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We report the new results of continuously tunable coherent VUV/XUV radiation produced by twophoton resonantly enhanced third harmonic generation and four-wave sum-mixing in 2 , H2 , CO, and 4 molecules. By use of the radiation from molecules combine to the radiation generated from Xe, Kr, Ar, and Ne atoms we have developed a reasonably practical VUV/XUV tunable coherent radiation source. The theory analysis, experimental results, and the experimental system, as well as the key techniques are presented in this paper. There are many exciting applications of coherent vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) sources in photochemistry, material science, atomic physics, and life sciences, but to become practical, these applications require a convenient laser source that can be easily asembled and that can operate in the laboratory with a minimum of attention. We report the new results of the generation of tunable coherent VUV/XUV radiation by two-photon resonantly enhanced third harmonic generation (THG) and four-wave sum-mixing (FWSM) in N2, H2 , CO , and 4 molecules. By use of these radiation combine to the radiation generated from Xe, Kr, Ar, and Ne atoms which we have produced previously, a reasonably practical VUV/XUV tunable coherent source has been developed and sued to the applications. The system is described as follows, the pumped source is the pulsed dye laser pumped by Nd:YAG laser, which has a linewidth of about 0.1 cm -i, was frequency-doubled by a BBO crystal. The output of the BBO crystal, acting as the input beam, was focused into the center of a molecular gas beam which is provided by a pulsed gas jet. The generated radiation of the third harmonic and the four-wave sum-mixing were separated from the irput laser by a VUV monochromator and was detected by an electron multiplier. All the detail results including the generated VUV/XUV radiation spectrum will be presented in our report. Moreover, we calculated the two-photon resonant TH and FWSM intensity of these molecules, it shows that five branches, namely O,P,Q,R, and S branches can be produced from part of the molecules, but the Q branch usually has a very weak third order susceptibility, and the intensity of the other four branches are mainly . governed by the rotational population of the ground state. Due to the expansion cooling of the gas jet, the population is always concentrated on a few rotational levels of the ground state, thus only a few rotational lines can be obtained in each branch. As an example, we consider CO molecule and suppose J = 4 is the most populated level and the population has a Boltzman distrbution, the temperature of the gas beam at the laser focus is estimated to be I I OK . Thanks to the rotaional cooling effect, the TH spectrum is greatly simplified and the rotational lines can be easily assigned, and the intenensities of the resonant lines which have the optimum population can be significatly improved. Also the TH and FWSM conversion efficieny are found to be of the same order or a little smaller than that in a three-phton resonant THG process in inert gas, this proves that these molecules are an efficient nonlinear medium for generating bright, tunable coherent VUV/XUV radiation.
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