Depletion spectroscopy of ultracold $\nu_x=0$ $^{85}$Rb$_2$ molecules trapped in a crossed optical dipole trap

In this work, we have loaded $\nu_x=0$ $^{85}$Rb$_2$ ultracold molecules into a crossed optical dipole trap from a standard magneto optical trap using a single light beam. Such beam is composed of a single frequency coherent light source, which is responsible for short range PA of cold rubidium atoms, and an incoherent broadband light source which transfers the molecules in different vibrational levels ($\nu_x$) of the singlet-ground-state X, into $\nu_x=0$, through optical pumping. The molecules were observed, by REMPI technique, through 11 transitions from the $\nu_x=0$ $X^{1}\Sigma_{g}^{+}$ ground state to the $2^{1}\Sigma_{u}^{+}$ excited state in the 20853-20985~cm$^{-1}$ energy range. Due to the bandwidth of the REMPI laser we were unable to resolve the rotational distribution of the $\nu_x=0$. Therefore, we have performed depletion spectroscopy in the $\nu_x=0$ trapped molecules using a diode laser at 682 nm to drive transitions from $v_X=0$ to $v=0$ of the $b^{1}\Pi_{u}$ potential. The pulsed dye laser frequency was set at the largest peak at 20966.9 cm$^{-1}$. The experimental depletion spectrum, which is in good agreement with theoretical predictions, allows us to determine that 75\% the $\nu_x=0$ molecules are in $J$= 0, 1 and 2 rotational states.