2-PHOTON-INDUCED FLUORESCENCE

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 effe ct of vibrations and substitution, particularly within the framework o f pseudoparity rules. A section is devoted to biological molecules and the emerging use of two-photon-induced fluorescence anisotropy. Relev ant theoretical results are discussed, with emphasis on quantum chemic al predictions of vibronic coupling and substituent effects on two-pho ton absorptivity and tensor properties of individual molecules. This c hapter includes higher-order spectroscopy, and a limited number of thr ee- and four-photon studies are discussed.