Absolute numbering of asymptotic vibrational levels of diatomic molecules from cold-physics experiments

We present a simple method for determination of absolute vibrational numbering of isolated near dissociation levels in diatomic molecules, usually observed in cold-physics experiments. The method is based on the isotope shift and works even when energies of only two levels from one isotopologue and one level from another isotopologue have been measured. It is demonstrated with data from recently reported precise measurements of binding energies of levels lying close to the dissociation limits in ultracold Yb2, CsYb, RbSr, and RbYb molecules. Its predictions agree with those of much more elaborate multi-isotope potential curve fitting.