The effect of hydrogen iodide on the ignition of hg-free metal-halide lamps

Summary form only given. Trace amounts of moisture adsorbed on metal-halide salt pellets during lamp fabrication may react with the salts during initial lamp light-up to form volatile hydrogen iodide (HI). HI remains stable in Hg-free metal halide lamps during operation delaying the diffusion of hydrogen through silica burner walls. Nonetheless, Hg-free lamps can develop high ignition voltages due to the presence of HI, which adversely affects lamp operation. This work discusses the thermochemistry and spectroscopic measurements which demonstrate that HI is indeed formed in the reaction of salts and moisture. Experimental confirmation of increased breakdown voltage (lamp ignition) with increasing concentration of HI (at the ppm level) was obtained in discharge cell experiments. Prior work has shown that electron attachment by HI is a fast process, with a reaction occurring on every collision between an electron with an HI molecule. Our hypothesis is that the average electron energy gain and avalanche multiplication is thus retarded, with increasing voltage needed for successful breakdown in moisture contaminated lamps.