Rotational laser spectroscopy of the $X^1Σ_g^+\rightarrow B^1Π_u$ transition of $\text{Rb}_2$ molecule in a supersonic beam: As good as it gets
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 $X^1Σ_g^+(v''=0)\rightarrow B^1Π_u(v'=1,2)$ and $X^1Σ_g^+(v''=1)\rightarrow B^1Π_u(v'=1)$ bands, with a spectral resolution of $3.3\times10^{-4}$~cm$^{-1}$. Although restricted to the $v'=1$ and $v'=2$ vibrational levels of the excited $B^1Π_u$ state, the measurements extend previous work by Amiot and Vergès [Chemical Physics Letters 274, 91 (1997)] through substantially higher resolution and dense low-$J'$ rotational data for all three isotopologues. A global least-squares analysis combining the new and published data, yields improved Dunham coefficients for the excited $B^1Π_u$ state, significantly refines rotational and rovibrational coupling constants. In addition, the $Λ$-doubling constants of $B^1Π_u$ state were determined for the three isotopologues.