Triboluminescence and the potential of fracture surfaces

Triboluminescence (TrL) arises on the fracture of many solids. Spectral evidence suggests that the TrL of the majority of materials is due to electrical discharge. Gas discharge is also invoked to explain the TrL of certain materials in which there is no spectral evidence for it. A technique for determining if the TrL of a material is due to discharge, independent of spectral information, is described here. Electrostatic probes are used to monitor the potential of the fracture surfaces of a crystal during cleavage. The signal from a photomultiplier reveals if the TrL is due to discharge between the fracture surfaces. The use of this technique is illustrated through studies of sucrose, sodium chloride, and a highly TrL (and highly photoluminescent) terbium complex. Oppositely charged fracture surfaces are found in sucrose; however, the sense of the electrification is opposite to that predicted by the existing theory of piezoelectric charging. It was confirmed that the TrL of both sucrose and sodium chloride is associated with electrical discharge. In contrast, the TrL of the terbium complex commenced prior to electrical discharge. This demonstrates that the TrL of this material is not due to the fluorescent re-emission of the ultraviolet component of the discharge spectrum, as had been suggested previously.