Back to basics: What exactly is an emission spectrum?

[rwooduk]

This will probably end up being a stupid question with a simple answer, but if you don't ask and just accept things then where's the understanding?

Anyway, the spectrum of emission is usually shown as some form of Intensity vs Wavelength, my question is how does it relate to time and how is it measured?

Does it vary with time or is it an instantaneous measurement of all the wavelengths emitted in the process? Does the wavelength change?

I'm presently doing a dissertation on sonoluminescence and it occurred to me that my (very) basic knowledge of spectra is lacking, I've included a graph of the spectra for various gases (the gas in the bubble as it collapses and emits light)

Any comments would be welcome!

Thanks.

 

[mal4mac]

Don't such simple gasses have a constant emission spectrum? The sun doesn't change colour, you get the same wavelength of light beating down each day.

 

[rwooduk]

Don't such simple gasses have a constant emission spectrum? The sun doesn't change colour, you get the same wavelength of light beating down each day.

Thanks for the reply, so all the wavelengths are emitted at once in effectively one beam of light? and the spectrum is an instantaneous imprint that does not vary with time?

 

[Nugatory]

Thanks for the reply, so all the wavelengths are emitted at once in effectively one beam of light? and the spectrum is an instantaneous imprint that does not vary with time?

Yes. A graph such as the one you posted applies to a source that is not changing over time, so we can take multiple measurements at multiple frequencies at our leisure, and then plot them all on a single chart to get the curve.

If you have a time-varying source, then you need either a way to make multiple measurements in a time that is small compared to the rate of change (so you effectively get a snapshot) or you need to run the experiment multiple times on a similar setup.

 

[rwooduk]

Yes. A graph such as the one you posted applies to a source that is not changing over time, so we can take multiple measurements at multiple frequencies at our leisure, and then plot them all on a single chart to get the curve.

If you have a time-varying source, then you need either a way to make multiple measurements in a time that is small compared to the rate of change (so you effectively get a snapshot) or you need to run the experiment multiple times on a similar setup.

Excellent, that's just what I needed to know, thanks!

 

[sophiecentaur]

I have heard of situations in which the spectrum changes during a chemical change and that can be used to study the actual timescale of the change. For instance, in MRI, it is possible to study the spectrum (although admittedly, the absorption spectrum) [edit - as it changes in time] and to identify the composition of tissue as well as the density.
If you were studying the spectrum of received radiation from a distant source that was changing with time, the spectrum would probably not appear to be 'changing' because the whole source would probably not be changing at the same time. The effect would be more likely to be a broadening of the spectrum - more than could be explained by the temperature of the source.

 

[Andy Resnick]

<snip>

Any comments would be welcome!

Thanks.

Emission spectra (and conversely, absorption spectra) can vary with time, sometimes by quite a lot, especially in the case of semiconductors due to the dynamic processes present.

To your specific question, there's a bit of literature out there:

http://scitation.aip.org/content/asa/journal/jasa/116/4/10.1121/1.4785139
http://ieeexplore.ieee.org/xpl/abstractKeywords.jsp?arnumber=4332307