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

In situ Fourier transform infrared (FTIR) spectroscopy and mass spectrometr y have been used to investigate the gas phase of an expanding thermal argon plasma into which hydrocarbons are injected. With both techniques it is po ssible 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 adva ntages and disadvantages. To determine absolute densities of different spec ies in the plasma by means of mass spectrometry it is necessary to deconvol ute the obtained mass spectra, whereas by means of FTIR spectroscopy differ ent species are easily recognized by their infrared absorption at specific positions in the absorption spectrum. With mass spectrometry the gas compos ition is measured locally, i.e. close to the gas extraction point. On the o ther hand with FTIR spectroscopy all particles in the infrared beam are inc luded in the measurement. When both techniques are combined most of the ind ividual disadvantages cancel, leading to a very powerful plasma diagnostic tool. Aside from the power resulting from the combination of both technique s, FTIR spectroscopy data also contain information on the temperature of th e stable species inside the plasma. A method to extract the rotational part icle temperature (within 100 K) by means of infrared absorption spectra sim ulation is presented.