Giant Helium Dimers Produced by Photoassociation of Ultracold Metastable Atoms

We produce giant, purely long-range helium dimers by photoassociation of metastable helium atoms in a magnetically trapped, ultracold cloud. The photoassociation laser is detuned close to the atomic 2³S₁-2³P₀ line and produces strong heating of the sample when resonant with molecular bound states. The temperature of the cloud serves as an indicator of the molecular spectrum. We report good agreement between our spectroscopic measurements and our calculations of the five bound states belonging to a 0_u⁺ purely long-range potential well. These previously unobserved states have classical inner turning points of about 150a₀ and outer turning points as large as 1150a₀.