The paper reports on a new measurement method of the s-wave scattering length of metastable helium atoms by photoassociation (PA) technique. Lightshifts of the PA resonance lines are measured, which results from the coupling of a bound state in the excited state potential and the continuum of scattering states above the ground state potential. Ultracold molecules are formed by photoassociation from spin-polarized metastable helium atoms, which are trapped in a magnetic trap and cooled down to a temperature of 16 /spl mu/K by evaporative cooling technique. The PA laser is red detuned to the atomic D0 line (2/sup 3/S-2/sup 3/P/sub 0/) and scanned around the bound state levels in the purely long-range 0/sub 0//sup +/ potential to provide precise PA spectra.
We present a new measurement of the
s-wave scattering length
a of spin-polarized helium atoms in the
2 3S1 metastable state. Using two-photon photoassociation spectroscopy and dark resonances, we measure the energy ${E}_{v=14}=\ensuremath{-}91.35\ifmmode\pm\else\textpm\fi{}0.06\text{ }\text{ }\mathrm{MHz}$ of the least-bound state
v=14 in the interaction potential of the two atoms. We deduce a value of $a=7.512\ifmmode\pm\else\textpm\fi{}0.005\text{ }\text{ }\mathrm{nm}$, 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 $1\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$.