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

– We observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped He atoms in the metastable 2S1 state. A pair of ultracold spinpolarized 2S1 helium atoms is excited into a molecular bound state in the purely long range 0+u potential connected to the 2 S1−2 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 2S1 atoms. A linear dependence of frequency-shifts is found as a function of the PA laser intensity and measured for several ro-vibrational levels in the 0+u potential. Comparison with a theoretical analysis provides a good indication for the s-wave scattering length a of the quintet (5Σ+g ) potential, 6.6 nm < a < 7.7 nm, which is significantly lower than 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) 2S1 state. Since the laser cooling technique was developed and provided sub-millikelvin ultracold atomic samples, the photoassociation of such atoms has been of much interest and provided detailed information on the inter-atomic interaction or collisional properties [1]. A detailed study of the PA spectra has been performed for several ultracold thermal gases [2–6] or in condensates [7, 8]. The measurement of the PA spectra also enables very precise measurement of the s-wave scattering length [4, 9, 10], a crucial parameter for understanding the collisional properties of the ultracold atoms or molecules and the dynamical behavior of the condensate. One can even change the scattering length through optical Feshbach resonance [11–13], which is very useful as it can be applicable to any species of atoms in either optical or magnetic trap. One of the interesting phenomena in PA is the light induced frequency shift of the PA spectra. The laser light couples the excited molecular bound state with continuum of scattering states and ground molecular bound states, which results in the shift of the PA resonance curve when the laser frequency is swept over the resonance. As demonstrated in ref. [8,14] for Na and ref. [15,16] for Li, the light shift of the PA spectra is clearly visible at moderate laser intensity