Optical third-harmonic generation at 2314 using a focused ruby laser beam (6943 ) has been observed in a number of gases. A theory of third-harmonic generation by focused beams is presented which is used to derive atomic (or molecular) third-harmonic coefficients from the experimental data. An ambiguity in choice of a sign leads to two alternative sets of coefficients. One set is preferred on the basis of general agreement with data given in or derived from the literature: Theoretical calculations of the third-harmonic coefficients, measurements of the Kerr effect and theoretical dc hyperpolariabilities. For helium the measured absolute value is χHe=(7×3±)×10-39 esu/atom. For other gases the experimental values of χ in units of 10-39 esu/atom and scaled to χHe=4.0 (a theoretical value due to Sitz and Yaris which is thought to be reliable to about 1%) are: Ne 8.9 ± 1.5; Ar 126 ± 20; Kr 386 ± 75; Xe 979 ± 150; H2 80 ± 12; CO2 156 ± 23; N2 107 ± 11. Some data for (CH2)2, Cl2, O2, air, and glass are also presented. © 1969 The American Physical Society.
