Micro-gas-discharges as source of cathodic electroluminescence in non-aqueous solutions

The cathodic light emission observed during electrolysis of solutions of simple inorganic salts (e.g. LiClO₄, NaBF₄ and Tb³⁺ salts) in solvents such as dimethylformamide, dimethylsulphoxide, and POCl₃ at potentials higher than 6 V has been analyzed using spectroscopic, microscopic and electroanalytical techniques.Experimental results indicate that insulating (“passive”) layers on the cathode play an essential role in the mechanism of this electroluminescence. High current densities in the pores of these layers heat the solution and finally lead to local vaporization and decomposition of the solvent, thus creating insulating gas-filled cavities in the pores. If the applied voltage is sufficiently high, an electrical breakdown can develop across this cavity producing the observed luminescence phenomena.The luminescence emission spectra are indicative of excited gaseous species. Insulating model-layers obtained by glow-discharge polymerization were deposited on the cathode and used as models for electrodes with passive layers. Light emission was significantly more reproducible at such electrodes and allowed us to obtain reproducible voltage—current—light curves which support the proposed mechanism. The luminescence from Tb³⁺-ions occurs by energy transfer from the gas discharge.