RESUMO
We present a new measurement of the s-wave scattering length a of spin-polarized helium atoms in the 2(3)S1 metastable state. Using two-photon photoassociation spectroscopy and dark resonances, we measure the energy E(nu)=14= -91.35+/- 0.06 MHz of the least-bound state nu = 14 in the interaction potential of the two atoms. We deduce a value of a=7.512+/-0.005 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 micros.
RESUMO
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 2(3)S1-2(3)P0 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 150a(0) and outer turning points as large as 1150a(0).
RESUMO
We have observed a Bose-Einstein condensate in a dilute gas of 4He in the (3)2S(1) metastable state. We find a critical temperature of (4.7+/-0.5) microK and a typical number of atoms at the threshold of 8 x 10(6). The maximum number of atoms in our condensate is about 5 x 10(5). An approximate value for the scattering length a = (16+/-8) nm is measured. The mean elastic collision rate at threshold is then estimated to be about 2 x 10(4) s(-1), indicating that we are deeply in the hydrodynamic regime. The typical decay time of the condensate is 2 s, which places an upper bound on the rate constants for two-body and three-body inelastic collisions.
