Persistant Luminescence

Tangent_tracker

Active member
Thought folks might appreciate this... On a trip down Rowter Hole last week, one of our team brough a fairly powerful UV torch with them and we had much fun illuminating whichever parts looked interesting...

And some of the calcite covered rocks didn't dissapoint! Check out the persistance of the glow on these rocks... I think the increase in brightness is the camera adjusting for exposure, but other than that it was just as impressive in real life!

 

paul

Moderator
Years ago we noticed the same effect when taking photos in the Hall of Thirty in Otter Hole. We were using the "set camera to 'B' - turn all lights off - click camera shutter button and hold down for a few seconds while assistants fire flashguns manually" method, someone noticed the green glow of a near by piece of stal as we still had all our lights turned off.

All photo-taking was abandoned as the flashguns were fired near stall to choruses of "Wow!", etc.
 

Fulk

Well-known member
This is called ‘phosphorescence’ and it’s a quantum effect.

Electrons in atoms and molecules are ‘quantized’, which means that they can only have certain well-defined energies – it’s as though a car can only be driven at 10 mph, 20 mph, 30 mph . . . and not at, say 27.436 mph. Electrons can gain or lose energy by ‘jumping’ from one energy level to another – a so-called ‘quantum leap’ (QL); so next time you see a news headline banging on about ‘a quantum-leap forward for the treatment of cancer (or whatever)’, remember that a quantum leap is an infinitesimally small change in the energy of a sub-atomic particle in an atomic, molecular or ionic system; a ‘quantum leap forward for the NHS’ is nonsense – journalistic bollocks.

The energy that is gained or lost by an electron when it undergoes a QL is in the form of a photon (particle) of electromagnetic radiation, and can be anything from low-energy, long-wavelength radio waves to high-energy, short-wavelength gamma rays or X-rays, taking in visible light en route.

If you shine a flashgun at a formation, then an electron in some substance in there (I don’t know whether it’s the actual calcite or an impurity) can absorb a quantum of high-energy, ultra-violet radiation, and thus find itself in a high energy level. It happens that the gaps between energy levels get smaller as you go up the scale, as illustrated schematically in the diagram.

Text Box: Increasing energy
So – imagine our little electron, somewhere inside a stalactite, minding its own business happily whizzing round in its ground state orbital (domain) (A); suddenly it gets zapped by a photon of UV light and undergoes a QL from level A to level X. At this higher energy, the system is intrinsically unstable and starts to lose energy, but instead of re-emitting a quantum of UV light of the same frequency that it absorbed, it emits a series of low-energy (radio-frequency / infra-red?) quanta, as it ‘trickles’ back down through the energy levels. When it finally gets to level B, its first excited state, the last gap, from level B to level A, is the largest gap on the ladder, and it corresponds to visible light – specifically in the case of stalagmitic material, green light.

Phosphorescence is pretty much the same as fluorescence that you observe under UV lights in night-clubs and so on – the difference being that fluorescence is an immediate phenomenon that stops as soon as the UV light is turned off (as far as the human eye, at any rate, is concerned), whereas phosphorescence takes a few seconds to happen, and so can be observed by shutting your eyes, firing a flashgun at a formation, and immediately opening your eyes.
 

andrewmcleod

Well-known member
and if you've been to the right kind of caving parties, you will have seen this long-lived phosphorescence effect via small particles containing strontium aluminate doped with europium applied to caver's skin... ;)
 

Chocolate fireguard

Active member
In the mid 1990s we were caving in the Jura, France.
One of the caves had an entrance pitch, 30/40ish metres into a big chamber, and in that there was an impressive cluster of stalagmites, well over head height.
I put a slave unit in (or perhaps behind) them and when I tested it the slave went off and then within less than a second there was a bright green (I think) flash.
It only happened that once, and I assume it was some sort of fluorescence, but with all the energy released at once. I have never found any reference to the phenomenon.

I don't know the name of the cave as I gave the guide book away years ago. We were camping in Ornans and I doubt it was far away.
That group of stal is plainly marked on the topo.

