We have observed the spectrum of long range molecular states of singly excited ${\mathrm{Na}}_2$ formed by photoassociation during ultracold associative ionization collisions of laser-cooled Na atoms. The low kinetic energy of the collision allows us to obtain very high resolution spectra and to observe states close to the dissociation limit. The spectrum shows regular vibrational series characteristic of the 1/${\mathit{R}}^3$ long range portion of the $1_{\mathit{g}}$ potential (asymptotically ${\mathit{S}}_{1/2}$+${\mathit{P}}_{3/2}$) as well as molecular hyperfine structure within these peaks. We extract potential parameters and compare with recent theory.

• Received 7 June 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.2200