Formation, detection and spectroscopy of ultracold Rb2 in the ground X1Sigmag+ state

We form ultracold ground-state Rb2 molecules by photoassociating pairs of atoms in a magneto-optical trap into the 0u+ state, which decays radiatively into high vibrational levels of the X1Sigmag+ 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 21Sigmau+ 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 23Piu 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.