Nonresonant two-photon electronic spectroscopy of polyatomic molecules is reviewed for the period since 1979. Emphasis is placed on studies that expose patterns in the two-photon fluorescence (also ionization, optoacoustic) excitation spectra of aromatic hydrocarbons and the effect of vibrations and substitution, particularly within the framework of pseudoparity rules. A section is devoted to biological molecules and the emerging use of two-photon-induced fluorescence anisotropy. Relevant

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... tical results are discussed, with emphasis on quantum chemical predictions of vibronic coupling and substituent effects on two-photon absorptivity and tensor properties of individual molecules. This chapter includes higher-order spectroscopy, and a limited number of three-and four-photon studies are discussed. CALLIS (TPE) is now used in a wide variety of studies, including rotationally resolved spectra, time-resolved anisotropy, fluorescence microscopy, photochemistry, chemical analysis, chromatographic detection, and in vivo sampling, to name a few. Friedrich (1) and Birge (2, 3) have published introductory expositions of TPE, and Lin et al (4) have completed a comprehensive treatment of its theory and techniques. This review is concerned with two-photon (and some three-and four-photon) electronic state spectroscopies, wherein neither of the photons alone is resonant with an energy eigenstate of the molecule. Its primary focus is experimental fluorescence excitation spectra, but it also includes discussion of important studies that employ ionization, acoustic, thermal lensing, and photochemical detection. While some effort is made to recognize useful developments in experimental and theoretical techniques, emphasis is on innovations and insightful applications to the understanding of larger molecules, including those encountered in biophysical studies, occurring since the review by Friedrich & McClain (5) in 1980. This of course excludes a massive amount of work in the area of diatomic molecules as well as atomic and solid state physics, and the usual apology is extended for other omissions, whether due to painful choice or to an imperfect search. Regardless of the type of molecule and means of detection, one of the most useful tools associated with multiphoton excitation has continued to be a type of polarization ratio, , defined as the ratio of absorptivities using circularly and linearly polarized excitation light (6):