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Item Quantitative multiphoton absorption spectroscopy(Montana State University - Bozeman, College of Letters & Science, 2018) Mikhaylov, Alexander Evgen'evich; Chairperson, Graduate Committee: Aleksander RebaneMultiphoton absorption (MPA) is an intriguing photophysical process that has been found useful for diverse areas of science. Low probabilities of MPA processes in molecular systems require to use intense photon fluxes that can be generated by femtosecond laser sources. Performing MPA measurements with sufficient precision requires a detailed characterization of the photon flux values that poses many technical difficulties. However, if the MPA values are determined accurately, then a quantitative analysis of the data obtained can be used to derive information about many important molecular parameters. As an example of MPA spectroscopy we discuss here two-photon absorption (2PA) methods. 2PA spectroscopy is applied to elucidate the structure-property relationships in different types of molecular systems including Pt-based chromophores, pyrrolo-pyrol complexes, organometallic ferrocene compounds, porphyrin oligomers and DNA base fluorescent analogs. Such spectroscopic analysis allows to shed some light on questions of inversion symmetry breaking effects and charge transfer on molecular scales. The measured 2PA spectra are also used to test several approximations for 2PA cross section expressions by comparing the values of molecular parameters derived from the measured 2PA data with other methods and literature data. We employ different experimental methods including two-photon excited fluorescence and nonlinear transmittance to measure 2PA spectra in a broad range of excitation wavelengths. The success achieved with 2PA based methods stimulates interest in three-photon absorption (3PA) measurements. We report on a progress achieved in 3PA measurements using three-photon excited fluorescence. This method is used to measure 3PA spectra of common fluorescent standards including Fluorescein and Rhodamine 6G and other organic dyes in different solvents. Effects of third harmonic generation and solvents absorption are discussed concerning the reliability of the 3PA measurements.