Frequency shifts of photoassociative spectra of ultracold metastable Helium atoms : a new measurement of the s-wave scattering length

Abstract

We observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped ${}^4$He${}^{\ast }$ 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 $2^3{S}_1-2^3{P}_0$ 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 (${}^5\hat{I}{\pounds }_g^{+}$) potential, $a=7.2\pm 0.6$ nm, which is significantly lower than most previous results obtained by non-spectroscopic methods.

7 pages, 4 figures