Study of the Born-Oppenheimer Approximation for Mass-Scaling of Cold Collision Properties

TL;DR

This study investigates the limitations of the Born-Oppenheimer approximation in predicting cold collision properties by analyzing isotopomers of potassium molecules and finding that small mass-dependent corrections significantly impact collision predictions.

Contribution

The paper provides experimental evidence that a mass-dependent correction to the interaction potential is necessary for accurate modeling of molecular levels, highlighting limitations of the Born-Oppenheimer approximation.

Findings

Mass-dependent correction term of 10^{-6} improves potential accuracy.

Correct level structure reproduction requires this correction.

Potential corrections can significantly affect cold collision properties.

Abstract

Asymptotic levels of the A $^1\Sigma_u^+$ state of the two isotopomers $^{39}{\rm K}_2$ and $^{39}{\rm K}^{41}{\rm K}$ up to the dissociation limit are investigated with a Doppler-free high resolution laser-spectroscopic experiment in a molecular beam. The observed level structure can be reproduced correctly only if a mass dependent correction term is introduced for the interaction potential. The applied relative correction in the depth of the potential is $10^{-6}$, which is in the order of magnitude expected for corrections of the Born-Oppenheimer approximation. A similar change in ground state potentials might lead to significant changes of mass-scaled properties describing cold collisions like the s-wave scattering length.