In atomic physics, two-photon absorption (TPA or 2PA), also called two-photon excitation or non-linear absorption, is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state (usually the ground state) to a higher energy, most commonly an excited electronic state.
In atomic physics, two-photon absorption (TPA or 2PA) refers to the process where a molecule absorbs two photons simultaneously, either of identical or different frequencies, to transition from its ground state to an excited electronic state.