Study of the Born-Oppenheimer approximation for massscaling of cold collision properties

Ultracold collisions of atoms are often described by the s-wave scattering length. Also, positions and widths of Feshbach resonances characterize these collisions. Full information about the scattering wavefunctions is given by interaction potential curves, which can be determined by molecular spectroscopy. Adding neutrons to one of the nuclei does not alter the potential curve (Born-Oppenheimer approximation, BOA). It is possible to derive eigenvalues, Feshbach resonances, and the scattering length for different isotope combinations from a single potential simply by changing the reduced mass if the BOA is fulfilled. We study the validity of the BOA for asymptotic levels of the potential curve describing the A 1Σ+u state of K2 (correlated to the 4s1/2+4p1/2 asymptote) by comparing spectra of the two isotopomers 39K2 and 39K41K obtained by high resolution laser spectroscopy in a molecular beam