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November 2011 - Fluorescence vs Phosphorescence

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Written by Administrator
Saturday, 22 October 2011 14:12

Even nowadays, in the 21st century, the difference between fluorescence and phosphorescence as defined underneath is in the minds of many engineers, trainers, etc.

• Fluorescence is an emission of light that disappears with the end of excitation.
• Phosphorescence is an emission of light that goes on for some time after the end of excitation.

We are happy to have been in contact with Bernard VALEUR, a physicist and a chemist, emeritus professor in the Conservatoire National des Arts et Métiers (CNAM, as per its French acronym. One of the most prestigious French engineers schools) in Paris (France).
Bernard VALEUR is a renowned specialist of molecular fluorescence, and he is known throughout the world, thanks to the books he released on this topic (1) (2) (3).

Indeed, as Bernard VALEUR writes, "the difference is based on experimental data dating back to the 19th century!"

In fact, "we now know that in both cases, the emission lasts longer than the excitement; using only the emission time is not right. There are fluorescent species, the life of which is long and in the order of magnitude of the lifetimes of compounds, whose phosphorescence is short (some tenths of microseconds to some microseconds). Francis Perrin (4) presented the first theoretical distinction between fluorescence and phosphorescence: as for phosphorescence, the excited molecules pass through an intermediate state before emitting photons, contrary to what occurs during a fluorescent emission."

We sent Bernard Valeur some questions: 

First question
In fluorescent PT and fluorescent MT, we do not see how emission could last longer than excitation.
If we excite the fluorescence of the fluorescent penetrant or magnetic fluorescent detection media, for 5 minutes by example, and then we turn off the UV-A source, immediately we no longer see the fluorescence. Therefore, emission does not last longer than excitation.
This would mean that the fluorescent dyes, and optical brighteners used in PT and the fluorescent pigment used in MT are an exception to this rule.

Bernard VALEUR
The term "immediately" applies to your visual perception. The time resolution of your eye is very inadequate to see the fluorescence decay after illumination ceases since it occurs at times ranging from some scores of picoseconds to some hundreds of nanoseconds depending on the compounds(1,2). Physically, it is more accurate to say that the excitation lasts longer than the excitation, even if our eyes do not see the decay. Meters may measure it at a nanosecond and even a picosecond scale.

Second question
In a PT Level 3 training handbook, it is written:
"Fluorescence is a special case of photoluminescence.
Luminescence is the property that some substances show: they emit electromagnetic radiation when excited by an initial input of energy. This excitation may be an irradiation by photons (X, γ, UV); it is then called photoluminescence (X-ray screens). It may be an exposure to particles (α, β); it is called the radioluminescence.
When the photoluminescence occurs immediately after excitation (t = 10-8 s) and stops with it, it is called fluorescence.
Phosphorescence is a phenomenon that does not stop with the excitation; the emission occurs with a delay that may range from a few tenths of a second to several days. Phosphorescence depends on the temperature: the delay decreases as the temperature rises. Phosphorescence is often related to chemical and biological reactions (glow-worms ...)."

Bernard VALEUR
This paper on fluorescence and phosphorescence has many inaccuracies, atop the fact that the fluorescence does not stop with excitation.

• There are phosphorescent compounds, the life of which is about one microsecond, therefore much shorter than a few tenths of a second. Furthermore, the inorganic compounds, the phosphorescence of which lasts several days, are very common, while phosphorescence lasting for several days is, in my opinion, exceptional.

• It is not only phosphorescence that depends on temperature. The fluorescence of organic compounds in solution as well. Some people use the criterion of temperature dependence to distinguish fluorescence and phosphorescence, a point that applies only to inorganic compounds for which an increase of temperature releases the trapped electrons.

• The phosphorescence in no way is linked to chemical and biochemical reactions. When the luminescence occurs during a chemical reaction, this is called chemiluminescence. For a biochemical reaction, it is bioluminescence. In particular, the glow-worms are not phosphorescent but bioluminescent.

Note added in February 2012

After reading the works referenced in (1) (2), we have written a paper titled Dyes and fluorescent penetrants. The books' author, Bernard VALEUR, gave the complementary pieces of information and the explanations necessary to understand the phenomena involved. He also made the critical reading of our manuscript.

Bernard VALEUR pointed out that, on several occurrences, we used the ''the fluorescence excitation of the dyes'' wording. He does not agree!

His explanation is very clear … and so simple: "the dyes are excited, and their fluorescence comes as a result of this excitation. A phenomenon cannot be excited!''

We think it useful to bring you this piece of information because we are not alone, especially in PT and MT, to make this mistake, which is written in many training courses and documents.

As an example, in the 2010 edition of the ASME Boiler and Pressure Vessel Code (5), the paragraph T-777.3 is titled: "Fluorescent Magnetic Particles with Other Fluorescent Excitation Wavelengths".

References

(1) Bernard VALEUR, Mário Nuno BERBERAN-SANTOS, Molecular fluorescence, Principles and Applications, 2nd Edition, April 2012, ISBN-10: 3-527-32837-8, ISBN-13: 978-3-527-32837-6, Wiley-VCH Verlag GmbH & Co. KGaA, Boschstraße 12, D-69469 Weinheim (Germany): On this Website.

(2) Bernard VALEUR (2004), Invitation à la Fluorescence Moléculaire, ISBN 2-8041-4597-2, © De Boeck & Larcier s.a., 2004, éditions De Boeck Université, rue des Minimes 39, B-1000 Brussels (Belgium).

(3) Bernard VALEUR (2005), Lumière et luminescence. Ces phénomènes lumineux qui nous entourent, ISBN 978-2-7011-3603-5, Collection Bibliothèque scientifique, Éditions Belin, 8 rue Férou, F-75006 Paris (France).

(4) Francis PERRIN (1926, 1929), La fluorescence des solutions. Polarisation. Vie moyenne des molécules dans l'état excité (Journal de Physique, 1926, vol. 7, pp. 390-401, and thesis, Annales de Physique, 1929, vol. 12, pp. 169-275).

(5) ASME ASME's Boiler and Pressure Vessel Code (BPVC), West Caldwell, New Jersey, USA, 2010 Edition.

Last Updated ( Sunday, 10 June 2012 14:20 )