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Spark spectroscopy

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Tesladownunder

Thu May 13 2010, 05:07PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

iJim wrote ...

I do a little spectroscopy on plasmas...

So what are the bright visible lines that we see that make up a bluish high current spark like the above. I have wondered what is giving the major lines that we see.
Does the corona that you refer to, have a different spectrum from a spark?

radiotech

Thu May 13 2010, 05:58PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

This is a Lafayette F359 'hand spectroscope. The focus is adjustid by the slider on the side.

iJim

Fri May 14 2010, 08:55AM

Registered Member #2662 Joined: Fri Jan 29 2010, 10:14AM
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Posts: 36

Tesladownunder wrote ...

iJim wrote ...

I do a little spectroscopy on plasmas...

The spectrum of a corona or glow discharge differs a lot from that of an arc... they are relatively low density plasmas with high electron energies resulting in lots of narrow emission lines from excited gas atoms. Arcs are very dense in comparison, more electrons means more collisions which leads towards equilibrium conditions (very hot plasma!), in this situation spectral lines are broadened by collisions and you get a continuum.

From your spectrum I'm guessing the plasma falls somewhere between a glow and arc discharge... you see discrete lines, for example in the blue region which is nitrogen emission, similar to the spectra I showed. However, you also see some continuum combined with strong lines in the green and red. As you suggested, these are most likely from metal sputtered from the electrodes and excited in the gas phase. Possibly, if your air is quite humid or you happen to be breathing on it whilst taking the photo then the lines could be hydrogen emission at 486nm and 656nm, but it's unlikely.

Tesladownunder

Sat May 15 2010, 12:50AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

iJim wrote ...

... However, you also see some continuum combined with strong lines in the green and red. As you suggested, these are most likely from metal sputtered from the electrodes and excited in the gas phase.

I don't agree. The discrete lines right across the view have to be Oxygen/Nitrogen. They are not the metal lines which only appear a few mm from the electrodes as dots on the edge of the spectrogram. The spark is just too long and transient for the metal ions to travel far. There are ways around this by altering the time characteristics of the spark to make it a continuous discharge, however.
Thank you for the explanation of the continum spectra although I don't reallly understand it as just broadening of spectral lines. I would understand black body radiation of gas at 15,000C though and you will need a hot spark for that. I also understand the arc channel temp remains relatively constant as power goes up - just the spark channel size increaases.
When you said corona, I thought you may have meant corona in air rather than a low pressure gas discharge so we were talking about different things.
I still don't know which of the bright lines is oxygen and which is nitrogen. Of course the experiment is easy to do. I will just take a pic of a spark in oxygen alone.
Hopefully this weekend.

Tesladownunder

Sun May 16 2010, 11:33AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I have set up a spark in Oxygen next to one in air to compare spectra. The lines in the Oxygen are all oxygen and should be present in air (= 21% Oxygen). Any lines present in air but not Oxygen are likely to be Nitrogen lines.

PS guess who has learnt how to do labelling in photos? I use ACDSee pro 3.0 for photo storage and processing.

1274008496 10 FT88931 Hvspectroscopyoxygensetuptubelabelled

1274008496 10 FT88931 Hvspectroscopyoxygensetuprunninglabelled

1274008496 10 FT88931 Hvspectroscopyoxygenleftairrightlabelled

Bored Chemist

Sun May 16 2010, 01:00PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Some of those lines might be oxides of nitrogen, but I doubt it. You could always try running a spark in clean nitrogen.
I guess you would need to try argon to be a bit more certain.

Tesladownunder

Sun May 16 2010, 04:19PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

The setup makes comparing two gases easy but it is fairly low resolution and the spectra I obtained had multiple sparks in the plastic tube. I was a bit oncerned that I might ignite the tube if I pushed it too hard with sparks in oxygen.
I did try a razorblade slit setup using both my normal 18-75 lens and also with my 180mm lens but didn't really get satisfactory results.
Gases here are only availably by buying the regulator and hiring the cylinders so I can't do a lot of trying different gases. I guess I could just keep some the next time I get some LN2.
I would like to get Argon and Hydrogen sometime for fiendish experiments, finances permitting. Getting a balloon full of helium is easy and I think Nitrogen is used for medical equipment. I will have to explore that at the hospital.
Still this was just a bit of fun for a weekend and I need to move on to more pressing things.

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