We are recently working with a plasma mixture of argon and CF4 gases produced in atmospheric pressure by microwave plasma torch. We are doing optical emission spectroscopy (OES) of this plasma.
We have got all the major and minor argon lines as expected. But we could not find any trace of fluorine in the spectrum. Though we have got strong CN violet systems and C2 swan bands indicating that CF4 is cracked inside plasma. My problem is where and how fluorine is coming out from plasma? I know that CF, CF2, and CF3 has emission in UV region (150-300 nm). But we did not get any clear molecular band in that region.
Does anybody have experience working with CF4 OES spectrum? If yes, please help us to understand why this is happening here and how are we missing fluorine presence ?
OES interpretation at atmospheric pressure is always a tricky task. You have a very collisional plasma and various departures from equilibrium. So there can be various explanations why you don't see various species like you would expect at low pressure. For instance atomic F states are probably quenched almost completely by heavy particles collisions with Ar and other species... but they could recombine fast to form other species as well... it will depends much of the type of plasma you are using too. Did you try absorption measurements to try to detect directly any of the species you mentioned?
One thing is sure however: not detecting a species by OES does not mean that you don't have large quantities of it in its ground state.
Hi Emile, we are using a microwave plasma torch at atmospheric pressure. We are mixing Ar + CF4 gases to produce plasma.
We did not do any absorption measurements and directly took the emitted spectrum through a quartz window. Our flame length is about 4-5 cm and we placed optical fiber at two different locations. Spectrum is almost same.
Sorry, I did not understand what you said about quenching of atomic F states. We are very new to this field. We mainly do PECVD and electrical probe plasma diagnostics. Can you please help a little more ?
In the pure SF6 radiofrequency discharge we have seen many F I lines - the strongest is about 685 nm. Maybe you haven't got enough fluorine? CN and C2 bands are very strong...
At low pressure, I have performed measurements for F* emission at 703.8 nm and CF2* at 252.0 nm. Are you sure that your window is UV-transparent? How do you explain the strong CN* emission without N2?
Cedric Jaoul, most argon gases contain some admixture of nitrogen, which can be enough for CN band to be seen - it is not so surprising. Atmospheric pressure can mean also open source, nitrogen can then spread from everywhere around.
Dear Ewa, as I was assuming some possible issues, it was not a real question ;-). I wanted to know more details on the experimental conditions because gaz impurities or open-air plasma could explain the "consumption" or the "cover" of fluorine.
Hello Ewa and Cedric,
Thanks for these discussions. Actually as Ewa said, we are using commercial Ar cyllinder and operating microwave plasma torch in atmospheric pressure. We guess both of these may be source of N2 for CN bands.
As for window part, initially we had toughened glass window as we had no idea that CF* emissions in UV range. Now however, we are using QUARTZ window, so it is UV transparent. Our spectrometer can go from 200-1000nm. We have recently taken a spectrum and I could say more after analysis tomorrow.
Thanks for replying.
Fluorine is the third element in the periodic table having the highest ionization energy about 17.422 eV, and if you look at the strong Fluorine atomic lines in the NIST data base, they are all laying in the NIR region (670-780 nm) and all the given atomic spectral lines are having excitation energies around 14.6 eV. If you are looking for these F I lines then probably you may not able to get lines intensities with reason signal to noise ratios in argon plasma.
I am suggesting here few things which probably helps you -
1- If you are looking for these available spectral lines, the probability of excitation of these spectral lines in argon plasma is very low (see detail given in 2. & 3.), and if you are using alternative plasma gas (Ne or He) then detector you are using for spectral measurement should be sensitive in the region (670-780 nm).
2- The lowest atomic energy level of F (12.69 eV) is about 1 eV above from the argon metastable levels (11.55 & 11.72 eV), so there is no possibility that the collision of metastable atoms of plasma gas can be helpful in OES investigation of Fluorine atomic lines. Furthermore, there is also no possibility of charge transfer process in the case of argon plasma.
3- In any case, if you are interesting in emission spectrum of fluorine, then use of neon or helium as plasma gas would be useful instead of argon. The ionization energy of Ne and He are 21.564 eV and 24.587 eV respectively. More both gases are having higher metastable levels, therefore, there is more probability of getting excited emission lines for OES study in presence of these plasma gas instead of argon as plasma gas.
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