State-Selective Detection of Ultracold High-$v$ Ground-State Rb$_2$, and Spectroscopy of the (2)$\,\,^1\Sigma_u^+$ State

We have used single-color photoassociation (PA), followed by radiative stabilization, to form ultracold Rb$_2$ molecules in either the metastable triplet state, $a\,\,^3\Sigma_u^+$, or the true ground state, $X\,\,^1\Sigma_g^+$. Individual near-dissociation levels of the $X\,\,^1\Sigma _g^+$ state from $v''$=111 to $v''$=117 have been detected by resonant multiphoton ionization (REMPI) through the (2)$\,\,^1\Sigma_u^+$ state. The (2)$\,\,^1\Sigma_u^+$ state, which we have observed for the first time, has been studied over the wavelength range from 585 nm to 609 nm. The observed vibrational levels are determined to range from $v'$=26 to $v'$=60. Similarly, we have formed $a\,\,^3\Sigma_u^+$ molecules by PA and detected them via REMPI through the (1) $^3\Pi_g$ state. This work was supported by the National Science Foundation.