Understanding the vibrational distribution in CO$_{\mathrm{2}}$ microwave plasma for production of carbon neutral fuels, using time resolved in-situ spectroscopy

A microwave plasma could prove to be a cost effective way of converting CO$_{\mathrm{2}}$ to CO. The efficiencies of such a reactor have been shown to be very high, up to 90{\%}. It is currently understood that the preferable vibrational excitation by plasma electrons plays a key role in the efficient CO$_{\mathrm{2}}$ conversion. In the case that Vibrational-Vibrational (VV) relaxation times are much shorter than Vibrational-Translational (VT) relaxation times, molecules are vibrationally excited via intermolecular collisions until the dissociation energy is reached. As the VT-relaxation rate increases with temperature, a low temperature is needed to promote an overpopulation of high vibrational levels. To reduce the temperature, The microwave power was pulsed. Raman-scattering was employed to measure the temperature in the radial center and sides of the plasma, over an axial distance of a few centimeter. The infrared absorption spectrum of the CO$_{\mathrm{2}}$-plasma is recorded using an in-situ step-scan FTIR spectrometer. The absorption bands of higher vibrational levels are visible lower wavenumbers, down to 2000 cm$^{\mathrm{-1}}$. This enables us to look at the evolution of the densities of the vibrational levels. It was found that the vibrational temperature increased during plasma ON-time.