Photoassociation of cold Ca atoms

Photoassociative spectroscopy using laser cooled atoms proved to be a powerful tool for the determination of atomic properties such as excited state lifetimes and ground state scattering lengths. The most abundant isotopes of alkaline earths show a non-degenerate ground state with no hyperfine splitting, which is expected to simplify their photoassociation spectra to a great extent. This advantage in the theoretical description is paid for by difficulties in the experimental observation of photoassociation. The ensembles of laser cooled alkaline earths are less cold and less dense than those of e.g. alkalines which were used in most of the photoassociation experiments until now. Photoassociation of Ca is achieved by exciting atomic pairs to the B/sup 1//spl Sigma//sub n//sup +/ excited state of Ca corresponding to an atomic pair 4s/sup 2/ /sup 1/S/sub 0/+4s4p /sup 1/P/sub 1/ at large nuclear distances R. The dimer is bound by the resonant dipole interaction. Due to acceleration in this R/sup -3/-potential, atoms decaying at a smaller nuclear distance usually have sufficient kinetic energy to leave the magneto-optical trap. By measuring the trap loss dependent on the frequency of a photoassociation laser red-detuned to the /sup 1/S/sub 0/-/sup 1/P/sub 1/ transition we resolved the vibrational and rotational structure of the molecule. From the positions of the observed vibrational levels, corrected for the partially unresolved rotational structure and the thermal energy distribution of the ground state atoms, the atomic radiation decay rate /spl gamma/=2.180(11)/spl middot/10/sup 8/ s/sup -1/ of level 4s4p /sup 1/P/sub 1/ is derived with an uncertainty reduced by nearly one order of magnitude compared to previous measurements.