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Author Topic: Spectrophotometry  (Read 2252 times)

Offline harryneild

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Spectrophotometry
« on: 06/02/2006 23:12:53 »
In order to determine the chemical composition of the sun, Kirchhoff and Bunsen (I think) examinined the dark bands (fraunhofer lines) produced when the light from the sun was diffracted using a triagular prism into the spectrum. Some of these lines were quite distinguishable and thick, which i thought went against what i had learnt.

I thought that the lines would be almost indistinguishable because of the specificity of the absorption of photons by atoms. Because an electron on a certain energy level will only absorb a very precuse wavelength of radiation, why would these dark lines be so prominent and not be extremely thin.

I would appreciate any answers to this. Thank you!


 

Offline Soul Surfer

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Re: Spectrophotometry
« Reply #1 on: 07/02/2006 00:03:30 »
There are several reasons why spectrum lines have a noticable width.

The first is the fact that all spectrum lines have a natural width defined by the uncertainty principle and the length of time an atom will stay in its excited state before it emits the photon  (emission and absorbtion are complementary).  The shorter the period the wider the line.

The next is the effect of magnetic fields  (Zeeman effect)  most spectrum lines consist of several possible spin related transitions lumped together and applying a magnetic fields causes these states to seperate in energy this broadening the line or even splitting it into seperate elements.

The next is the temperature broadening. The atoms in a gas are all moving quite quickly and in random directions (Typical velocities even at room temperature are apound 1Km/sec) and the doppler effect causes the lines to be further broadened.

Finally there is pressure broadening. (this is frequently the most pronounced effect).  As pressure increases the atoms in the gas are pushed closer and closer together so the electromagnetic fields start to interfere with each other and change the energy levels slightly and so broaden the spectrum lines.

The spectra of white dwarf stars are frequently almost featureless because the pressure of their atmospheres is so high that the absorbtion lines are broadened so much as to be virtually indistinguishable from the background continuum.

You can also see examples of pressure broadening in modern street lamps.  Old fashioned sodium lamps were an intense harsh yellow and it was almost impossible to see different colours because they emitted almost all their light in two very closely spaced yellow spectrum lines (the D lines).
 most modern sodium lighting is a soft golden colour and more natural because the gas discharge is in a small translucent ceramic cylinder at high temperature and pressure.  This significantly broadens the lines and allows colours to be distinguished.  Why you can see different colours when all the light is still quite a narrow range of the yellow spectrum is another and very interesting story.

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Offline harryneild

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Re: Spectrophotometry
« Reply #2 on: 07/02/2006 23:34:18 »
Thanx Soul_Surfer, i hadn't thought of any of them factors (i havent even heard of most of them before) except from the temperature broadening which i wasnt sure about because i thought the time distortion that the atoms feel due to velocity would make up for the  doppler effect that you described. I guess i was wrong.
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Re: Spectrophotometry
« Reply #2 on: 07/02/2006 23:34:18 »

 

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