To understand ultracold collisions between neutral atoms it is necessary to have accurate knowledge of the interatomic interaction potentials at extremely long range, out to many times the atomic radius. One approach is to study diatomic molecules in highly excited vibrational states, since there is a high probability of finding the atoms at fairly large distances from one another as they reach their maximum excursion. However, state-of-the-art techniques of laser spectroscopy must be used to excite and measure these states.

In the July 11th issue of *Physical Review A*, Stephen Falke and coworkers at the Leibniz Universität Hannover and Christian Lisdat at the Physikalisch-Technische Bundesanstalt have measured, with MHz precision, the energy gaps between several asymptotically high vibrational levels in the electronic ground state of the ${}^{39}{\text{K}}_{2}$ molecule. By combining this data with earlier measurements, the authors can fit new ground-state potential functions and calculate accurate scattering lengths for each of the isotopic combinations of ${}^{39}\text{K}$, ${}^{40}\text{K}$, and ${}^{41}\text{K}$ atoms. Their findings are sufficiently accurate that they can observe small deviations from the Born-Oppenheimer (adiabatic) approximation that is conventionally used to describe neutral atom collisions.

The exquisitely detailed experimental and analytical work illustrates the influence of advances in atomic and molecular physics that have been highlighted in several recent Nobel Prizes, including femtosecond frequency combs, iodine-stabilized lasers, and quantum degenerate gases. It will also contribute to continuing progress in molecular Bose-Einstein condensates and degenerate Fermi gases, which is opening new vistas in molecular and chemical physics. - *Keith MacAdam*