Two-photon absorption and excited state absorption properties of an organic dye PSPI

The reason why there is a 90 nm red-shift for the highest upconversion wavelength compared with the strongest nonlinear absorption wavelength is given. A three-level energy model is proposed for an organic dye, trans-4-[p-(pyrrolidinyl)styryl]-N-methylpyridinium iodide (abbreviated as PSPI). It is the excited state absorption (ESA) that leads to the reduction of the upconversion efficient at wavelength shorter than 950 nm. The three-level energy model is proved to be right by using the femtosecond pumped two-photon excited spectrum. The TPA cross-sections and the ESA coefficients at various wavelengths are calculated using the measured nonlinear transmittance and upconverison efficiencies. The largest TPA cross-section sigma(2)' is 14.1 x 10(-48) cm(4) s/photon at 960 nm. The experimental results show that the electrons are excited to the same excited states for both the TPA and one-photon absorption processes. (C) 2004 Elsevier B.V. All rights reserved.