Experimental evidence for an isotopic effect in the formation of ultracold ground-state rubidium dimers

We perform a comparative study of the formation of ultracold rubidium dimers in their ground state with both 85Rb and 87Rb isotopes. Ultracold rubidium molecules are created through photoassociation of ultracold atoms into rovibrational levels of the 0+u(5S1/2 + 5P1/2) excited state, which interact through resonant coupling (Dion et al 2001 Phys. Rev. Lett. 86 2253) with rovibrational levels of the 0+u(5S1/2 + 5P3/2) excited state. The photoassociated levels decay by spontaneous emission down to rovibrational levels of the X1Σ+g ground state. The resonant coupling is expected to enhance the formation of ultracold molecules in low-lying ground-state levels. By modelling the ionization spectrum of these ultracold molecules, we show that this coupling is larger for 87Rb2 molecules than for 85Rb2 molecules.