Improved fitting equation for frequency-resolved femtosecond hyper-Rayleigh scattering experiments

One approach to the experimental determination of the molecular second-orde r nonlinear polarizability, or the first hyperpolarizability, of fluorescen t species by hyper-Rayleigh scattering is based on high-frequency demodulat ion of the time-delayed fluorescence contribution to the immediate scatteri ng signal [Rev. Sci. Instrum. 69, 2233 (1998)]. For typical fluorescence li fetimes of less than a nanosecond a detection bandwidth of more than 1 GHz is necessary. This bandwidth has not yet been realized. Measurements at suc cessively higher modulation frequencies are performed instead. A fitting of the apparent hyperpolarizability as a function of the modulation frequency then reveals the inherent hyperpolarizability without the fluorescence con tribution. An improved fitting function has been derived, resulting in the elimination of a small systematic error and in the reduction of the larger statistical uncertainty in the deduced value. Possible implications of the improved accuracy and precision are discussed. (C) 2000 Optical Society of America [S0740-3224(00)01110-3].