2-photon photoassociation of ultra-cold metastable helium

In the vicinity of Bose-Einstein condensation, the collisional properties of a dilute gas of metastable helium He∗ in the state 23 S1 are governed by the rate of ionizing Penning collisions and the s-wave scattering length a. Thus, we measured a by a 2-photon photoassociation experiment, which allows to create a molecule with two metastable atoms He∗ in the least bound state of the interaction potential between two metastable atoms which are spin polarized. The measure of this bound state using the atoms-molecules dark resonance allows to deduce the scattering length a, which is determined with a large accuracy (a = 7.5105 ± 0.0025 nm), and which depends not much on theory : it is a direct measurement. More ones, the lifetime of molecules formed by 2-photon spectroscopy is linked to Penning ionization between the two bound He∗ atoms. Its measurement allows to understand the mechanism of these collisions.