https://doi.org/10.1351/goldbook.D01610
The first (second, ...) derivative [ , ... ] of the as a function of wavenumber, . Wavelengths may be used in place of wavenumbers but the shape of the derivative spectra will be slightly different. When derivative spectra are obtained at low temperature, they are called first (second, ...) derivative
absorption spectrum
of a molecule is defined as the first (second, ...) derivative, absorbance
low temperature absorption spectra
(specifying the solvent, temperature and solute concentration).