Second-Harmonic Generation in Atomic Hydrogen

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The generation of second-harmonc radiation in atomic hydrogen at 121.6 run was investigated with and without an applied electric field. Experiments were carried out without the application of an electric field and the mechanism responsible for second-harmonic generation was studied. It has been shown that the incident radiation at 243 nm wavelength iodm the hydrogen atoms by fl"photon absorption and the resultin charge distribution produces an electric-field, the charge-separation field. This electric-field breft the symmetry of the isotropic gas and results in radiation at 121.6 nm.Second-harmonic generation is forbidden in an isotropic medium under the electric dipole approximation. Previous observations of second-harmonic generation in atomic gases without any applied fields were explained by involving contributions of higher-order moments, or intensity gm(hents of the Lum beam, or collisional effects, or charge-separation fields. The simple electronic structure of atomic hydrogen and the carefully chosen experimental conditions in the present work ehmted the possibility of other explanations of the last one. Values of charge separation field were obWW experimentally under various conditions, and were shown to be conffitent with theoretical estimates derived in this work.