Spectra are organized into three categories: organic dyes, fluorescent proteins, and intrinsic molecules. The data are "action" cross sections, that is the product of the fluorescence quantum yield and the absolute 2P cross section. If one assumes that the single photon quantum yield is the same as the 2P quantum yield (a standard assumption), then the action cross section represents the probability that a two-photon excitation event will lead to the emission of a fluorescence photon. This is useful for estimating the number of photons generated in the focal volume. 2P cross sections are measured in Goeppert-Mayer units (GM), where 1 GM = 10^-50 cm⁴s per photon per molecule. These data were measured by comparison to fluorescein at pH 11, which was our reference standard using data from Xu & Webb (1996 JOSA B 13:481-491).

Comparison to other published data: although the line shapes will be similar, the GM values presented here may differ from other published values. This is due to differences in measurement conditions, methods and cross-section reference standards, and for fluorsecent proteins (FPs), differences in the measured concentration. Not all FPs mature and fold into fluorescent species, so normalization based on protein concentration can be inaccurate. FP cross section data here was normalized to the concentration determined by Fluorescence Correlation Spectroscopy (FCS), which is only measuring proteins that are fluorescent.