Stimulated Raman scattering experiments have been performed in H(2) and in H(2)-Ar mixtures with a UV-preionized XeCl discharge laser (308 nm) as the pump. The energy conversion efficiency from the pump laser beam to Stokes beams has been investigated as a function of Ar concentration and pumping energy. It has been found that, in the H(2)-Ar mixture with a 50% Ar concentration, the energy conversion efficiency to the first Stokes beam was more than 80% higher than that obtained in H(2) at the same total pressure and pump energy (60 mJ).