Europhys. Lett.
Volume 72, Number 4, November 2005
Page(s) 548 - 554
Section Atomic and molecular physics
Published online 14 October 2005
Europhys. Lett., 72 (4), pp. 548-554 (2005)
DOI: 10.1209/epl/i2005-10275-y

Frequency shifts of photoassociative spectra of ultracold metastable helium atoms: A new measurement of the $\mth{s}$-wave scattering length

J. Kim, S. Moal, M. Portier, J. Dugué, M. Leduc and C. Cohen-Tannoudji

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

received 26 July 2005; accepted in final form 20 September 2005
published online 14 October 2005

We observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped $\chem{^4{He}^*}$ atoms in the metastable 23S1 state. A pair of ultracold spin-polarized 23S1 helium atoms is excited into a molecular bound state in the purely long-range 0u+ potential connected to the 23S1-23P0 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 23S1 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.

32.80.Pj - Optical cooling of atoms; trapping.
33.20.Ea - Infrared spectra.
34.20.Cf - Interatomic potentials and forces.

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