Formation of Electronically Excited Ions in Electrode Processes: Electroluminescence of Trivalent Rare-Earth Ions in Liquid Solutions

Abstract

Trivalent cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, thulium, and ytterbium electroluminesce in liquid phosphorus oxychloride solutions at room temperature. Trivalent terbium electroluminesces also in dimethyl sulfoxide and propylene carbonate. The emission is due to the known inner ``f'' subshell transitions of the trivalent ions, originating from established metastable states. The electroluminescence spectra of the ions range from the ultraviolet (in the case of Gd3+), through the visible (Ce3+, Pr3+, Sm3+, Eu3+, Tb3+, Dy3+, and Tm3+), to the infrared (Nd3+, Yb3+). The emission originates at the surface of the negative electrode and becomes observable, in the case of green emitting Tb3+, at about 6 V dc. The electroluminescence is explained by the following mechanism: Application of an external potential attracts the Helmholtz and Gouy Chapman layers of solvated cations towards the surface of the cathode. This causes a Schottky lowering of the barrier to electron escape and allows electrons to escape from the cathode into the electrode/electrolyte interface. The electrons are accelerated across the positive ion rich interface and acquire enough energy to excite the luminescent cations by inelastic collisions.