Xanthenes and Other Laser Dyes

In xanthenes, even if all one-photon allowed transitions are also two-photon allowed, the shape of the bands and their relative intensities are very different in the IPA and 2PA spectra [76,78]. This is not the case for other laser dyes and chromophores, for which the two spectra are almost identical (if represented as a function of the total transition energy), showing peaks in the same position and with very similar band shapes. Some example of chromophores in this category are coumarin 307 [78], coumarin 102 [80], 7-hydroxycoumarin [81], lucifer yellow [78], and cascade blue [78]. 

It is generally assumed that the fluorescence emission spectrum of a compound is the same after the compound undergoes IPA or 2PA excitation. This has been confirmed experimentally in a number of cases [74,78], but there 

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