Fitting line shapes in photoassociation spectroscopy of ultracold atoms: Auseful approximation - art. no. 012501

Line shapes observed in photoassociation spectroscopy of ultracold (T < 1 m K) atoms are asymmetric and the peak of the line is shifted by a temperatur e-dependent amount from the zero-temperature threshold. In principle, calcu lating the line shape requires a full knowledge of both the ground- and exc ited-state potentials. We demonstrate here that an approximation based on t he Wigner threshold law for scattering yields a relatively simple expressio n which allows one to extract line positions and linewidths directly from t he data. The formula contains a few parameters (threshold frequency, linewi dth, temperature, height) and does not require a detailed knowledge of the molecular potentials involved. We illustrate the utility of this approach w ith examples for Na photoassociation at 500 mu K and also discuss the limit ations of the approach.