Abstract: We address two experimental issues concerning the measurement of depolarized Hyper-Rayleigh scattering (HRS). The first issue concerns with the possibility of two-photon induced fluorescence (TPF). We observed that the depolarization of TPF from crystal violet, which is an octupolar molecule with a very short fluorescence lifetime, is identical to that expected theoretically for HRS. This makes distinguishing the two processes difficult. The second issue concerns with the way second harmonic light is collected in a HRS measurement. We developed a simple procedure to ensure the depolarization ratio measured would correspond to the ideal 90$DGR scattering from a point source. We measure the depolarized HRS of p-nitroaniline (PNA) using this improved technique. The result indicates that the hyperpolarizability tensor of PNA has essentially the characteristics of a rod-like structure, with a single nonvanishing diagonal component $beta$-333$/, where the 3- axis corresponds to the polar direction. This is in contrast with previous-theoretical calculations, which predicted non- negligible values for some of the off-diagonal components. A plausible explanation of the experimental result is suggested based on a model of the local field correction factors.!18
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