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Thread: Little question...

  1. #1
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    Default Little question...

    I have a 40 mW green laser. At one day I noticed that a "glow in the dark"-star give no more light when I illuminated it with my laser. Can anybody explain this?

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    It is simply because the energy per quantum (photon) is insufficient to excite the material. I'm guessing it is just a normal green "glow in the dark" item, probably zinc sulfide based. You need to excite it with a shorter wavelength than your 532nm green laser. A blue or violet laser would readily excite the material as they have a sufficiently high energy per photon.

    As a side note there are "glow in the dark" pigments that can be excited by green 532nm light. I have such a pigment that glows orange.

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    OK, but why does the star give no more light even when it was fully recharged? I don't get this one...

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    Oh, I see what you mean now. Hmm... that is a very interesting phenomenon that I had not noticed before. It seems that even a red (660nm) laser will quench the phosphorescence. I have reproduced the phenomenon with 4 different pigments and they all display the same behavior.

    I have only studied a little quantum mechanics, but I will do my best to explain it. Maybe we have some physicist here who can do better (I'm a chemist myself).

    Phosphorescence (the scientific term for the phenomenon behind [most] "glow in the dark" products) occurs because a portion of the electronically excited material enters an excited triplet state through intersystem crossing. Transitions between triplet states and singlet states are forbidden, which basically just means they take a long time. As the material slowly relaxes to its (singlet) electronic ground state light is emitted; this is phosporescence.

    Now, it is obvious that your green laser somehow manages to facilitate the relaxation process, but how? Well, it helps to think about energies here. To go from the triplet state to the singlet ground state a little energy is required, even though the triplet state is higher in energy and should spontaneously relax (but it cannot, because it is forbidden). This energy is normally provided by inherent thermal vibrations in the material. As you have observed however, it can also be provided by an external light source.

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    Geeeez as if we didnt all already know that off by heart ...

    My pipe drum sergeants always telling me off for my sloppy triplets...

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    I'm struggling myself to understand it... Actually, since it is quantum mechanics it cannot be understood.

    Here is the very simple explanation for it, using a classical mechanics analogy. Now even with illustrations. Consider a ball in the landscape below. The higher up it is the higher its energy. As it is drawn the ball is stuck in the so called triplet state and cannot escape it because it simply doesn't have the energy (speed) to overcome the hill (E[2]). This describes a very cold phosphorescent material. Heat could be introduced into this model as an earthquake of sorts. As the ground shakes there is a chance the ball will bounce out of its "pit". In the process it will lose energy which is emitted as light. Irradiating it with light however, is more like shooting it with a cannon; it will certainly bounce out of its pit.

    Hope that clears it up.

    Oh, I did another experiment using an IR (780nm) laser. This is much more interesting as you can see the phosphor starts to glow intensively as it is hit by the IR radiation (which is perceived as very weak). You could never observe this effect by the naked eye using for example a green laser as the phosphorescent emission would drown in the reflection from the laser.
    Attached Thumbnails Attached Thumbnails phosphor.gif  


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    I'm digging up my old thread to say thank you tocket for that information, I'm using it in my thesis. I hope I may quote you?
    Anyway, that is a good explanation, I even understand it!

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    You're very welcome. Of course you can use it in your thesis. I'm a little curious what the topic of the thesis is though.

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    Quote Originally Posted by tocket View Post
    You're very welcome. Of course you can use it in your thesis. I'm a little curious what the topic of the thesis is though.
    I would call it "Quantum mechanics cannot be understood and this proves it"

    -Gene

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    I think its the same principle that those IR viewing cards work on. The material is "charged" by the ambient light, and any IR beam causes the material to release all of its energy at once, making it appear to glow.

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