We present a new measurement of the s-wave scattering length a of spin-polarized helium atoms in the 2(3)S1 metastable state. Using two-photon photoassociation spectroscopy and dark resonances, we measure the energy E(nu)=14= -91.35+/- 0.06 MHz of the least-bound state nu = 14 in the interaction potential of the two atoms. We deduce a value of a=7.512+/-0.005 nm, which is at least 100 times more precise than the best previous determinations and is in disagreement with some of them. This experiment also demonstrates the possibility to create exotic molecules binding two metastable atoms with a lifetime of the order of 1 micros.

Abstract. -We observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped 4 He * atoms in the metastable 2 3 S1 state. A pair of ultracold spinpolarized 2 3 S1 helium atoms is excited into a molecular bound state in the purely long range 0 + u potential connected to the 2 3 S1 −2 3 P0 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 S1 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 Σ + g ) potential, a = 7.2 ± 0.6 nm, which is significantly lower than most previous results obtained by non-spectroscopic methods.Introduction. -The present article describes a photoassociation (PA) experiment with ultracold helium atoms in the metastable (τ ∼ 8000 s) 2 3 S 1 state. Since the development of laser cooling techniques which provide sub-millikelvin ultracold atomic samples, the photoassociation of such atoms has been of much interest and provides detailed information on the inter-atomic interaction and collisional properties [1]. The measurement of the PA spectra allows very precise measurement of the s-wave scattering length [2-5], a crucial parameter for understanding the collisional properties of the ultracold atoms or molecules and the dynamic behavior of the condensate.One of the interesting phenomena in PA is the light induced frequency shift of the PA spectra. As demonstrated in Ref. [6,7] for Na and Ref. [8,9] for Li, the light shift of the PA spectra is clearly visible at moderate laser intensity for the ultracold samples. The observed shifts are described by a theoretical calculation based on the theory developed by Bohn and Julienne [10] or Simoni et al. [11]. The laser light couples the excited molecular bound state with the continuum of scattering states and the ground molecular bound states, which results in the shift of the PA resonance curve when the laser frequency is swept over the resonance. The dependance of the shift on a has two origins. First, in the limit of large and positive scattering length a, the energy of the least bound-state E LBS in the ground state potential

We present an exhaustive analysis of the light-induced frequency shifts of the photoassociation lines of ultracold metastable 4 He * atoms in a magnetic trap. The measurements of the shifts of several vibrational levels bound in the purely longrange J = 1, 0 + u potential linked to the 2 3 S 1 − 2 3 P 0 asymptote were reported in a previous paper [1], and are analyzed here. The simplicity of this system makes it very appropriate for a detailed study. Indeed, the purely long-range character of the excited potential allows one to calculate exact excited molecular wavefunctions and to use asymptotic expansions at large internuclear distances of the ground state wavefunctions appearing in Franck-Condon type integrals. Moreover, the number of collisional channels to be considered is strongly reduced by the absence of hyperfine structure for 4 He * and the use of polarized ultracold atoms and polarized light. This allows us to derive semi-analytical expressions for the shifts showing explicitly their linear dependences on the s-wave scattering length a of spin polarized metastable 4 He * atoms. This explains how it is possible to derive the measurement of a from these shifts.