Crystalloluminescence

Charge and electric field fluctuations in aqueous NaCl electrolytes

Crystalloluminescence, the long-lived emission of visible light during the crystallization of certain salts, was first observed over 200 years ago; however, the origin of this luminescence is still not well understood. The observations suggest that the process of crystallization may not be purely classical but also involves an essential electronic structure component. Strong electric field fluctuations may play an important role in this process by providing the necessary driving force for the observed electronic structure changes. The main objective of this work is to provide a basic understanding of the fluctuations in charge, electric potentials, and electric fields for concentrated aqueous NaCl electrolytes. Our charge analysis reveals that the water molecules in the first solvation shell of the ions serve as a sink for electron density originating on Cl(-). We find that the electric fields inside aqueous electrolytes are extremely large (up to several V/Å) and thus may alter the ground and excited electronic states in the condensed phase. Furthermore, our results show that the potential and field distributions are largely independent of concentration. We also find the field component distributions to be Gaussian for the ions and non-Gaussian for the O and H sites (computed in the lab frame of reference), however, these non-Gaussian distributions are readily modeled via an orientationally averaged nonzero mean Gaussian plus a zero mean Gaussian. These calculations and analyses provide the first steps toward understanding the magnitude and fluctuations of charge, electric potentials, and fields in aqueous electrolytes and what role these fields may play in driving charge redistribution/transfer during crystalloluminescence.

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.

Laser-induced shock wave can spark triboluminescence of amorphous sugars

We investigated the triboluminescence (TL) of sugars using an innovative experimental approach: the laser-induced shock wave (LISW) technique. We found that the LISW could induce very bright TL in crystalline sugars, the intensity of which was shown to be 10(5) times higher than that obtained by conventional manual hand rubbing. We also applied the LISW technique to amorphous sugar samples. Although it was supposed that TL could not be excited in amorphous solids of sugars having isotropic structures, we revealed that LISW could induce bright TL in amorphous sugars similar to that induced in crystalline sugars. On the basis of the experimental results showing the dynamic behavior of the sample fracture combined with those of the TL, we discuss these novel TL mechanisms in sugars. We believe that the shock wave technique opens a new channel for investigating the nature of TL.