The Excitation Spectra of Two-Photon Induced Fluorescence in Xanthene Dyes

Two-photon excitation spectra of xanthene dyes in ethanol were measured by observing fluorescence emission after excitation by light with wavelengths from 780 to 1260 nm. The normalized spectra showed large two-photon induced fluorescence at higher energies and two moderate peaks near the one-photon absorption band. These features agreed well with the two-photon excitation spectra reported by other authors. The peaks near the one-photon absorption band may be due to a two-photon allowed state overlapping the lowest singlet state or to excitation to the lowest singlet state. We believe the latter is the case; while a two-photon excitation is formally symmetry forbidden, both interaction with the counterion in solution as well as vibronic coupling will remove the symmetry, allowing this transition. Comparison of the spectra of dyes with slightly different structures supports this explanation. The input power dependence of fluorescence irradiance was also measured and some deviation from the expected quadratic dependence was observed in Rhodamine 6G and Rhodamine B.