For investigating diatomic and triatomic molecules of astrophysical interest in the vacuum UV using the methods of laser spectroscopy we have set up several systems for generating coherent vacuum UV sources. Since we are interested in tunable and narrow banded (0.1 - 6. 01 cm ) sources it is advantageous to employ the methods of nonlinear optics.

Recently, experiments of the third harmonic generation in the Rb-Xe system. have been performed and a quantitative interpretation up to the highest conversion efficiencies of a few percent has been given. On the basis of these experiments we have investigated the optimum conditions for third harmonic generation in isotropic media. It has been possible to solve the third order problem analytically and it can be shown that the intensity conversion efficiency depends in the limit of large input intensities only on the wavelength dependent nonlinear susceptibilities of the nonlinear medium. For a Gaussian beam it can be shown that a Na-Xe system, for example, is capable of providing an energy conversion efficiency of up to 40% for the Nd:glass laser.