Formation, detection and spectroscopy of ultracold Rb₂ in the ground X¹Σ_g⁺ state

We form ultracold ground-state Rb₂ molecules by photoassociating pairs of atoms in a magneto-optical trap into the 0_u⁺ state, which decays radiatively into high vibrational levels of the X ¹Σ_g⁺ state. Sensitive and vibrationally state-selective detection is achieved by means of resonantly-enhanced two-photon ionization with a pulsed laser. Frequency scans of the detection laser reveal a long vibrational progression to a previously unobserved electronic excited state, which we identify as the 2¹Σ_u⁺ state. Most of its vibrational spectrum is in excellent agreement with predictions based on ab initio potentials, although the lowest vibrational levels exhibit strong perturbative mixing with the triplet 2³Π _u state. The detection method reported here, with minor variations, should be effective for the entire potential well of the X state. In this work no transitions are observed from vibrational levels above v ≤ 118, but this turns out to be a limitation not of the detection method but rather of the photoassociative formation scheme, due to re-excitation of the highest-v levels by the same photoassociation laser that produces them.