Different real and imaginary components of the resonant third-order polarization revealed by optical heterodyne detected transient grating spectroscopic studies of crystal violet: Model and experiment

We carried out heterodyne detected transient grating measurements on a triphenylmethane (TPM) dye molecule, crystal violet, in ethanol, to investigate the origin of the apparently contradictory wavelength dependence observed in pump-probe and homodyne detected transient grating experiments. The measurements were performed at two different wavelengths and the decay profiles of the real and imaginary components of the third-order polarization were found to be strikingly different at both wavelengths. A simple model that accounts for the additional contribution from hot ground state absorption is proposed. The model successfully explains the major features observed in the experiments and provides a consistent picture for the underlying dynamics in this type of system. The striking differences between the decay profiles and wavelength dependencies of the real and imaginary components of P⁽³⁾ observed in the TPM molecules predominantly originate from the different wavelength dependence of the amplitudes of the various contributions, rather than from changes in the time scales or the observation of new contributions at particular wavelengths.