Molar Napierian absorption coefficient divided by the Avogadro constant. When integrated against the logarithm of wavenumber (or frequency) it is called
the integrated absorption cross-section.
PAC, 1990, 62, 2167
(Glossary of atmospheric chemistry terms (Recommendations 1990))
on page 2169
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2226
