https://doi.org/10.1351/goldbook.S05857
Same as @R05041@, $$M$$. It is the integration of the @R05046@, $$P$$, leaving a source over the solid @A00346@ and over the whole @W06659@ range.
Source:
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2276 [Terms] [Paper]