser spectroscopy of the ${}^{85}{\mathrm{Rb}}_2$, ${}^{85}{\mathrm{Rb}}^{87}\mathrm{Rb}$, and ${}^{87}{\mathrm{Rb}}_2$ isotopologues has been performed in a supersonic molecular beam using a continuous-wave (cw) tunable diode laser. A total of 958 rovibronic transitions were recorded up to $7{\mathrm{cm}}^{-1}$ below the vibrational band heads of the $X^1{\Sigma }_g^{+}(v^{\prime \prime }=0)\to B^1{\Pi }_u(v^{\prime }=1,2)$ and $X^1{\Sigma }_g^{+}(v^{\prime \prime }=1)\to B^1{\Pi }_u(v^{\prime }=1)$ bands, with a spectral resolution of $3.3\times 10^{-4}$~cm${}^{-1}$. Although restricted to the $v^{\prime }=1$ and $v^{\prime }=2$ vibrational levels of the excited $B^1{\Pi }_u$ state, the measurements extend previous work by Amiot and Vergès [Chemical Physics Letters 274, 91 (1997)] through substantially higher resolution and dense low-$J^{\prime }$ rotational data for all three isotopologues. A global least-squares analysis combining the new and published data, yields improved Dunham coefficients for the excited $B^1{\Pi }_u$ state, significantly refines rotational and rovibrational coupling constants. In addition, the $\Lambda$-doubling constants of $B^1{\Pi }_u$ state were determined for the three isotopologues.