The energy ecient excitation of CO2 in atmospheric pressure plasmas
may be a method to generate solar fuels from renewable energies. This energy eciency
can be very high, if only specic states of the molecules in the plasma are populated
creating a strong non-equilibrium. This requires a specic design of the plasma source,
method of plasma excitation and choice of gases and admixtures. In this paper, nonequilibrium
excitation and dissociation of CO2 in an atmospheric pressure helium RF
plasma jet is analysed for varying absorbed plasma power and admixture levels of
CO2. The concentrations of CO2 and of CO, as well as the vibrational and rotational
temperatures of the possible degrees of freedom of the molecules are evaluated by
Fourier transform infrared spectroscopy (FTIR). The molecular rotational vibrational
spectra are modelled based on Maxwell-Boltzmann state populations using individual
temperatures for each degree of freedom. A strong non-equilibrium excitation of CO2
and CO has been found. Whereas the rotational temperatures are 400 K or below, the
vibrational temperature for CO reaches values up to 1600 K and that of the asymmetric
vibration of CO2 of 700 K. The dependence of these excitation temperatures on plasma
power and admixture level is rather weak. The mass balance, the energy and conversion
eciency are consistent with a very simple chemistry model that is dominated by CO2
dissociation via Penning collisions with helium metastables. A conversion effciency up
to 30 % and an energy effciency up to 10 % is observed in the parameter range of the
experiment.