Abstract
We show that the two-photon resonances in the third-order susceptibility can be exploited to yield two-photon spectra of molecular gases at moderately high spectral resolution. This form of spectroscopy does not depend on the occurrence of processes (such as fluorescence of photoionization) leading to indirect methods of two-photon absorption. The method is direct and leads in principle to values for a two-photon cross section. Comparisons of two-photon and coherent anti-Stokes Raman resonances leads to ratios of Raman and two-proton cross sections independent of the laser powers and spatial characteristics. The technique is documented with rotationally resolved spectra of SO 2 and NO. A value for |α xx α yy | of 1.5 × 10 −52 cm 6 was measured for the O 12 (6 built:1 2 ) component of the (A) 2Σ ← (X) 2 Π 1 2 transition of nitric oxide.
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