Hi,
I am investigating a H2+CH4-Plasma with OES. I am trying to find out what the origin of the processes behind the OES peaks are. I start by igniting a 200 sccm H2 plasma with 800V and ~150W. Then I add some CH4 (5 sccm, 10 sccm, 20 sccm,...) and I observe the H alpha, beta and gamma peaks, namely the Balmer-series. With increasing CH4 amount in the reactor the Balmer series is decreasing. And now I want to understand what is happening:
1. One possibility is that the amount of H is decreasing and thus there are no transitions possible for the emission. But for CH4 having 4 hydrogen atoms I don't think that this will be the case right?
2. The energy that is needed to excite electrons from the second shell to higher shells in order for them to relax again is used for the decomposition of the CH4 molecules. Thus the emissionpeaks decrease because the transition is getting less probable. The energy is more efficiently used for the decomposition of the CH4 molecules.
I got these two possible explanations at the moment. Can you help me with that?
Thank you very much
It's both, actually.
For the first part of your own explanation: You have indeed four H atoms bound to the methane but they have totally different energy levels than the H2 or H atom, so (as you assumed correctly) you would have to invest some energy first to decompose the methane and then excite the resulting H to your Balmer-series transition levels.
However, it gets even more complicated because you won't just decompose CH4 and form C and 2 H2 but there is a lot of organic chemistry going on in the plasma, which means that you will get even more complex organic compounds that also do not have the Balmer transitions (it might be alcohols, polymers, etc...).
Hi
In order to understand what is the main source of H alpha in your plasma you need to characterize its microscopic parameters, electron density and temperature. The chemistry of H2/CH4 plasma is rather complex and there are hundreds of papers devoted to it. As an example, may I recommend to you our 1993 and 2011 papers: J. Appl. Phys. 73 (2), 15 January 1993, JOURNAL OF APPLIED PHYSICS 110, 053302 (2011)
In the first one we address the origin of Ha and in the second one the decomposition of methane in a RF plasma...
A full description of the processes leading to Ha formation can be found in the Eirene data base, http://www.eirene.de/
W.J. Goedheer already suggested, and I also believe the main cause of the emission decrease is a reduction in the electron temperature when CH4 is added. If your plasma is sustained at atmospheric pressure, then the EEDF (electron energy distribution function) is expectedly Maxwellian. Then a small reduction in the plasma temperature will result in a significant decline of the energetic electrons capable of exciting the Balmer transitions. A total number of electrons will also decrease at lower temperature.
Thank you for your answers.
So if I understand it right, due to the CH4-molecules in the plasma, the average kinetic energie of the elctrons will decrease due to impacts, making the transitions in the hydrogen less probable.
Please note tat not only the excitation of H2 decreases at lower temperatures but also the formation of these molecules from methane decreases (cracking) ...
Okay I get the decrease of the balmer series with the increase of CH4. But if I observe the plasma power I see an increase in plasma power for higher amounts of CH4 (see attached file). What could be the reason for the increase in plasma power while the Balmer-Serie keeps goinf down with increasing CH4 amount?
If we have to summarize the gas-phase chemistry of H2/CH4 plasmas, then I will summarize it as, in the presence of a small addition of CH4 in hydrogen plasma, at fixed discharge conditions:
a) the electron temperature will decrease due to additional collisional processes of 1-C neutral species (CH4, CH3, CH2, CH and C) and 2-C species (C2H6, C2H5, C2H4, C2H3, C2H2, C2H and C2) with energetic electrons in the plasma.
b) Electron density will increase because all the interesting C-containing species have lower ionization energies than the hydrogen atom (13.6 eV) and hydrogen molecular ions (15.4 eV), therefore it is more likely that the electron density will increase with the addition of CH4.
c) Gas-temperature will increase because the thermal conductivity of CH4 is significantly lower than the H2; therefore heat is thus less efficiently transported to the side wall. This will confine to hotter plasma core and correspondingly higher gas temperature leading to thermal dissociation of H2 molecules.
d) H-atom density will increase significantly in H2/CH4 plasmas due to an increase of dissociation of hydrogen molecules (e.g EI dissociation, thermal dissociation and also due to CH4 dissociation kinetics via successive dehydrogenation processes).
Above all, it is
necessary to keep in mind that if you are using DC plasma then deposition/contamination of carbon on the wall/electrodes will considerably affect intensity changes.
Best wishes,
SM.
@SM: thank you very much for your contribution. At this moment I would totally agree with all of your answers. I explained it the same way you stated in my thesis. It is good to know that I wasn't completely wrong about it :)
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