Measurements of the electrical to VUV conversion efficiency in a xenon dielectric barrier discharge lamp by oxygen actinometry

Within the plasma of high-pressure Xe₂* excimer lamps, measurements of the intrinsic efficiency for the conversion of electrical input power into lambda~172nm output are notoriously difficult to undertake experimentally. In particular, the spatially integrated VUV output is difficult to determine with reasonable accuracy. Direct measurements of the far-field VUV irradiance from the lamp require either evacuated or inert-gas filled photon propagation paths to prevent photo- absorption by O₂ that occurs for lambda< 190nm¹. However, by using controlled flows of pure oxygen through an external cell surrounding the lamp, the total VUV output power may be measured by means of O₂ actinometry. In this paper, a medium-scale Xenon excimer lamp was fabricated using Suprasil 310 fused-silica tubing fitted with VUV transmitting fine metal mesh electrodes to allow the VUV photons to exit the plasma region with minimal attenuation. The performance of the Xenon excimer lamp was investigated using electrical excitation from custom-built high-voltage pulsed power supplies with fast pulse slew rates (<1mus) and at elevated pulse repetition frequencies (3-30kHz). The results obtained for a range of operating conditions (Xe pressure, pulse voltage amplitude, pulse frequency) clearly demonstrate that even in pulsed operation, the lamp can operate in two distinct modes of efficiency with well-defined transition or switching points. Analysis of the measured Ozone production rates from VUV photolysis indicate the highest intrinsic efficiency for VUV production within the plasma was 38% (Xe = 225 mb, pulse rate = 2.77kHz for input power loadings 5-10 W). This compares with ~60% reported by Hitzschke and 56% - 62% from detailed modelling of the discharge kinetics in Xe₂* excimer lamps3.