FLUORESCENCE OF COUMARINS AND XANTHENES AFTER 2-PHOTON ABSORPTION WITH A PULSED TITANIUM-SAPPHIRE LASER

Fluorescence emission after two-photon absorption of coumarins and xan thenes in an alcoholic solution was measured in the tuning range of a femtosecond-pulsed titanium-sapphire laser (750-840 nm). Xanthenes, wh ich have a low one-photon absorption in the near UV, show a higher flu orescence signal after two-photon absorption than the UV-excitable cou marins. When fluxes of 10(28) photons/(cm(2) s) are used, the two-phot on absorption cross sections for xanthenes are 1 order of magnitude hi gher than the two-photon absorption cross sections of the coumarins. A bsolute cross sections have been estimated for three coumarins and thr ee xanthenes. For the xanthenes a significant wavelength-dependent dep arture from the expected fluorescence intensity square law was observe d. The coumarins follow the square-law dependence. The consequences of the findings are discussed for analytic and diagnostic methods. An es pecially important result is that the resolution in two-photon microsc opy of xanthenes is worse than expected.