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Use of in situ FTIR spectroscopy and mass spectrometry in an expanding hydrocarbon plasma

In situ Fourier transform infrared (FTIR) spectroscopy and mass spectrometry have been used to investigate the gas phase of an expanding thermal argon plasma into which hydrocarbons are injected. With both techniques it is possible to determine the depletion of the precursor gas and the densities of new species formed in the plasma, each of the techniques with its own advantages and disadvantages. To determine absolute densities of different species in the plasma by means of mass spectrometry it is necessary to deconvolute the obtained mass spectra, whereas by means of FTIR spectroscopy different species are easily recognized by their infrared absorption at specific positions in the absorption spectrum. With mass spectrometry the gas composition is measured locally, i.e. close to the gas extraction point. On the other hand with FTIR spectroscopy all particles in the infrared beam are included in the measurement. When both techniques are combined most of the individual disadvantages cancel, leading to a very powerful plasma diagnostic tool. Aside from the power resulting from the combination of both techniques, FTIR spectroscopy data also contain information on the temperature of the stable species inside the plasma. A method to extract the rotational particle temperature (within 100 K) by means of infrared absorption spectra simulation is presented.