Resonant generation of even-order harmonics in metal vapors

The second and fourth-harmonic generation of laser radiation in Mg vapor is reported at the forbidden 3s(21)S0 - 4s(1)S0 transition. The input frequency-tunable radiation is produced b either an N2-laser or a ruby-laser-pumped dye laser. The output is filtered and analyzed with a monochromator and detected by a photomultiplier and an oscilloscope. The output wavelength for both cases is 229.9 nm. The conversion efficiency for the second-order harmonics is as high as 0.1-0.01 percent and for the fourth one, 10 to the -6th - 10 to the -8th percent. The experimental studies reveal some processes that break the symmetry of the gaseous medium and that make the even-order nonlinearities allowed.