Blue-detuned photoassociation in Rb$_2$*

We report on the observation of blue-detuned photoassociation as proposed in [1] and references therein.``Blue-detuned" refers to the location of vibrational levels --- energetically above the corresponding atomic asymptote. $^{85}$Rb atoms in a MOT were photoassociated to levels of the $1\,^3\Pi_g$ state a few hundred wavenumbers above the $5S+5P_{3/2}$ limit. These transitions were found to be strong even though they occur at short internuclear separations ($R_e$=$10\,a_0$). Levels of the $1\,^3\Pi_g$ state were detected by resonantly enhanced multiphoton ionization with time-of-flight spectroscopy (REMPI-TOF). We have observed most vibrational levels of the $1\,^3\Pi_g$ state belonging to all its spin-orbit components ($0_g^+,0_g^-,1_g,2_g$). Some of these levels spontaneously decay preferentially to the $v=0$ level of the $a\,^3\Sigma_u^+$ state (FCF$>0.3$). Therefore in Rb$_2$ it is possible to populate ground ro-vibrational levels of the $a\,^3\Sigma_u^+$ state using just one photoassociation laser.\\[4pt] [1] M.-L. Almazor \emph{et. al.}, Eur. Phys. J. D \textbf{15} 355 (2001).