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March 28, 2024, 07:56:18 pm

Author Topic: Can absorption and emission spectrums be separate?  (Read 1974 times)  Share 

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BakerDad12

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Can absorption and emission spectrums be separate?
« on: May 13, 2020, 07:22:07 pm »
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Hi guys, another question. If a continuous spectrum is sent into a cooled gas, the electrons in each atom will 'jump' shells. This is the energy that is absorbed. For an emission spectrum, light is released when the electrons jump down. So my question is, when the continuous spectrum is sent into the cooled gas and the atoms jump up and absorb certain wavelengths, do they jump down and emit those wavelengths again as well? Or do they just absorb it?

Also, why  does it have to be a cooled gas?

Thanks for the help!

Einstein_Reborn_97

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Re: Can absorption and emission spectrums be separate?
« Reply #1 on: May 13, 2020, 07:53:35 pm »
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So my question is, when the continuous spectrum is sent into the cooled gas and the atoms jump up and absorb certain wavelengths, do they jump down and emit those wavelengths again as well? Or do they just absorb it?
Yes, when a continuous spectrum is sent through an unexcited gas, the gas absorbs the same unique wavelengths that it emits when it is excited, leaving dark lines in the spectra. Remember, when the gaseous atoms absorb photons, they become 'excited' - some of their electrons jump to a higher energy level. The atoms are not in a stable state. Eventually, the electrons will drop back down to the lowest possible energy level, emitting the photons (the atoms are now said to be in their 'ground' state).

Also, why  does it have to be a cooled gas?
Well, the gas has to be stable, 'unexcited', i.e. atoms are in their ground state. If not, it will have an emission spectra that is not from the continuous spectrum that you are observing. I think that the cool temperature of the gas corresponds with the fact that it is stable and not excited. If it were warmer, it would possess heat energy which would come from photons.
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BakerDad12

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Re: Can absorption and emission spectrums be separate?
« Reply #2 on: May 13, 2020, 08:59:58 pm »
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Hi, thanks for the response. I'm still confused on this though: if a continuous spectrum is sent through the unexcited gas, will it produce an emission spectrum and an absorption spectrum or just an emission spectrum?

Einstein_Reborn_97

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Re: Can absorption and emission spectrums be separate?
« Reply #3 on: May 13, 2020, 09:24:44 pm »
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Hi, thanks for the response. I'm still confused on this though: if a continuous spectrum is sent through the unexcited gas, will it produce an emission spectrum and an absorption spectrum or just an emission spectrum?
Yes. When the continuous spectrum is sent through the gas, some of its wavelengths are absorbed by the gas and the light that doesn't get absorbed and passes through are seen on the absorption spectrum. It will look like the continuous spectrum except it will exhibit missing wavelengths (dark lines - Fraunhofer lines). Meanwhile, the wavelengths that were absorbed will then be re-emitted, in random directions, and when passed through a spectrometer, you'll see a discontinuous spectrum that consists of only the emitted wavelengths - emission spectrum.
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BakerDad12

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Re: Can absorption and emission spectrums be separate?
« Reply #4 on: May 13, 2020, 09:46:37 pm »
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Right, so you can never get an absorption spectrum by itself. But you can get an emission spectrum by itself by using discharge tubes or heating the atoms up?

Einstein_Reborn_97

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Re: Can absorption and emission spectrums be separate?
« Reply #5 on: May 13, 2020, 10:25:12 pm »
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Right, so you can never get an absorption spectrum by itself.
You can. For example, if you have a really accurate prism or grating spectrometer, careful examination of reflected sunlight will reveal dark lines in the spectra - the absorption spectra. I'll see if I can get a clearer explanation of how to get an absorption spectrum. Basically, if you view light through a spectrometer, you will be able to see the different wavelength components in the light that is not absorbed. So, if some wavelengths are absorbed by a gas, the wavelengths of the light that passes through is what will be seen as the absorption spectrum.

But you can get an emission spectrum by itself by using discharge tubes or heating the atoms up?
Yeah, when light emitted from a low-pressure gas discharge tube, excited by a high voltage, is passed into a prism spectrometer it splits into specific wavelengths that appear as different coloured lines of light in the spectrometer.
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BakerDad12

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Re: Can absorption and emission spectrums be separate?
« Reply #6 on: May 14, 2020, 08:17:45 am »
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Hi thanks for the response. I'm so confused because my teacher said you can't have an absorption spectrum without an emission spectrum? So when a continuous spectrum is sent through a say gas cloud in space, it will absorb and also emit in different directions. I suppose what I'm asking is when using atomic absorption spectroscopy, do scientists still rely on absorption and emission, or have they found a way to only extract absorption? For example, in that example you mentioned, or in the link below, they say this is how you analyse absorption spectrums.  What I'm asking is, in this scientific setting, do the absorption spectrums still give off emission spectrums? Or do they cool the gas so it doesn't give off an emission spectrum, but they still get the absorption spectrum?

(https://easychem.com.au/monitoring-and-management/manufactured-products/atomic-absorption-spectroscopy-aas/)

Einstein_Reborn_97

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Re: Can absorption and emission spectrums be separate?
« Reply #7 on: May 24, 2020, 09:05:44 pm »
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Hi thanks for the response. I'm so confused because my teacher said you can't have an absorption spectrum without an emission spectrum? So when a continuous spectrum is sent through a say gas cloud in space, it will absorb and also emit in different directions. I suppose what I'm asking is when using atomic absorption spectroscopy, do scientists still rely on absorption and emission, or have they found a way to only extract absorption? For example, in that example you mentioned, or in the link below, they say this is how you analyse absorption spectrums.  What I'm asking is, in this scientific setting, do the absorption spectrums still give off emission spectrums? Or do they cool the gas so it doesn't give off an emission spectrum, but they still get the absorption spectrum?

(https://easychem.com.au/monitoring-and-management/manufactured-products/atomic-absorption-spectroscopy-aas/)
Oops...I totally didn't forget about this :P

What your teacher says is right, for the spectrum to be termed an "absorption spectrum", there must be some missing wavelengths which have been absorbed by the gas cloud; these wavelengths WILL be emitted elsewhere.
Important point to note here: you won't be measuring an emission spectrum from something like the sun's radiation or passing incandescent white light through an unexcited gas...the wavelengths which have been absorbed will be re-emitted in random directions. It's more feasible to measure the wavelengths that pass through the gas and create the absorption spectrum. From this, you can generate an emission spectrum with the missing wavelengths. You can then use this emission spectrum to identify the gases that the light was passed through, e.g. the gases that make up the sun.

The only instance (at least, the only one I'm aware of) where you would be directly finding emission spectra is with the spectral analysis of light emitted from a low-pressure gas discharge tube, excited by a high voltage. This is because the spectral lines that show up on the emission spectrum are not as a result of the gaseous atoms re-emitting photons that it absorbed from an incandescent light source...they are from the electrons of the gaseous atoms being excited by the electrical energy and then releasing this energy as photons of light when they fall back to their stable ground state. The emission spectrum that is created by exciting low-pressure gas in a discharge tube with a high voltage is unique to the gaseous element in the tube, e.g. neon, hydrogen, etc. Using these unique elemental 'signatures', scientists (us too) can then identify the gas element(s) that, for example, make up the sun or is present in a discharge tube, etc.

I hope I've answered your questions. If not, fire some more ;)
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