Influence of terms that are of
\(2^{\rm{nd}}\) and higher order in the
normal coordinate \(Q\) in the expansion of the electric dipole moment.
See also: electrical harmonicity
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 744 (https://doi.org/10.1515/pac-2019-0203)