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8 May 1997 Nonlinear decomposition and two-photon fluorescence of molecules in sensitized tissues
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Generation of reactive photoproducts from the chromophores and destruction of biomolecules can be realized with high efficiency if the irradiation of the solutionis carried out by an intensive laser radiation, when two-step light absorption and photoreactions from highly excited electronic states of molecules are taken place. We have observed nonlinear decomposition of sensitizers in tissues upon picosecond laser radiation by fluorescence method. The subsequent change of the autofluorescence spectra of sensitized tissues has ben observed. Fluorescence spectra of tissues sensitized by tumor-localizing sensitizers hematoporphyrin derivative (HpD) chlorin e6 was registered using fiber-optic laser spectrofluorometer. It has been shown that initial rate of photobleaching is depended quadratically on the irradiation intensity. Two- photon excited fluorescence from tissues, sensitized by chlorin e(subscript 6$., and HpD has ben registered during excitation by picosecond YAG:Nd laser. These spectra coincided with that at one photon excitation, however, the background autofluorescence was absent. Red and near infrared radiation allows to increase the light penetration depth and as a result one can enhance the sensitivity of fluorescence diagnosis and the efficiency of photochemical treatment of tumors.
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Vladimir A. Hovanessian and Asatur A. Lalayan "Nonlinear decomposition and two-photon fluorescence of molecules in sensitized tissues", Proc. SPIE 2972, Optical Methods for Tumor Treatment and Detection: Mechanisms and Techniques in Photodynamic Therapy VI, (8 May 1997);

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