We report on spectroscopic measurements of the Rb SD,,, state performed using two-photon excitation with a resonant intermediate state. With the two beams (5S,/, -+ 5P,/, at 780nm and 5P,/, + 5D,/, at 776nm) counterpropagating, the spectra are Doppler-free and the signals are very robust, even in a room temperature vapor. This is due to the near match of the transition wavelengths. The 5D,/, hyperfine constants have been measured and compared to previous determinations. We have also performed the excitation in a sample of laser-cooled atoms and compare these results to those obtained at room temperature. The fact that the 5P,/, + 5D,/, transition is narrow, robust, diode-laser accessible, and relatively immune to AC-Stark shifts, may make it a promising candidate for a frequency reference at 776 nm.

We report a direct measurement of the Na 2 X 1 ⌺ g ϩ ground-state dissociation energy. Three spectroscopic measurements are combined to directly yield the energy of the lowest (vϭ0) vibrational level relative to the free-atom limit. The X 1 ⌺ g ϩ (vϭ0→31) splitting is measured by double-resonance spectroscopy; X 1 ⌺ g ϩ (v ϭ31)→A 1 ⌺ u ϩ (vЈϭ165) by laser-induced fluorescence; and free atoms→A 1 ⌺ u ϩ (vЈϭ165) by photoassociation of ultracold atoms. The result is D 0 ϭ5942.6880(49) cm Ϫ1 . The uncertainty is a factor of 6 improvement over a previous ͑indirect͒ determination. ͓S1050-2947͑96͒50608-6͔

We demonstrate a number of two-colour spectroscopy techniques where the first step is the photoassociation of laser-cooled and trapped sodium atoms to form bound states of Na 2 . High-resolution (∼ 0.001 cm −1 ) spectra of Na 2 are obtained. Spectra of the 0 − g (3S + 3P 3/2 ) 'purely long-range' state demonstrates the use of two-colour spectroscopy to open up an ionization channel for detecting the occurrence of photoassociation. The utility of photoassociation as a technique for producing cold molecular samples in a well defined rovibrational state is demonstrated with spectra showing bound → bound and bound → free (Condon internal diffraction) transitions after the photoassociation. Starting from states in the 0 − g and 1 g (3S + 3P 3/2 ) potentials, these transitions include upward transitions to autoionizing potentials dissociating near 3P + 3P and downward transitions back to the 3S + 3S ground-state potentials. Spectroscopic constants for six levels of a doubly excited 1 u state of Na 2 are given.

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