Spectroscopy of Na₂ by photoassociation of laser cooled Na

Recently, several groups, have used photoassociation of ultracold atoms to study transitions from free atoms to bound molecules. Photoassociative spectroscopy involves illuminating a collection of laser cooled and trapped atoms with a tunable probe laser. When the probe laser is resonant with transition to an excited molecular state, molecules are formed and detected either by ionizing the molecules or by monitoring changes in the number of trapped atoms. We use a 'dark spot' magneto-optical trap which stores sodium atoms at approx..6 mK and approx.10¹¹ cm^-3 predominantly in the F = 1 ground state. We study photoassociation of sodium by observing both ionization (see Fig. 1) and trap loss (see Fig. 2). Figure 1 shows a vibrational series (1 g dissociating at S + P_3/2) which spans 170 cm^-1. The features in the trap loss signal that do not appear in the ion signal (see Fig. 2) are due to the 'pure long range' (0 g^-) state which has an inner turning point of 55 a₀, and thus, poor overlap with the ion potential at the wavelength of interest. These data represent the first spectra of this state; the analogous state of rubidium has been studied recently by a similar technique. In addition to the spectroscopy of the excited molecular state, we obtain information about the ground state wave functions. In particular, the relative peak heights in our spectra are determined by the Franck-Condon overlap of the ground state and excited state wave functions. With theoretical input, we may determine the s-wave scattering phase shift and scattering length. This is a crucial parameter for the experimental realization of Bose-Einstein condensation, which is currently not well characterized; even its sign is unknown.