CIBJO releases Gemmological Special Report: considers process of separating measurable facts from opinion; See Gemological Articles below.
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 Post subject: Think, why don't ya? (fluorescence)
PostPosted: Fri Apr 21, 2006 12:21 pm 
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Maybe this is a start in a small series of "why".
It is not novice proof, but I'm more than happy to go into it.

A lot of gemology students struggle with the concept of fluorescence while there is little need to. The fact that they battle their brains over it however puts them ahead of those who can't be bothered.

What you will learn: light with high energy enters a gemstone, some energy get's lost inside the stone (converted), the remainder gets emitted and if the emitted energy is between 1.77eV and 3.1eV we can see it as fluorescence.

Before I attempt to explain the simple mechanism that triggers fluorescence I must correct the following statement:
"in many gemstones there are tiny trace elements that can take this invisible ultraviolet light and slow it down enough that it becomes visible light ... it takes the longer wave UV light and generates the red color ... It takes the high energy UV light and slows it way, way down, until the color is at the bottom of the visible spectrum of waves....which means you get the color at the bottom end of the visible spectrum...which is red!" .
I read that somewhere on the internet (if it was you who wrote it and want copyright, let me know and I'll put your name under it for credit).

Let's examine that statement.
The "Velocity of light = Frequency x Wavelength", so if the wavelength goes up so does the velocity (use the basic 10 = 2 x 5 equation for this and jiggle a bit with the numbers .. as in 12 = 2 x ? ).
This shows that larger wavelengths travel at higher speed, so there can not be a case of "slow it down".
Red light at 700nm travels faster than blue light at 400nm.
If one would slow down longwave UV light (at for instance 366nm), it would result in shortwave UV (or atleast something shorter that falls outside the visible light spectrum).
If anything, the fluorescence, in this logic, would be a result of the "speeding up" of light, which is also incorrect (although the result is true).

So what does happen?
The anwer is conversion of energy.

In order to understand this, one should be aware that light is a form of energy and that every wavelength of light carries a specific amount of energy and that the amount of energy is opposite proportional to the wavelength.
That was a mouthfull, but I mean that the longer the wavelength, the lesser energy it carries.
For instance red light with a wavelength of 700nm has 1.77eV of energy and blue light of 400nm carries 3.1eV of energy (eV = electron energy = a very small amount of energy .. google for it).
So the shorter the wavelength, the more energy.

You can easily calculate the amount of energy a certain wavelength carries with the formula: Energy = 1240 / wavelength .

One can not alter this amount energy, but it can be converted.
As an example: when you drive in your car, the car has energy. When you hit a tree, this energy is lost from the car (because you parked your car there). However the energy from the car is not lost from space .. instead it got converted to other types of energy. In this case mostly heat. (Don't try this at home).

A similar thing happens inside some gemstones.
When energy in the form of UV light (lets say longwave UV at 366nm with 3.4eV of energy) enters the stone, some electrons absorb this energy and get elevated to a higher energy state (google for "jablonski diagram"). Then through "internal conversion" some of the energy of the UV light gets "lost" ("converted to something other than electromagnetic energy" is better) and the remainder of the energy gets emitted. When this remainder of energy is, lets say, 1.77eV, then the stone will emit red light (because 1.77eV corresponds with 700nm = red light).
So 1.63eV of energy got "lost" (3.4 - 1.77 = 1.63).

Now what happens to the lost energy? That gets absorbed inside the gemstone, or transformed to heat .. several options.

And there you have it, it's all about energy that get's converted.
For more investigation, google for "Stokes Law" and "Stokes Shift".

To recap: light with high energy enters a gemstone, some energy get's lost inside the stone (converted), the remainder gets emitted and if the emitted energy is between 1.77eV and 3.1eV we can see it as fluorescence.

That is why we should ask ourselves "why?" when we read something we don't understand. And remember, if you really can't make any sense from the answer given .. it is usually because the one who answered didn't understand the concept or money is involved.


p.s.: if you really think about it, then I'm sure you came up with the "why doesn't frequency change?".
That has to do with Planck's law: Energy = Plancks constant x Frequency or Energy = Plancks constant x (Velocity/Wavelength) .

QED


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PostPosted: Fri Apr 21, 2006 3:40 pm 
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Why do some elements, like iron, in incredibly minute concentrations, totally inhibit fluorescence?


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PostPosted: Fri Apr 21, 2006 4:02 pm 
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Iron absorps energy at the lowest exicted stage affecting fluorescence, but not too much so it would affect the body colour as I understand it.

Thus if the Iron grabs energy, lets say another 0.5eV, from the electron, that would result in the emmision of 1.27eV of energy. Which corresponds with 927nm (infra-red light) .. and that we can't see either. (3.4 - 1.63 - 0.5 = 1.27)

Does that sound logical?


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PostPosted: Sun Apr 23, 2006 10:04 am 
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Hi Doos

Can you explain in similar detail why some stones continue to fluoresce (phosphorescence) after the higher energy has been removed

Frank


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PostPosted: Sun Apr 23, 2006 12:22 pm 
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Sure, the mechanism is basically the same.

