Photoassociation in an ultracold gas of metastable Helium
Prof. Michèle Leduc (Laboratoire Kastler Brossel, Ecole Normale Supérieure, Paris)

Helium 4 atoms in the metastable 2 ³S₁ state (He*) carry an internal energy of 20 eV, which is responsible for efficient ionizing Penning collisions. However, if the atomic gas is spin-polarized, one observes a strong inhibition of the Penning collisions. That is why a spin-polarized gas of He* could undergo Bose Einstein Condensation after evaporative cooling in a magnetic trap. We will present Photoassociation (PA) experiments performed in this ultra-cold gas at temperatures in the μK range, close to the BEC transition. We first produced giant helium dimers by exciting molecular bound sates in a purely long-range molecular potential, with a PA laser beam red detuned from the 2 ³S₁- 2 ³P₀ line at 1083 nm. For these previously unobserved states, the classical inner turning points are so large that the molecule destruction by ionisation is strongly suppressed. Thanks to an original ´calorimetric´ detection, the molecular ro-vibrational spectrum of these dimers is measured with a great accuracy and gives an excellent agreement with theoretical prediction incorporating the dipole-dipole interaction with retardation effects. Recent PA experiments will also be presented, in which frequency shifts of the molecular spectra can be interpreted in term of the scattering length, a parameter which rules the dynamical behaviour of the condensate and can be modified by light.