Laser-assisted binding of ultracold polar molecules with Rydberg atoms in the van der Waals regime

We study ultracold long-range collisions of heteronuclear alkali-metal dimers with a reservoir gas of alkali-metal Rydberg atoms in a two-photon laser excitation scheme. In a low density regime where molecules remain outside the Rydberg orbits of the reservoir atoms, we show that the two-photon photoassociation (PA) of the atom-molecule pair into a long-range bound trimer state is efficient over a broad range of atomic Rydberg channels. As a case study, we obtain the PA lineshapes for the formation of trimers composed of KRb molecules in the rovibrational ground state and excited Rb atoms in the asymptotic Rydberg levels n²S_j and n²D_j, for n=20-80. We predict atom-molecule binding energies in the range 10-10³ kHz for the first vibrational state below threshold. The average trimer formation rate is order 10⁸ {rm s}^-1 at 1.0 μK, and depends weakly on the principal quantum number n. Our results set the foundations for a broader understanding of exotic long range collisions of dilute molecules in ultracold atomic Rydberg reservoirs.