Observation of the direct S-2 -> S-0 two-photo the hyperpolarizability of crystal violet (CV) scattering measurement

We present the observation of the direct S-2 --> S-0 two-photon fluorescence (TPF) of crystal violet (CV) in methanol solution. The S-2 --> S-0 emission spectra of CV with excitation wavelengths from 730 to 900 nm are obtained for the first time. These spectra present three clear features: the broad S-2 --> S-0 TPF (370-480 nm) peaked at 408 nm, the hyper-Rayleigh scattering (HRS) at half of the excitation wavelength, and the hyper-Raman scattering with a Stokes shift about 1415 cm(-1) from the HRS wavelength, which can be attributed to the phenyl groups in the CV molecule. The two-photon excitation spectra measured from this S-2 --> S-0 emission is peaked at 374 +/- 2 nm. This TPF emission has the same depolarization ratio as the HRS signal, indicating a lifetime much shorter than the rotational relaxation lifetime. Subtraction of the TPF contribution leads to a hyperpolarizability value of CV at 800 nm, which is much smaller than that previously reported. Moreover, with the ability to directly measure HRS of the pure solvent, the hyperpolarizability value of CV also has much higher precision