Deviations from Born-Oppenheimer mass scaling in spectroscopy and ultracold molecular physics

TL;DR

This paper examines deviations from the Born-Oppenheimer approximation in molecular spectroscopy and ultracold physics, highlighting the significance of nonadiabatic effects and field shifts in isotope variations of diatomic molecules.

Contribution

It provides a comparative analysis of ab initio and spectroscopic BOB functions, and introduces a semiclassical model for BOB effects on ultracold molecular states.

Findings

Mass shifts dominate for light nuclei up to potassium.

Field shifts are significant for rubidium and strontium, dominant for ytterbium.

Discrepancies between ab initio and empirical BOB functions suggest nonadiabatic contributions.

Abstract

We investigate Born-Oppenheimer breakdown (BOB) effects (beyond the usual mass scaling) for the electronic ground states of a series of homonuclear and heteronuclear alkali-metal diatoms, together with the Sr$_2$ and Yb$_2$ diatomics. Several widely available electronic structure software packages are used to calculate the leading contributions to the total isotope shift for commonly occurring isotopologs of each species. Computed quantities include diagonal Born-Oppenheimer corrections (mass shifts) and isotopic field shifts. Mass shifts dominate for light nuclei up to and including K, but field shifts contribute significantly for Rb and Sr and are dominant for Yb. We compare the {\em ab initio} mass-shift functions for Li$_2$, LiK and LiRb with spectroscopically derived ground-state BOB functions from the literature. We find good agreement in the values of the functions for LiK and LiRb at their equilibrium geometries, but significant disagreement with the shapes of the functions for all 3 systems. The differences may be due to contributions of nonadiabatic terms to the empirical BOB functions. We present a semiclassical model for the effect of BOB corrections on the binding energies of near-threshold states and the positions of zero-energy Feshbach resonances.