The Hyperpolarizability of Benzene Measured in the Presence of Absorption

The second hyperpolarizability of a molecule, y, mediates a wide range of non-linear optical processes [I] as well as providing a sensitive probe into molecular electronic structure [2] _ Gas-phase electric-fieldinduced second-harmonic generation (ESHG) measurements at present provide the most accurate determinations of the non-resonant y [3-g] _ Measurements have usually been made at a single, isolated frequency [3,4] but mare recently measurements have also been made systematically as a function of frequency [S-9] _ Measurements of frequency dependence allow one to address the mechanisms underlying y_ Since resonant reponses in general provide more detailed information about molecular structure than does the non-resonant response, we have attempted to apply our technique to a case where either one-photon or two-photon resonances occur. Benzene was c*hosen as a suitable candidate because its electronic excitations are sufficiently low-lying to be accessible using a visible laser (for ESHG). We report here the results of measurements of -y for benzene at X = 5 145 nm. Benzene shows onephoton absorption at the second harmonic of the laser frequency employed [lo] so io general y will be complex. We outline the special methods used to make measurements for an absorbing vapor sample, and also compare our results with those obtained from a recent, similar experiment [l l] _