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Author Topic: Why does ZnS show fluorescence?  (Read 5271 times)

Offline Tintin_Triton

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Why does ZnS show fluorescence?
« on: 29/09/2009 13:51:23 »
Okay, I know it shows fluorescence, i.e. it emits light when Cathode rays strike onit, But my Question is why does it happen like that.
I mean if someone could explain to me the processes involved and all, It iwll be a great help!
 


 

Offline Bored chemist

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Why does ZnS show fluorescence?
« Reply #1 on: 29/09/2009 18:34:26 »
Pure ZnS doesn't fluoresce. The stuff used as a phosphor has a little cadmium and silver added to it. These impurities are stuck in the latice and the electrons associated with them have a different set of permitted energy levels from the Zn atoms.
The electrons from the cathode rays (or UV light) will promote electrons to a higher energy level associated with one of these impurity atoms. When the electrons fall back down they give out light.
 

Offline chris

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Why does ZnS show fluorescence?
« Reply #2 on: 29/09/2009 22:13:09 »
Zinc sulphide doped with copper was the first phosphor I think. In the early 1900s scientists discovered that mixing it with radium produced a nice bright glow that was ideal for clock hands and faces. The zinc sulphide glowed because the decay products from the radium excited the electrons, as explained above.
 

Offline chris

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Why does ZnS show fluorescence?
« Reply #3 on: 29/09/2009 22:15:08 »
BC - can you explain why the electrons are promoted to the energy levels associated with the impurities? Does not the zinc have higher energy states that its electrons could occupy?

Chris
 

Offline Bored chemist

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Why does ZnS show fluorescence?
« Reply #4 on: 30/09/2009 18:46:46 »
The ZnS does have energy levels that the electrons could be bumped up into but these lie a bit higher- the light involved would be in the UV. Also since all the ZnS would be the same, it would be easy for the energy of an excited electron to be transfered to the next Zn atom along. That transfer would sometimes "miss" and lead to the energy being degraded to heat.
With a small number of impurity atoms the excited electrons are exssentially "stuck" until they let the energy out as light.
 

Offline chris

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Why does ZnS show fluorescence?
« Reply #5 on: 02/10/2009 14:05:48 »
Thanks, that's very helpful.

chris
 

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Why does ZnS show fluorescence?
« Reply #5 on: 02/10/2009 14:05:48 »

 

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