Appendix

What is luminescence?

            Throughout our section concerning light-producing experiments we have used the terms ‘incandescence’, ‘thermoluminescence’, and ‘chemiluminescence’.  In this Appendix we will now explain what the various types of luminescence really are.  The following text was mainly sourced from a wonderful book m luminescence by A.K. Campbell [1].

            Everyone knows that when solids and liquids get very hot they emit light.  This is incandescence.  Boyle, in 1688, carefully described five similarities between burning coal and ‘shining wood’, the latter now known to be caused by a luminous fungus.  Both produced their own light, but only in the presence of air, and both could be extinguished by removal of air.  However, whereas ‘a live coal’ was irreversibly extinguished by withdrawing air for a few minutes, ‘shining wood’, on the other hand, could easily regenerate its light, even if air was not re-admitted for half an hour.  Furthermore, ‘live coal was actually and vehemently hot’, whereas ‘shining wood’ was ‘not sensibly lukewarm’.

            By 1794, J. Hutton had proposed the term ‘incandescence’ (Latin, incandare - to become white) to describe the emission of light by a body heated to a high temperature.  The limits of colour recognition for the human eye are approximately 400 nm (violet) to 750 (red), though under extreme light intensities this response range can sometimes be extended to 350-900 nm.  In incandescence a faint red glow can be detected at 525^oC(798 K).  As the temperature rises the light becomes dark red then turn yellow, and becomes increasingly white as blue light contributes to the spectral emission (Table A1). Further theoretical detail can be found in physics text books, under the headings of ‘black-body radiation’ and the ‘Stephan-Wien Law’.

 

Table A1.  Typical colours of incandescence.

Approximate temperature (oC)	Colour observed
525	faint red
700	dark red
900	cherry red
1100	dark yellow
1200	bright yellow
1300	white
1400	blue white

 

            The discovery of the ‘Bolognian Stone’ in 1603, by an Italian cobbler and amateur alchemist Vincenzo Cascariolo, was to lead to the characterisation of another group of phenomena, classified by Wiedemann, but not until 300 years later, as luminescence (Latin, lumen - light).  The Bolognian stone, made of a heavy spar (barium sulphate) could ‘imbibe’ the light from the sun and re-emit it in the dark.  A large number of inorganic and organic compounds have now been identified which also luminesce after being irradiated by ultraviolet or visible light.  Wiedemann, in his original paper of 1888, proposed that a ‘luminescent substance’ was one which ‘becomes luminous by the action of an external agency which does not involve an appropriate rise in temperature’.  Wiedemann initially distinguished six types of luminescence and designated them by a prefix: photoluminescence, caused by absorption of light; electroluminescence, produced in gases by an electric discharge; thermoluminescence, produced by slight heating; triboluminescence, as a result of friction; crystalloluminescence, as a result of crystallisation; and chemiluminescence, the result of chemical processes.  Other have since been added to the list (Table A2).

            The key difference between incandescence and luminescence is not so much heat, but rather whether the physical process necessary for light emission involves transitions in electronic energy levels within atoms or molecules, in the case of luminescence, or transitions in energy levels between atoms or molecules, in the case of incandescence.  The energy levels can be displayed diagrammatically by an ‘energy well’ diagram, or more simply by the Jablonski diagram, first introduced in the 1930’s (Fig. A1).

            Thus luminescence is concerned primarily with the emission of visible or near-visible radiation (200-1500 nm) when electrons in excited orbitals decay to their ground state, the light arising from the potential energy of electronic transitions within atoms or molecules.  In the many types of luminescence (Table A2), the prefix identifies the energy source responsible for generating or releasing the light.  Incandescence, on the other hand, is light arising from losses in kinetic energy between atoms or molecules.  This energy is usually supplied initially as heat.  Heat, for example, can generate electronically excited states in the gas phase, giving rise to pyroluminescence.  In contrast, in thermoluminescence, gentle heating, usually 100-500 ^oC, can provide the activation energy necessary to release electronic energy, initially absorbed from ultraviolet and visible light or subatomic particles.  This occurs, for example, in a crystal or liquid containing impurities.  Absorption of a photon by the major component can excite an electron to a higher energy level.  A ‘hole’ is left behind which, since it has lost a negatively charged electron, behaves like a positive charge.  A positive ‘hole’ attracts an excited electron, the association being known as an exciton.  But the exciton can jump between molecules in the crystal.  In a pure crystal, the energy of the exciton on reassociation of the ‘hole’ and the electron is dissipated as heat.  However, if there are dislocations, structural boundaries, or chemical impurities within the crystal, the exciton may be trapped there for long enough to allow thermoluminescence to occur.  Now, when the crystal is heated, the energy of the exciton can be emitted as light, the emission spectrum being characterised by the impurity rather than a main component.

