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 absorptio
n band may be due to a two-photon allowed state overlapping the lowest sing
let state or to excitation to the lowest singlet state. We believe the latt
er is the case; while a two-photon excitation is formally symmetry forbidde
n, both interaction with the counterion in solution as well as vibronic cou
pling will remove the symmetry, allowing this transition. Comparison of the
spectra of dyes with slightly different Structures supports this explanati
on. 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.