Investigation of amplification process on the third Stokes line of H ₂ under ultraviolet laser irradiation

An amplification process was investigated in the third stimulated Raman scattering (SRS) line of H2 excited with a 266 nm laser beam generated from the fourth harmonic of a Nd: YAG laser. The unexpected intensity enhancement observed at the third Stokes SRS line around 397.8 nm is attributed to the seeding of the self-generated H-ε Balmer line at 397 nm of atomic hydrogen by pumping the H₂ molecule with a high-energy laser pulse at 266 nm. It is worth mentioning that in our case the SRS spectrum of H₂ showed a quite different intensity pattern from the usual SRS spectra of hydrogen. The pulse energy and pressure dependence of all the SRS lines in general and the third Stokes SRS line in particular were investigated, and in all respects the amplified SRS line at 397.8 nm manifested completely different characteristics that have not been reported in previous publications. The conversion efficiency (CE) of all the SRS lines in the hydrogen 266 nm SRS spectrum was also estimated, and 36% CE was achieved at the 397.78 nm line. To support our claim for amplification at the third Stokes line by seeding of the H-ε Balmer line of atomic hydrogen, a comparative study was also carried out by pumping hydrogen gas with 355 nm (less energy per photon) and 266 nm laser beams. It is worth noting that amplification of the third Stokes SRS line was observed only with the 266 nm pump laser, where dissociation of H₂ and excitation of atomic hydrogen take place, and not with the 355 nm pump laser.