A strategy towards the next generation of low pressure discharge lamps

Summary form only given:In this study we describe the design of a strategy based on a number of selection rules that led us to the identification of materials that could potentially replace mercury and play the role of the active medium in low-pressure discharge lamps. The selection rules focus on the emission and thermodynamic characteristics of species as well as safely considerations and the strategy is by no means a complete solution but a logical approach to the solution based on available sources and databases. We experimentally tested the first candidates with promising results and we observed trends in the periodic table in terms of emissions. Finally a description of the limitations posed by the stoichiometry of the plasma reactions is given. The authors feel that applying the concept of high pressure metal halide lamps to the low pressure regime is most likely the way to rediscover low pressure discharges and substitute mercury. So the focus was on molecules with sufficient vapour pressure so that they reach a critical pressure in low pressure discharge conditions. The molecules must contain at least one atom with useful emissions. This is a key point of the strategy as the authors believe that atomisation of molecules employed in low pressure discharges occurs to a significant extend and the atomic emissions contribute a lot to the total spectral output. The spectroscopic results obtained in this study justified this selection rule. Another key point is the very specific spectral range defined as useful. Scanning of the available atomic and molecular emission databases led us to a number of metal halides of which some were tested experimentally. Specifically the halides of aluminium and gallium were employed as active media in standard tubes with argon as buffer gas and operated under ac mode. The preliminary spectral information revealed what indeed our design of the selection rules aimed at and predicted, which is for the metal atoms to emit their atom- c lines in the blue part of the visible spectrum and for some molecular bands to also appear in the same region covering as much of the visible spectrum as possible. This is perfectly demonstrated with aluminium chloride where the atomic aluminium line at 396 nm is seen to dominate the spectrum and a continuum that resembles the human eye sensitivity curve covers the entire visible region. In the case of the gallium halide compound there is no continuum in the visible but the three atomic lines dominating the blue region is a promising result that encourages us to investigate further. The results presented here are promising enough and confirm previous claims that the halides of both metals could form the basis of future lighting products that could replace the conventional low pressure fluorescent lamps.