Simultaneous (coherent)
absorption of two
photons (either same or different
wavelength), the energy of excitation being the sum of the energies of the two photons. Also called
two-photon excitation.
Note:
This term is sometimes also used for a two-step absorption.
Source:
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 308