The only other thing I remember is that there was a 40 gallon oil drum in there that was full of spent carbide, and while we were there a group of French cavers came down to haul it out and replace it with an empty one.
 

Cantclimbtom

Well-known member
There is a lead mine somewhere in N Wales (unfortunately not for naming in public forum, sorry Please Don't) where a section particularly coated white like someone threw buckets of milk every way in the passage. I meant to get a UV torch when I visited but didn't get round to it getting the torch. This is inspiring me that I need to get a really powerful UV torch and return to try it there
Has anyone tried it in that location?

(before anyone says I didn't say which one, I assure you above... I did!)
 

wormster

Active member
Oooohhhh I know, I know, and, YES it does indeed glow ina dark, as do some of the formations in Minera, there are spots of fluorescence in St Cuthberts and Wheel Pit Swallet on Mendip!!
 

Fulk

Well-known member
As far as I'm aware, phosphorescence of stal is quite common; every time I've tried to make it happen, it happened.
 

cap n chris

Well-known member
Been doing this in caves for decades; A Level Physics covered the topic effectively, known about it since then.
 

JasonC

Well-known member
Visitors to Stump Cross are lent UV torches to show up calcite veins between rock joints, which enlivens otherwise rather dull sections of passage. Many of these veins are entirely missed under normal light.
More obvious formations can be disappointing re phosphorescence, possibly because just a small amount of dirt can prevent the effect occurring.
 

Tangent_tracker

Active member
This is called ‘phosphorescence’ and it’s a quantum effect.

Electrons in atoms and molecules are ‘quantized’, which means that they can only have certain well-defined energies – it’s as though a car can only be driven at 10 mph, 20 mph, 30 mph . . . and not at, say 27.436 mph. Electrons can gain or lose energy by ‘jumping’ from one energy level to another – a so-called ‘quantum leap’ (QL); so next time you see a news headline banging on about ‘a quantum-leap forward for the treatment of cancer (or whatever)’, remember that a quantum leap is an infinitesimally small change in the energy of a sub-atomic particle in an atomic, molecular or ionic system; a ‘quantum leap forward for the NHS’ is nonsense – journalistic bollocks.

The energy that is gained or lost by an electron when it undergoes a QL is in the form of a photon (particle) of electromagnetic radiation, and can be anything from low-energy, long-wavelength radio waves to high-energy, short-wavelength gamma rays or X-rays, taking in visible light en route.

If you shine a flashgun at a formation, then an electron in some substance in there (I don’t know whether it’s the actual calcite or an impurity) can absorb a quantum of high-energy, ultra-violet radiation, and thus find itself in a high energy level. It happens that the gaps between energy levels get smaller as you go up the scale, as illustrated schematically in the diagram.

View attachment 16134So – imagine our little electron, somewhere inside a stalactite, minding its own business happily whizzing round in its ground state orbital (domain) (A); suddenly it gets zapped by a photon of UV light and undergoes a QL from level A to level X. At this higher energy, the system is intrinsically unstable and starts to lose energy, but instead of re-emitting a quantum of UV light of the same frequency that it absorbed, it emits a series of low-energy (radio-frequency / infra-red?) quanta, as it ‘trickles’ back down through the energy levels. When it finally gets to level B, its first excited state, the last gap, from level B to level A, is the largest gap on the ladder, and it corresponds to visible light – specifically in the case of stalagmitic material, green light.

Phosphorescence is pretty much the same as fluorescence that you observe under UV lights in night-clubs and so on – the difference being that fluorescence is an immediate phenomenon that stops as soon as the UV light is turned off (as far as the human eye, at any rate, is concerned), whereas phosphorescence takes a few seconds to happen, and so can be observed by shutting your eyes, firing a flashgun at a formation, and immediately opening your eyes.
Thanks for the explanation. I knew about electrons jumping energy levels and emitting light, but what is it that makes the effect last so long that we can witness it exactly? Also, it did seem to be referred to as luminescence on the articles I read, which I suppose is an easy way of describing the effect, but of course referring to phosphorescence as the underlying mechanism...
 

Fulk

Well-known member
but what is it that makes the effect last so long that we can witness it exactly?

