For the detection of hydrogen atoms in plasma devices by the resonance scattering technique, a coherent tunable narrow-band source at the Lyman α(L
α: 121.6 nm) line is developed. L
α radiation is generated by frequency tripling of a Xe Cl-laser-pumped dye laser in phase-matched Kr and Kr/Ar mixtures. The condition for phase matching and the effect of Ar addition are consistent with the theoretical predicts of the nonlinear optics. For a 1-MW input at 365 nm, 6.6 × 10
9 photons (1.1W, 10ns) per pulse at the L
α line are generated in Kr at 80 Torr. The detection sensitivity of hydrogen atoms by the resonance scattering technique at the L
α line is briefly discussed.
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