Deep ultraviolet and visible crystalloluminescence of sodium chloride

A protocol has been developed for production of intense crystalloluminescence (XTL) from sodium chloride in aqueous solution by selective doping with transition metal salts (Ag(+), Cu(2+), and Dy(3+)). The method was used to record complete, fully dispersed deep UV-visible (200-650 nm) XTL spectra of sodium chloride for the first time. The results show conclusively that the emissions are due to dopant cations in the NaCl lattice, with no evidence for emission directly from NaCl, e.g., by triboluminescence resulting from crystalline fracture. The UV components of the XTL spectrum are attributed to single cation substituents (Ag(+) and Cu(+)), and a strong visible component (~415 nm) of the XTL is attributed to emission from silver-pair centers, (Ag(+))(2). The nature of the electronic transitions of the dopant cations is discussed. The results suggest that the timescale for transformation of a cluster to the crystalline phase is rapid, with efficient relaxation to the lowest excited electronic states of the dopant cation. This transformation is followed by photoemission of the cation in the nascent crystal.