16 - , um generation by C 0 2 - pumped rotational Raman scattering in H 2

In 1976 Byer proposed using stimulated rotational Raman scattering in hydrogen gas as an efficient method of frequency conversion in the infrared.' In particular, it was noted that stimulated Raman scattering in para-H 2 gas with a CO2 laser pump generates 16-gm radiation, which is of potential use for UF 6 isotope enrichment. It was subsequently learned that others have also suggested stimulated rotational Raman scattering in the infrared.2 Recently Frey et al. 3 have generated 16-Am output at the 1-mJ level by vibrational Raman scattering in H2 and N2 pumped by a rubypumped dye laser. We have successfully generated 50 mJ of 16.9-gm radiation by four-wave mixing-assisted stimulated rotational Raman scattering. We used a 2-J, 70-nsec CO2 laser source to pump a 25-pass, 4-m-long cell filled with 3 atm of p-H 2 gas obtained from a liquid H2 source. Based on theoretical calculations' and rotational Raman threshold, scaled from careful measurements at 1.064 gim, we predicted a C0 2 -pumped rotational Raman threshold in our multipass cell of 1.7-2.6 J in a 70-nsec gain-switched pulse. Our available CO2 pump energy was not adequate to reach stimulated Raman threshold by amplification of spontaneous emission. However, when 16.9-gm radiation generated by fourwave mixing was injected into the multipass cell, the Raman gain was sufficient to lead to significant pump depletion and a peak photon conversion efficiency of 40% to the Stokes wave at 16.9,gm. The four-wave mixing process was first described for interactions in crystals4 and later extended to gases5'6 as a means for generating widely tunable infrared radiation. 7 Sorokin et al. 8 have demonstrated four-wave mixing in H2 gas using a CO2 laser source as a method for 16-,m generation. The equations governing both four-wave mixing and stimulated Raman scattering are given by9