We report observation of a pressure-induced extra resonance in four-wave mixing (PIER4 signal) using single-mode continuous-wave dye lasers. The narrow laser linewidth permits for the first time a quantitative verification of the frequency, intensity, and pressure dependence of the PIER4 signal predicted by the theory of the nonlinear susceptibility ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$.

• Received 9 February 1981

DOI:https://doi.org/10.1103/PhysRevA.24.623