Excitation resulting from successive or simultaneous absorption of two photons by
an atom or molecular entity. This term is used for successive absorption only if some
of the
excitation energy of the first photon remains in the atom or molecular entity before absorption of
the second photon. The simultaneous two-photon absorption can also be called
biphotonic excitation.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2282