Luminescence of rare gas crystals at high excitation densities for VUV laser applications

Exciton densities of the order of 10¹⁸ cm^–3 are generated in 0.1–0.3 mm thick surface layers in an area of 10×20 mm² of optically clear rare gas crystals. The quantum efficiencies at 126 nm (Ar), 145 nm (Kr), and 172 nm (Xe) remain near 0.5 even for the highest excitation densities. The corresponding gain coefficients of 2.6 cm^–1 (Ar) to 18 cm^–1 (Xe) exceed those of high pressure gas lasers by a factor of 20. Stimulated emission is inferred by observing the line narrowing, the dependence of intensities and time courses on excitation density and amplification measurements. The net gain coefficient is reduced however to 0.5–1 cm^–1 by transient absorption of excited centers and scattering by irradiation induced defects. The results are analysed by a system of rate equations for the excitation, relaxation, quenching, and amplification processes. A peculiar temperature dependence of the quantum efficiencies and time courses is attributed to electron trapping at grain boundaries.