Talk:Candoluminescence

Copyrighted material?

The exact same text in this article can be found at: http://www.bookrags.com/wiki/Candoluminescence Is there a copyright issue here? M610 (talk) 17:42, 19 November 2008 (UTC)

• That website looks like one of the many that clone Wikipedia. Compare link for link (the ones that don't lead to ads, of cource), all the valid articles are identical. Nekura (talk) 00:12, 8 July 2010 (UTC)

Implausible definition

The definition as substances that at a certain temperature "emit light at shorter wavelengths than would be expected for a typical blackbody radiator" appears problematic, in (visible) light of the fact that, if I am not mistaken, it follows from the laws of thermodynamics that a body at a temperature T cannot give off at any wavelength more energy than a black body at that same temperature.

Perhaps what is meant is that the emission at shorter wavelengths relative to that at longer wavelengths is greater than in a black body. That is possible not by the emission being greater, in absolute terms, than that of a black body at shorter wavelengths, but by it being less, in absolute terms, at longer wavelengths. In other words, the light will appear whiter, or bluer, than would the light of a black body, not because there is greater emission in the shorter wavelengths, but because ther is less emission in the longer ones.

If that is the right explanation, it should be reformulated.

David Olivier (talk) 17:32, 28 September 2009 (UTC)

Say, I have a hot piece of lime (as used in limelight) at 2800 Kelvin, cooling down to, say, 2799 Kelvin in vacuum. From simple conservation of energy it directly follows that if it emitted 'less' energy at some wavelength than blackbody, it had to emit more energy than blackbody at some other wavelength, as the sum of total energy over all wavelengths is same. Ditto for limelight which gets N watts of power from oxyhydrogen flame. I think you have totally confused energy with power, or rather, radiance. Dmtrlk (talk) 07:51, 21 April 2010 (UTC)

Well... if the piece of lime emits less energy (per second) than a black body at some wavelengths, that may mean simply... that it will take more time to cool from 2800K to 2799K. Nothing constrains it to cool at a given rate!

The point is that, in total, it will emit more energy at a shorter wavelength than a black body would.

If limelight gets N watts from the flame, and is not a black body, then it will simply have to get hotter than it would have in order to reemit those N watts.

Or it could have larger surface area.

It remains that a body in equilibrium at temperature T cannot give off more light at any given wavelength than would a black body at temperature T. If it did, it would be possible to place between that body and a black body a filter transparent only to that wavelength, and by that means heat the black body to a higher temperature than T. That is clearly not possible.

David Olivier (talk) 09:38, 21 April 2010 (UTC)see

You're mixing up energy, power, radiance, spectral radiance, and nobody knows what else, in the "give off more light". If you want to say that limelight cannot have larger spectral radiance at any wavelength than a black body, that may very well be true, this does not mean that lime in a torch won't give off more light at shorter wavelength than blackbody in a torch.

You are mistaken. Black-body radiation is a spectrum, not a specific, single wavelength. A black-body emits all wavelengths, distributed around a peak intensity, and trailing off away from it. It is not concerned with the total amount of energy being emitted, nor the amount of energy at any specific wavelength, but just with the distribution of wavelengths. It is a idealized entity, and no object actually truly emits the exact spectrum, meaning the relative intensity of two wavelengths may differ from the black-body spectrum. This is exactly what the statement in the article says, and means. So there is nothing wrong with the statement as stated. Nekura (talk) 01:16, 8 July 2010 (UTC)

Hello, I think the article as it stands is wrong - I spent some time researching this today and read that its actually something to do with direct excitation of the rare earth material by radicals (IE OH radicals et al IIRC). If my understanding thus far is correct, combustion creates radical species like OH and the (potential chemical?) energy of such a radical is in the eV range (1 to 10 eV) which corresponds to thousands of K thermal energy(10,000 K and more). These radicals can directly excite atoms of say thorium (or molecules of thoria?) which, once in a higher energy state than thermal excitation alone can take them to in a flame of up to 2000k (for example), can revert to a lower energy state in the usual way with the emission of a quanta of radiation greater than thermal excitation could take them to. Sorry I can't add any more detail or explain it any better, my understanding of radicals is limited and theres very little on the web about the topic.

From http://www.ijs.speleo.it/pdf/72.593.39(1)_Sweet.et.al.pdf:

'The energy source for candoluminescence is the recombination of radicals in the flame on the mineral surface. The recombination energy is transferred to the luminescence center where it is emitted as the observed candoluminescence. Possible recombination reactions in the hydrogen flame include:

OH* → OH = 4.03 eV
H + H → H2 = 4.31 eV
H + OH → H2O = 5.5 eV
OH* + H → H2O = 9.17 eV

OH* is an excited state of the OH radical. The energy of short wave UV (253.7 nm) is 4.89 eV. Because the observed bright candoluminescence requires an efficient energy transfer, it seems likely that the 9.17 eV reaction is the excitation source'

So a free radical is a molecule which has unfilled electron shells, so its in a reactive state (think potassium for instance - its electronically balanced but very reactive because its outer shell is not full of electrons. At least thats how I will think of it until a chemist says otherwise!)

Now replace that view of potassium with a hydroxyl radical (OH). Its got the same number of protons as electrons so its electronically balanced, IE its not an ion. But its highly reactive like potassium because its not got a full outer shell of electrons. It can combine with a hydrogen radical in the flame to produce H2O. By doing so it gives up energy because the H2O configuration is of a lower energy than the combined OH and H configurations (a chemist would have to explain this since I only just understand it myself).

If thoria is present, OH and H are absorbed on to it from a flame as they are created by the thermal breakup and partial combustion of hydrocarbon atoms (assuming a hydrocarbon fuel). As they are absorbed, they combine with each other producing H2O. This gives up energy which is directly passed to the thoria molecules. I assume this is via vibrational energy transfer but don't know. Once the thoria molecule has absorbed part of the vibrational energy from the reaction that created the H2O molecule, an electron is knocked into a higher energy level by the 'shock' and subsequently decays to its normal state, emitting a photon with an energy corresponding to the energy which was transferred to it from the new H2O molecule.

This is different to thermal excitation of the thoria because the thermal 'impacts' from moving gas molecules which transfer the thermal energy to the thoria are of a lower energy. In a gas 1 eV = 11605K, a much higher temperature than is present in a flame. On the other hand the energy available via the free radicals is up to 9 eV or so according to the above reference. The transfer is probably only going to be partially efficient so a high reaction energy of 9 eV is said to be advantageous in the above reference.

The blue colour in a gas flame is apparently from recombination of the free radicals, but what the gas mantle must do I assume is recruit some of these free radicals and use them to directly excite the thoria.

What confuses me is if these free radicals are recombining in a flame anyway, why is the flame not brighter with blue and UV emission? Perhaps the combined free radical has a fairly long life and usually gives up its energy when in the gas of a flame by transfer to the other molecules of gas in the flame rapidly, whereas if the reaction between the free radicals is on a substrate of thoria, the thoria is able to scavenge the energy before its diluted into the bulk of the gas, and transfer it more efficiently into light.

This does appear to violate thermodynamic principles. I will try to patch it up.

David R. Ingham (talk) 07:33, 1 November 2011 (UTC)