Frequency shifts of photoassociative spectra of ultracold metastable helium atoms: A new measurement of the

We observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped $\chem{^4{He}^*}$ atoms in the metastable $2^3{S}_1$ state. A pair of ultracold spin-polarized $2^3{S}_1$ helium atoms is excited into a molecular bound state in the purely long-range $0_{u}^+$ potential connected to the $2^3{S}_1$-$2^3P_0$ 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 $2^3{S}_1$ atoms. We measure the frequency-shifts for several ro-vibrational levels in the $0^+_u$ 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 ($^5\Sigma_g^+$) potential, $a=7.2\pm 0.6\un{nm}$, which is significantly lower than most previous results obtained by non-spectroscopic methods