We calculate the rates of formation and detection of ultracold Cs2 molecules obtained from the photoassociation of ultracold atoms through the double-well 0g− (6S1/2 + 6P3/2) state. We concentrate on two features previously observed experimentally and attributed to tunnelling between the two wells (Vatasescu et al 2000 Phys. Rev. A 61 044701). We show that the molecules obtained are in strongly bound levels (v'' = 5,6) of the metastable a3 Sigma u+ (6S1/2 + 6S1/2) ground electronic state.