David B. Beard and George B. Beard 1970
Quantum Mechanics with Application
pps 214-217 states "it is reasonable ...proton masses >>> mass of the lone electron left." paraphrased.
But what of diatomic ions with more massive nuclei ? I've 2 references with potential energy diagrams (simplified in the case for Oxygen and its several isotopes) and another with Nitrogen.
I am not sure if these are theoretical or experimental determined in any case no mention is made of the "adibatic aproximation".
Thanks, Carl P. Hennessey