Frequency shifts of photoassociative spectra of ultracold metastable helium atoms: A new measurement of the -wave scattering length

Ecole Normale Supérieure and Collège de France, Laboratoire Kastler Brossel 24 rue Lhomond, 75231 Paris Cedex 05, France

Received: 26 July 2005
Accepted: 20 September 2005

Abstract

We observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped atoms in the metastable state. A pair of ultracold spin-polarized helium atoms is excited into a molecular bound state in the purely long-range potential connected to the - asymptote. The shift arises from the optical coupling of the molecular excited bound state with the scattering states and the bound states of two colliding atoms. We measure the frequency-shifts for several ro-vibrational levels in the potential and find a linear dependence on the photoassociation laser intensity. Comparison with a theoretical analysis provides a good indication for the s-wave scattering length a of the quintet () potential, , which is significantly lower than most previous results obtained by non-spectroscopic methods.