Determination of N2(X) vibrational level populations and rotational temperatures using CARS in a D.C. low pressure discharge

For the first time, vibrational level population distributions of a low pressure D.C. flowing discharge were measured up to ν = 14 using Coherent Anti-Stokes Raman Scattering (CARS). The vibrational populations of the ground state N2(X 1Σ+g, ν) were obtained by recording rotationally resolved CARS spectra of the different vibrational bands. In the discharge region, the vibrational distributions have a non Boltzmann behaviour, as expected. The vibrational parameters were found to lie in the range 4 000-5 000 K, and rotational temperature to be about 550 K. Similar results were observed in the post-discharge region up to ν = 10. The vibrational and rotational temperatures were about 3 000 and 350 K respectively. The problems experienced while recording CARS spectra in low density media are discussed and comparisons with previously published theoretical models are presented.