https://doi.org/10.1351/goldbook.A00366
  1. (in radiochemistry) An interaction between a particle and its
    antiparticle
    in which they both disappear.
    Source:
    Orange Book, 2nd ed., p. 211 [Terms] [Book]
  2. (in photochemistry) Two atoms or molecular entities both in an excited
    electronic state
    interact often (usually upon collision) to produce one atom or molecular entity in an excited
    electronic state
    and another in its ground
    electronic state
    . This phenomenon (annihilation) is sometimes referred to as
    energy pooling
    .
    See:
    singlet-singlet annihilation
    ,
    spin conservation rule
    ,
    triplet-triplet annihilation
    Source:
    PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2228 [Terms] [Paper]