Photoassociation of spin polarized Chromium

We report the homonuclear photoassociation (PA) of ultracold ${}^{52}\mathrm{Cr}$ atoms in an optical dipole trap. This constitutes the first measurement of PA in an element with total electron spin $\tilde{S}>1$. Although Cr, with its ${}^{7}\mathrm{S}_{3}$ ground and ${}^{7}\mathrm{P}_{4,3,2}$ excited states, is expected to have a complicated PA spectrum we show that a spin polarized cloud exhibits a remarkably simple PA spectrum when circularly polarized light is applied. Over a scan range of 20 GHz below the ${}^{7}\mathrm{P}_{3}$ asymptote we observe two distinct vibrational series each following a LeRoy-Bernstein law for a $C_3 / R^{3}$ potential with excellent agreement. We determine the $C_3$ coefficients of the Hund's case c) relativistic adiabatic potentials to be -1.83$\pm$0.02 a.u. and -1.46$\pm$0.01a.u.. Theoretical non-rotating Movre-Pichler calculations enable a first assignment of the series to $\Omega=6_u$ and $5_g$ potential energy curves. In a different set of experiments we disturb the selection rules by a transverse magnetic field which leads to additional PA series.