What is luminescence?

Bombarbing some form of energy at the surface of materials may result in the emission of radiation, other than heat, in the visible (400-700 nm), UV (<400 nm) or IR (>700 nm) - this is luminescence

 

Luminescence has 2 components:

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Phosphorescence example: The hands of the clock are painted with a phosphorescent ink, which absorbs light when illuminated and then radiates that light back out

Source: http://www.glassner.com/andrew/cg/research/fluphos/fluphos.htm

  White-light room Black-light room On the left is a room lit first by a normal indoor incandescent bulb; we can see things pretty well. (Note that the display dithering is not part of the original image). Then we turn off the bulb and turn on a "black light", which is really just a bulb that illuminates mostly in the blue and ultraviolet. Notice that almost everything goes blue (or black) except for the posters, which are painted with fluorescent ink. Those inks absorb the short-wavelength blue light, and re-radiate the energy at the longer green and yellow wavelengths. Without fluorescence, the posters would appear as blue as the rest of the room, rather than colored.

Source: http://www.glassner.com/andrew/cg/research/fluphos/fluphos.htm

 

The forms of incident energy:

Cathodoluminescence (CL)

visible light (400-700 nm) that is produced with the bombarding of material with energetic electrons

typically produced by the electron beam produced by an electron microprobe or SEM, or by cathodoluminescence microscopy attachment (CMA)

the nature of CL in material is a complex function of composition, lattice structure and superimposed strain or damage on the structure of the material

semi-quantitative, at best, but lots of information of material development

Electron beam bombardment

Interactions of the electron beam and sample

Qualitative image of the interaction of an electron beam with a flat solid.

Beam diameter (d) is typically 1-2 mm.

Due to interactions with the solid, only secondary electrons near the surface will be emitted - small volume observed by SEM detector i.e. good for surface features.

Backscattered electrons are emitted over a large volume and are a function of the mean atomic weight of the materials.

X-rays are produced over a larger volume, assuming the excitation potential is above the appropriate level. The enlarged volume is a quantitative analyses limit of resolution.

CL is commonly excited at greater depths (L), commonly 2-8 mm. This further reduces the resolution.

(modified after Marshall, 1988; Beaman and Isasi, 1972)

 

Luminescence involves excitation of an electron to a state of higher energy and then a return to the low energy state accompanied by the emission of a photon

 

Depends on the electron configuration and the lattice that holds the element or ion.

 

Causes of cathodoluminescence

Two types of CL:

Impurities – most common source of CL in minerals

Activators - trace elements (substitutional) that promote CL function of concentration such that CL increases to a point and then decreases (self-quenches) dependent on the lattice structure

Examples of activators: Mn2+, Fe3+, Ti4+, REE and others

Sensitizers (co-activators) – ions that must coexist with another impurity to luminescence. E.g. Pb2+ and Mn2+ produce in calcite to produce UV luminescence

Quenchers – ions that inhibit or eliminate CL in a mineral e. g. especially Fe2+

Instruments used for observation of CL

CL microscope attachment (CMA)

2. SEM-CL