I am interesting in this question and do not know right answer. Nonetheless I would like to comment current discussion.

1) Dissociation energy and threshold of dissociation by electron impact are not the same.

2) Absence of the spectra does not mean absence of the molecules. Absence of the molecular oxygen spectrum in visible range (300-900 nm) is usual for low pressure discharges. At atmospheric pressure it is detected seldom...

3) LTE spectrum is not suitable for interpretation of low-temperature DBD discharge.

4) I believe that the levels excited by electron impact are emitting in other spectral ranges (VUV and IR).

I think, you have to be a little bit more precise about your kind of discharge, but in principle the ionisation energy of O2 is 12 eV while the dissociation energy of the oxygen molecule is only 5.2 eV and the oxygen atom has 13.6 eV ionisation energy. Hence the O2 molecule will first be dissociated and then ionised.

However, you will have to look up the values for the excited states, but generally speaking the excitation energy has to be lower than the ionisation energy for a given particle. Maybe the the wavelength of the energy transitions does not lie within the spectral range of your spectrometer, but again, here it would be usefull to have a more detailed description of your experiment.

Right, I think that the relative low dissociation energy of O2 molecule is at the origin of your problem, thank to Johannes,

As far as I understand, you were asking about neutral oxygen species and their emission in the normal UV, which are easily excited and seen in low pressure plasmas. At ambient pressure, a possibilty could be that the participating states are not excited. But I would rather consider quenching, for which you should consider all molecules having states close to the ones of molecular oxygen. Therefore, it is also important at which type of discharge you were looking at. Consider also the pressure-dependent broadening.

Thanks all of you for answers

Here I am considering atmospheric pressure non-thermal air plasma generated by DBD technique. The emission spectrum is taken in the range of 300-900nm.

The O2 schumann-runge (B-X) bands fall in this range (175-535)nm. The estimated electron temperature for my system is around 2eV.

I think at this temperature these bands are either overlapped by stronger N2(C-B) bands (268-546). The attached document shows a simulation for LTE atmospheric pressure air plasma where at temperature above 5000K molecular oxygen bands are not seen and N2(C-B) & N2+(B-X) are seen.

I would like to know your views on this. Also the temperature dependence on emission characterstics has to be better understood.

I am interesting in this question and do not know right answer. Nonetheless I would like to comment current discussion.

1) Dissociation energy and threshold of dissociation by electron impact are not the same.

2) Absence of the spectra does not mean absence of the molecules. Absence of the molecular oxygen spectrum in visible range (300-900 nm) is usual for low pressure discharges. At atmospheric pressure it is detected seldom...

3) LTE spectrum is not suitable for interpretation of low-temperature DBD discharge.

4) I believe that the levels excited by electron impact are emitting in other spectral ranges (VUV and IR).

Dr. Andrei Pipa,

Can u please elaborate on (1) mentioned in your answer.

Hydrogen bond energy 4.52 eV

http://en.wikipedia.org/wiki/Bond-dissociation_energy

Threshold for dissociation by electron impact almost 10 eV

http://journals.aps.org/pra/pdf/10.1103/PhysRevA.12.1340 (see table1 and fig.2)

Dissociation energy related to separation of the nuclear to a certain distance. Incident electron could not efficiently interact with nuclear of the molecule. It would excite a new electronic state, which can be repulsive (or bound) and have often energy higher then dissociation level.

thanks Dr.Andrei

The spontaneous emission Einstein coefficients of molecular oxygen bands are very weak compared to molecular nitrogen. This is the main reason why it is very difficult to observed O2 bands in emission when some nitrogen gas is added. For low temperature plasma, N2 bands will dominate the plasma spectrum and O2 is not detected. On the other hand atomic O is usually easily seen even if only minute amounts of it are present in the plasma.

@Emile. At low pressure in pure O2 the molecular bands in VIS are not detected (Pearse and Gaydon). It should not be any influence of N2. From other side, I have seen very strong O2 bands at atmospheric pressure, so the transition probabilities should not be too weak, but experimental conditions were rather specific.

Dear Andrei,

It is not due to an influence of N2. I agree (although there might be some quenching going on there as well). I did not mean that. It is just that these O2 bands are not meant to be seen in most cases. And adding N2 will produce very strong features in the plasma spectrum that will make impossible to see any oxygen bands if there were any... i.e. the initial case where a pure O2 plasma is considered.

@Emile. I am agree, thank you for the extra comment.

One of the possible answer is the change of de-excition of excited state molecular O2. At low pressure, the main channel for de-excition is by spontaneous radiation transition, thus the emission spectrum is observed. Howerver, at high pressure, the collision possibility was greatly increased and the collision quenching of excited O2 became the main channel for de-excition, so the emission spectrum could not be observed.

self absorption is a complex and i no not see extreme case of self absorption

the experimental observation of molecular oxygen band depends on the production and quanching rate, the existance of large number of rare gas such as Helium may help the observation at atomospheric pressure.

Do not quote me, I talk from 20 yrs ago memories ... But ... usually molecular spectra are due to rotational/vibrational bands, which compared to electronic levels ecitations have quite low energy separation.  ie they are mostly in the IR and FIR band - which air tends to absorb quite easily, except a few windows - I believe one is around 5 micron, the other around 10 micron

@Ion Laurentiu Tobos  

Homo-nuclear molecules (N2, O2, H2) do not have ro-vibronic transitions, only electronic-vibration-rotational transitions. IR windows mainly related to H2O and CO2 absorption bands.  

The most common molecular band is the Schuman Runga band system which starts around 190 nm. There are no electronic states in oxygen that will emit in the visible region. The famous classic book by Gaydon and Pears is the best source to find the molecular bands of the diatomic molecules. In addition, a book by Herzberg "Spectra and Structure of di-atomic molecules" published by von nostrad.

best of luck

If somebody is still looking for visible spectra of molecular oxygen I recommend

"The Spectrum of Molecular Oxygen" by Paul H. Krupenie, J. of Physical and Chemical Reference Data 1, 423 (1972); doi: 10.1063/1.3253101

Some new papers on this topic appeared also in the 90-s.