Early Studies of Two-Photon Absorption in Organic Molecules

Early Studies of Two-Photon Absorption in Organic Molecules 3.1 

However, before the 1990s a significant number of papers on 2PA spectroscopy had already been published. Many of these are worth mentioning, not only because they laid the foundations for the field, but also because they addressed interesting spectroscopic issues for a variety of organic molecules. Indeed, at that time, 2PA was mainly used as a spectroscopic tool, in combination with the more traditional IPA and fluorescence approaches. 

The term 2PA spectrum will be used here and in the rest of the chapter to indicate the variation of the two-photon cross section as a function of wavelength, even when the data were obtained by monitoring the intensity of the fluorescence emission induced by two-photon absorption (that is when, strictly speaking, the two-photon induced fluorescence excitation spectrum was obtained). Also, unless otherwise specified, the spectra are reported as a function of the wavelength of the excitation beam and are degenerate 2PA spectra. 

In addition to providing a brief tutorial on 2PA spectroscopy, some of the examples discussed here can be useful in understanding the origin of the 2P activity and in estimating approximate 2PA cross sections of so-called conventional initiators which have been used by some investigators in microfabrication applications (see Sect. 5). 

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