Stimulated Raman scattering in helium with soft-x-ray laser radiation

We report calculations for stimulated Raman scattering (SRS) with pump photons from a soft-x-ray laser. The Raman transitions from the 1{sup 1}{ital S}{sub 0} ground state to the 2{sup 1}{ital S}{sub 0} and 3{sup 1}{ital S}{sub 0} metastable states of helium at 20.6 and 22.9 eV, respectively, are considered. We calculate {chi}{sup (3)} for pump photons with an energy close to the autoionizing resonances (2{ital s}2{ital p}){sup 1}{ital P} at 60.15 eV and ({ital sp},23+){sup 1}{ital P} at 63.66 eV. SRS gain coefficients are derived for x-ray laser lines in close resonance with the autoionizing levels, viz., the 3{ital p}(3/2,3/2){sub 2}{r_arrow}3{ital s}(3/2,1/2){sub 1} transition in neonlike selenium ({lambda}=20.64 nm) and the analogous transition in neonlike bromine ({lambda}=19.47 nm). It is shown that the Raman gain coefficient with experimentally realistic parameters is significant and that a high gain length product can be achieved even with pump intensities below 10{sup 11} W/cm{sup 2}. {copyright} {ital 1996 The American Physical Society.}