I made a few images that might make it easier to visualize. Note that this is just a simplified version and that I'm not a scientist.

Fluorescence:
Image

Above is what happens with fluorescence.

Phosphorescence:
Image

An electron gets excited (by the UV light) to a higher energy state.
The electron falls in a trap and must overcome the barrier to get back to the groundstate. As this takes longer, the effect is seen as an afterglow.
So basically it is just trapped energy that slowly gets released.

Electrons always want to be as close as possible to the nucleus of the atom, so they will do whatever they can to fall back to their groundstate.

Someone with more scientiific knowledge may step in anytime.


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PostPosted: Mon Apr 24, 2006 5:10 pm 
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Thanks Doos. I was wondering how you would explain it without the stairs,ball and seal analogy. At least we've lost the seal :) The diagrams are helpful though

Thanks again

Frank


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PostPosted: Tue Apr 25, 2006 8:58 am 
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Okay, so it was not good enough for you!

Here goes my attempt at a more scientific explanation.

Normal situation:
An electron gets exited to a higher energy state (orbital) by absorption of a photon.
The electron falls back to its ground state emitting a photon of the same energy. So no loss in energy.

Fluorescence:
The electron absorbs energy from the UV light source, gets exited to a higher energy state. The electron then undergoes vibrational relaxation (loosing energy) and emits the remainder of the energy. When this remainder of energy that is released is between 3.1 and 1.77eV .. we see it as fluorescence.

Phosphorescence:
The same thing as with fluorescence, except that the electron doesn't fall back to the groundstate immediatly. Instead it undergoes vibrational relaxation first and then (through intersystem crossing) it drops to a forbidden state. All the while loosing energy.
As the dropping from the forbidden state to groundstate is slow, we see it as an afterglow.

In the image in my previous post you can regard the trap as the forbidden state.

What really happens is that the spin of the electron changes (for instance from up to down spin). This means that in a molecul more than one electron has the same spin in certain orbitals. This is unfavourable and termed "triplet state" opposed to the usual "singlet state" in which the spins are opposite (see my post on Magnetism in this section).
So the trap in the diagram represents the forbidden or triplet state.

This may also explain why for instance a diamond has blue fluorescence and yellow phosporescence. Because yellow light is of lesser energy than blue light.

Hopefully one day I'll understand the details.


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PostPosted: Tue Apr 25, 2006 4:27 pm 
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It really was good enough for me doos. but every added morsel of knowledge fills me with glee so thanks for the long version...Vibrational relaxation sounds just fine. :)


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PostPosted: Thu Jun 29, 2006 6:43 am 
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I just found out that some tourmalines fluoresce.

According to Gem ID Made Easy:
Deep Red Tourmaline - SW = medium bluish/lavender ; LW = inert
Yellow (Tanzanian) - SW= pale yellow ; LW = orange

However, I checked one brownish orange tourmaline in my practice yesterday and I found one that is inert under LW and fluoresce a dull orange under SW.

Any ideas?
My instructor said that there may be a catalyst in it.
I can't find an answer in my books or maybe I haven't been looking hard enough :lol:

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PostPosted: Thu Jun 29, 2006 7:02 am 
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So does this mean that the red waves of a rainbow actually "appear" first, and that the other colors show up according to speed/wavelength until we see the full rainbow display?


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PostPosted: Thu Jun 29, 2006 10:34 am 
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Sidebar: Hey Gemma! 8) Great to see you back. :P


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PostPosted: Thu Jun 29, 2006 4:43 pm 
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Riniel, dunno .. ask her in about 2 weeks, might start a nice discussion.

Gemma, in theory yes I suppose.


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PostPosted: Thu Jun 29, 2006 6:34 pm 
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Hi Gemma, nice to see you around here.

Your question is legit, but, I think when we see graphics of spectrums or rainbows or refractive light it skews our perception of how fast this is really happening.

An example would be: Take a greyhound bus and paint a spectrum on the side of it. Now have it driven in front of you at the speed of light. Can you tell me which color you saw first?

Or paint six buses (seven if you want to include indigo) the various spectral colors and have them drive towards you at the speed of light, even stagger their speed a little. You wouldn't be able to say which one passed you first, unless you knew the order beforehand.

Of course we see the individual colors of the spectrum when light is passed through a prism but they resume their normal speed, even if not combined once they leave that medium. We don't see red, then orange then yellow etc. appear individually in order. Happens too fast.

Like I said, those little staggered colored rays of the spectrum we see depicted in educational graphics look so slow when they're not moving but, add some warp speed to them and everything changes.


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PostPosted: Fri Jun 30, 2006 3:46 pm 
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Riniel,

This report may be of use to you. The body color of the tourmaline and the SW UV reaction sounds similar.

http://www.gia.edu/gemsandgemology/1857 ... detail.cfm


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PostPosted: Fri Jun 30, 2006 10:01 pm 
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Thanks JB!!
Now I know!! I need to update my notes!!

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