The transition mechanisms actually involved in direct and indirect two-photon absorption (TPA) processes have been investigated in two different materials: ZnO and AgCl. It has been proven that the TPA dominant mechanism changes when the excitation energy is varied. In particular, in both materials and for both direct and indirect processes, transitions are of the allowed-allowed type near the energy gap and become of the allowed-forbidden type far from the gap.
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Catalano et al. (1986) studied this question.
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