Well, there's a question to ponder; if I could give you an answer, I'd probably win a Nobel Prize for physics!
 

mrodoc

Well-known member
When I discovered the Scurion I bought second hand some years ago had a UV setting I played with that and there is a photo in the Reservoir Hole book taken by putting the camera on a tripod and shining the lamp on some pure white stal. I also recall hearing of somebody in the WCC many years ago when we had film who designed a method for catching flashgun phosphorescence by keeping the shutter open but covering the lens between flashes to build up a photo. Here is the UV light cup formation.
 

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Serious question about safety as I dont work with UV on a regular basis.
I have looked at getting / building a UV to explorer rocks I gather the shorter the wavelength the more that things fluoresce?
but also the shorter the wavelength the higher the danger to the eye as we don't see it and so our eyes don't react so potentially blinding and thinking that underground your eye will be fully dilated increasing this risk ?
The guidance seems to be wear eye protection when working with >5mw UV sources https://electronics.stackexchange.com/questions/123674/are-uv-leds-really-dangerous
so those of you using these are you wearing eye protection ? if so does it interfere with seeing the fluorescence ? or is the exposer so small / short that your happy with the risk?
 

andrewmcleod

Well-known member
Thanks for the explanation. I knew about electrons jumping energy levels and emitting light, but what is it that makes the effect last so long that we can witness it exactly? Also, it did seem to be referred to as luminescence on the articles I read, which I suppose is an easy way of describing the effect, but of course referring to phosphorescence as the underlying mechanism...
It just means that the likelihood of an electron jumping back from the higher energy state to the lower energy state is quite low for some reason, so it takes a 'long' time.

From Wikipedia article on phosphorescence:
"There are two separate mechanisms that may produce phosphorescence, called triplet phosphorescence (or simply phosphorescence) and persistent phosphorescence (or persistent luminescence). Triplet phosphorescence occurs when an atom absorbs a high-energy photon, and the energy becomes locked in the spin multiplicity of the electrons, generally changing from a fluorescent "singlet state" to a slower emitting "triplet state". The slower timescales of the reemission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur relatively slowly in certain materials, absorbed radiation is reemitted at a lower intensity, ranging from a few microseconds to as much as one second after the excitation is removed.[3]

On the other hand, persistent phosphorescence occurs when a high-energy photon is absorbed by an atom and its electron becomes trapped in a defect in the lattice of the crystalline or amorphous material. A defect such as a missing atom (vacancy defect) can trap an electron like a pitfall, storing that electron's energy until released by a random spike of thermal (vibrational) energy. Such a substance will then emit light of gradually decreasing intensity, ranging from a few seconds to up to several hours after the original excitation."

So in the first case it's because the transition itself is 'forbidden' (but in quantum mechanics this usually just means something is unlikely rather than completely impossible). In the second case it's because you need a particular random amount of thermal energy to come along, which is unlikely.

So 'glow in the dark' materials will be exploiting the second case ('persistent phosphorescence') since the first case only lasts for at most a few seconds. The best glow-in-the-dark material today is strontium aluminate when it is 'doped' with elements such as europium. The small number of europium atoms create the defects in the crystalline lattice that are needed to make the 'traps' for the electrons. The energy level of the 'trap' needs to be in the visible spectrum.
 

wormster

Active member
Serious question about safety as I dont work with UV on a regular basis.
I have looked at getting / building a UV to explorer rocks I gather the shorter the wavelength the more that things fluoresce?
but also the shorter the wavelength the higher the danger to the eye as we don't see it and so our eyes don't react so potentially blinding and thinking that underground your eye will be fully dilated increasing this risk ?
The guidance seems to be wear eye protection when working with >5mw UV sources https://electronics.stackexchange.com/questions/123674/are-uv-leds-really-dangerous
so those of you using these are you wearing eye protection ? if so does it interfere with seeing the fluorescence ? or is the exposer so small / short that your happy with the risk?
A 5 watt uv LED placed in front of you shining at a rock on the floor whist you take a few snaps is hardly long term exposure is it now?? The main H&S concern is looking directly into a 500w lamp with the eye that didn't get blinded by looking into the Lazer beam.
 
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