GENERATION OF VACUUM-ULTRAVIOLET RADIATION IN H-2 BY NONLINEAR OPTICAL PROCESSES NEAR THE EF-STATE AND B-STATE RESONANCES

We report on the observation of several nonlinear optical processes in connection with two-photon excitation of H-2 by 193-nm radiation generated by a narrow-bandwidth ArF laser system: stimulated electronic hyper-Raman scattering, parametric four-wave mixing (PFWM), and stimulated resonant Raman scattering (SRRS). These processes lead to the generation of several intense lines in the ir and vacuum ultraviolet (vuv). vuv generation occurs by two interconnected schemes: first, a vuv wave at lambda approximately 109 nm is generated by PFWM (power up to 16 kW). This wave subsequently generates Stokes lines around lambda approximately 150 nm by SRRS. The frequencies of these waves are tunable over spectral intervals of approximately 20 cm-1 by phase matching with admixture of various gases (e.g., noble gases, N2, D2). With additional irradiation of ir laser radiation, four-wave mixing (FWM) takes place. In this case, continuously tunable vuv radiation (tuning range approximately 600 cm-1) can be generated with power in the 10- and 100-kW range. The tuning ranges are localized around the rovibronic resonances X-B. FWM is not possible on resonance. As in the case of PFWM, Stokes lines are generated by SRRS, tunable over intervals up to approximately 60 cm-1. The susceptibilities governing the processes involved are calculated. The agreement between the theoretical expectations and the experimental results is very good with respect to the frequency dependence. The calculated absolute values of the vuv intensities, however, turn out to be by far too high. This is due to the neglect of several limiting processes which will be discussed together with the experimental results. In addition to the processes mentioned above, several B-X resonance transitions exhibit a limited tunability (approximately 10 cm-1). It is proposed to explain this tunability in terms of stimulated two-photon emission.