2.4. Cathodoluminescence and Heat

Cathodoluminescence is the emission of visible light from a sample during electron bombardment and may be observed using the light optics of the microprobe. Resolution is about 1000 Å (the same as for X-rays; the diameter of the excitation jug). This effect is produced in materials with at least some semiconductor properties when incident electrons knock a photoelectron into the "conduction band" of a material resulting in a positively charged "hole." The free electrons recombine with the holes to produce radiated light or heat in the sample. Cathodoluminescence can be an intrinsic property of a material (e.g., scheelite) or the result of luminescent centers produced by trace impurities (often Mn or rare-earth elements) in a non-luminescent host (e.g., calcite). Minerals that are luminescent include K-feldspar, zircon, fluorite, diamond, apatite, and benitoite (blue); quartz (orange to blue); calcite (red-orange due to Mn²⁺ or Pb²⁺ activator); willemite (green); and enstatite (red due to Mn²⁺ activator or blue with no activator) and dolomite (blue).

Significant amounts of heat are produced with a sample because electron excitation of X-rays is not very efficient. Many low energy continuum photons and low-energy inelastically scattered electrons do not escape the sample and their energy is transformed into higher vibrational energies of the bonds (heat). The maximum temperature rise for a material can be expressed as:

 

Consider an accelerating voltage of 15 keV, a specimen current of 0.05 µA (= 50 nA), and a beam diameter of 1 µm. A material like copper with a high thermal conductivity (C_t = 4.01 W/cm°K) has DT = 0.9°C, whereas, calcite (C_t = 0.05) has DT = 72°C. Note that going to a beam diameter of 10 µm would drop this latter number by a factor of 10 to 7.2°C. Actually, epoxy will decompose ("burn") long before attaining DT in excess of 150°C. In contrast, most minerals can survive temperatures well in excess of 200°C. Values at 15 keV for some materials are given below (currents in nA).