State-selective detection by two-photon ionization and magnetic trapping of ultracold Rb$_{2}$ triplet state molecules

We have produced and detected ultracold $^{85}$Rb$_{2}$ in high vibrational levels of the lowest triplet state, $a{ }^3\Sigma { }_u^+ $, by one-color resonance-enhanced two-photon ionization through the $2{ }^3\Sigma _g^+ $state, in the transition energy range of 14000-17000 cm$^{-1}$. The cold molecules are formed by photoassociation followed by radiative decay into the $a{ }^3\Sigma { }_u^+ $state. Many levels corresponding to the$2{ }^3\Sigma _g^+ $,$2{ }^3\Pi { }_g^ $,$1{ }^3\Delta _g $, and $3{ }^1\Sigma { }_g^+ $ states have been observed for the first time, and the vibrational levels of the $a{ }^3\Sigma { }_u^+ $state have been assigned. Experimental spectroscopy agrees well with a new theoretical analysis. In particular, the measured vibrational spacings correspond very well with those calculated from the potential curves of the $a{ }^3\Sigma { }_u^+ $state and the $2{ }^3\Sigma _g^+ $state. The relative vibrational state populations are also consistent with the Franck-Condon factors. Additionally, we present evidence for the trapping of triplet $^{85}$Rb$_{2}$ molecules by the inhomogeneous magnetic field of our MOT. This work is supported by NSF.