Resonant four wave mixing in molecular crystals

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Abstract
Experimental studies are presented of the Raman and two‐photon resonant effects in the third order susceptibility for benzene, naphthalene, and biphenyl crystals at 1.6 °K. The experiments consist of measurements of the polarized coherent light dispersion at ω3=2ω1−ω2 when the crystals are irradiated with two tunable lasers at ω1 and ω2. The frequencies ω1 and ω2 are chosen such that ω1−ω2 and 2ω1 match vibrational and electronic resonances, respectively, of the materials. The four wave mixing results obtained under definite polarization conditions are used in association with Raman scattering cross sections to find values for the nonresonant background third‐order susceptibilities of the crystals and the two‐photon absorption coefficients of various vibronic transitions. In addition the large dynamic range of these experiments has allowed us to obtain homogeneous (Lorenzian) damping parameters (Γ) for a number of vibrational levels of the electronic ground and excited states. In terms of the trace (α t 2) and the anisotropy (β t ) the following results were obtained: Benzene 15450 1, α t 2 =8.6×10−51 (cm6 mol−1), β t 2?0, Γ=0.7 cm−1; naphthalene 15420 1, α t 2=2.8×10−50, β t 2 =4.1×10−50, Γ=5.5 cm−1; biphenyl B 3g ←A g , 0–0, β t 2=3.6×10−49, Γ=0.9 cm−1. These calibration points can be utilized to obtain the absolute strengths of each of the many two‐photon vibronic transitions observed previously in the two‐photon fluorescence of these crystals. The damping parameters yield vibrational relaxation times in the range 0.4 ps for naphthalene 1542, to longer than 12.5 ps for the naphthalene ground state mode at 1383 cm−1. It is proposed that 1542 relaxes by fission into an electronic and vibrational exciton.