 

Table A2.  A Classification of luminescence, based on E.N. Harvey.

Type		Basis of light emission	Example
A.	Associated with heating (distinct from incandescence)
1.	Candoluminescence	luminescence of incandescent solids emitting light at shorter wavelengths than expected	heat ZnO
2.	Pyroluminescence	luminescence of metal atoms in flames	yellow Na flame
3.	Thermoluminescence	luminescence of solids and crystals on mild heating (i.e., well below that necessary to produce incandescence)	heat diamond
B.	Associated with prior radiation (fluorescence - short lived; phosphorescence - long lived)
1.	Photoluminescence	irradiation by UV or by visible light	Bologna stone (BaSO4)
2.	Cathodoluminescence	irradiation by b particles (electrons)	television screen
3.	Anodoluminescence	irradiation by a particles (the nuclei)	zinc sulphide phosphor
4.	Radioluminescence	irradiation by g or X-rays	luminous paint
C.	Associated with electrical phenomena
1.	Electroluminescence and piezoluminescence	luminescence associated with electric discharges and fields	fluorescent strip light
2.	Galvanoluminescence	luminescence during electrolysis	electrolysis of NaBr
3.	Sonoluminescence	luminescence from intense sound waves in solution	ultrasonic probe in pure glycerol
D.	Associated with structural rearrangements in solids
1.	Triboluminescence	luminescence on shaking, rubbing, or crushing	gentle agitation of uranyl nitrate, UO2(NO3)2.6H2O
2.	Crystallo( = tribo-) luminescence	luminescence on crystallisation	HCl or ethanol addition to saturated alkali metal halide solutions (NaCl, KCl)
3.	Lyo( = tribo-)luminescence	luminescence on dissolving crystals	LiCl or KCl coloured by irradiation by cathode rays
E.	Associated with chemical reactions
1.	Chemiluminescence (oxyluminescence)	chemical reaction	luminol + H2O2 and peroxidase
2.	Bioluminescence (organoluminescence)	luminous organisms	O2 + luciferin-luciferase from the sea firefly (Vagula)

 

            In chemiluminescence, the electronically excited state is generated by a chemical reaction.  As a result, the excited molecule, the product of the reaction, has a different atomic structure from the initial substrate(s).  Here, too,  gentle heating can initiate the phenomenon.

            While the difference between incandescence and luminescence was a source of confusion for several centuries, more recently there has been even more confusion over the terms fluorescence and phosphorescence, together with their relationship to chemiluminescence.  The modern scientific usage of the term phosphorescence is ascribed to Johan Elsholtz who in 1677, applied it to substances or objects which, like the Bolognian stone, absorb light and re-emit light, after a delay, in the dark.  However, phosphorescence soon became a popular term for any kind of ‘cold light’, including that emitted by luminous organisms.  E. Becquerel carried out many experiments on luminescence in the mid-19th century, including the invention of a phosphoroscope for measuring the minimum duration of phosphorescence.  This led to the later discovery of radioactivity by his son Henri, and to the distinction between true phosphorescence and fluorescence.  The term fluorescence (Latin fluo - I flow) was first introduced by Sir George Stokes in 1853 to describe a phenomenon observed in 1845 by Sir John Herschel.  Herschel found that certain substances, e.g., quinine sulphate or calcium fluoride minerals (known as fluorspar), when exposed to ultraviolet radiation, gave off visible blue light.  However, the phenomenon was of very short duration since, unlike phosphorescent minerals, no light was observable immediately after the UV lamp was switched off.  Phosphorescence is most striking when it lasts for many minutes, though in 1858 Becquerel, using his phosphoroscope, reduced the time between irradiation and light emission in some phosphorescent substances to 0.1 ms. Yet for fluorescence, even this brief time interval is some four orders of magnitude greater than the lifetime of an excited fluor.  Unfortunately, the Oxford English Dictionary still defines ‘phosphorescent’ as ‘the property of shining in the dark’.

            In short, we can define the following terms.  Luminescence, - the emission of electromagnetic radiation in UV, visible and IR (light) from atoms or molecules as a result of the transition of an electronically excited state to a lower energy state, usually the ground state.  Fluorescence, -  luminescence from a singlet electronically excited state, and which is of very short duration after removal of the source of excitation.  Phosphorescence, - luminescence from a triplet electronic state that remains detectable, sometimes for considerable periods, after the source of excitation is removed.

 

Reference

1.    A.K. Campbell, Chemiluminescence: Principles and Applications in Biology and Medicine, Chichester, Eliis Horwood Ltd., 1988.