We study the porphyrin S1 → S0 fluorescence and the photosensitized singlet oxygen 1Δg → 3∑g phosphorescence, both originating from absorption of photons with energy less than the porphyrin S0 → S1 transition energy. By measuring the excitation intensity dependence of fluorescence at lowered sample temperatures, we are able to discriminate between two parallel processes of one-photon hot-band absorption (HBA) and simultaneous two-photon absorption (TPA). When the HBA and TPA contributions are comparable in magnitude, we use this new method to determine absolute TPA cross-section. We also demonstrate for the first time a singlet oxygen photosensitization via HBA in porphyrin. © 2003 Elsevier Science B.V. All rights reserved.
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