Integrating Research and Education > Geochemical Instrumentation and Analysis > CLTheory
Author Profile

Cathodoluminescence Theory

Darrell Henry, Louisiana State University

Bombarding the surface of a material with some incident radiation or particle may result in the emission of electromagnetic radiation beyond that produced by thermal black body radiation. This emission can be in the visible range (400-700 nm), ultraviolet (UV; <400 nm) and infrared (IR; >700 nm). This general phenomenon is known as luminescence. The types of luminescence is general distinguished by the type of incident radiation or particles and by the kinetics of the emission process. In the latter case, if the luminescent radiation occurs in <10-8 seconds after the incoming radiation ceases, it is a luminescent feature termed fluorescence. If the luminescent radiation continues to emit for >10-8 seconds (and sometimes much longer) after the incoming radiation ceases, it is a luminescent feature termed phosphorescence.

Cathodoluminescence (CL) is the emission of photons of characteristic wavelengths from a material that is under high-energy electron bombardment. The electron beam is typically produced in an electron microprobe (EPMA) or scanning electron microscope (SEM-CL) or in a cathodoluminesce microscopy attachment to a petrographic microscope (Optical-CL). The nature of CL in a material is a complex function of composition, lattice structure and superimposed strain or damage on the structure of the material. Different minerals exhibit fluorescent or phosphorescent kinetic behavior which can have an effect on the quality of the CL images, depending on the manner in which the image is obtained.


Solid-state band theory provides a way to explain the luminescence phenomenon. An insulating solid material (such as quartz or calcite) can be visualized as having a valence band and a conduction band with an intervening band gap (forbidden gap).

CL conceptualized with band theory
Figure 1. Generation of CL response in an insulator, as conceptualized with band theory.
[Top] Insulator with a wide band gap between the valence band and conduction band with hypothetical electron traps (horizontal lines) with the band gap. [Middle] Excitation of electrons from the valence band to the conduction band, leaving so-called "holes". [Bottom] Possible paths of electrons as they fall back to ground state of the valence band include: (left) electron falling directly back to the valence band, generally emitting UV radiation, (middle) electrons encountering a single trap, emitting CL proportional to the energy released from the temporary occupancy of the trap by the electron as it falls to the valence band and (right) electrons encountering multiple traps, emitting CL proportional to the energy released from the temporary occupancy of the traps by the electron as it falls to the next trap or valence band.



In a flat sample that is > 10 µm, the resolution of a CL image will be inherently reduced because of the significantly larger depth/volume in which the CL is being excited (see electron beam interactions).


Extrinsic luminescence results from impurities in the structure. The impurities generate luminescent centers and are most commonly transition elements, rare earth elements and actinide elements (due to the occurrence of valence electrons in either "d" or "f" orbitals, see below). These impurities are generally the most common source of CL in minerals. There are several categories of interactions that influence the character of the CL.

Geologic applications


In the related field of material science, CL is commonly used to determine the occurrence and distribution of impurities in a variety of materials such as Si-wafers used in semi-conductors, in other ceramic products, and in synthetic crystals (e.g Y- and REE-doped Garnets).


Boggs, S.,Jr. and Krinsley, D. (2006) Application of Cathodoluminescence Imaging to the Study of Sedimentary Rocks. New York, Cambridge University Press, 165 p.

Gotze, J., Plotze, M., and Habermann, D. (2001) Origin, spectral characteristics and practical applications of the cathodolumimescence (CL) of quartz - a review. Mineralogy and Petrology, 71, 225-250.

Marshall, D. J. (1988) Cathodoluminescence of Geological Materials. Boston, Unwin Hyman, 146 p.

Pagel, M., Barbin, V., Blanc, P. and Ohnenstatter, D. (2000) Cathodoluminescence in Geosciences. Berlin, Springer-Verlag.

Remond, G., Balk, L. and Marshall, D. J. (1995) Luminescence, Scanning Microscopy 9, Proceedings of the 13th Pfefferkorn Conference, Scanning Microscopy International, Chicago.

Waychunus, G.A. (1988) Luminescence, X-ray emission and new spectroscopies. in Hawthorne, F.C., editor, Spectroscopic Methods in Mineralogy and Geology, Reviews in Mineralogy, 18, 639-668.