Surface vibrational relaxation of N2 studied by CO2 titration with time-resolved quantum cascade laser absorption spectroscopy

A new method for determination of the wall de-excitation probability gamma(N2) of vibrationally excited N-2 on different surfaces exposed to low-pressure plasmas has been developed. A short dc discharge pulse of only a few milliseconds was applied to a mixture containing 0.05-1% of CO2 in N-2 at a pressure of 133 Pa. Due to a nearly resonant fast vibrational transfer between N-2(v) and the asymmetric v(3) mode of CO2 the vibrational excitation of these titrating molecules is an image of the degree of vibrational excitation of N-2. In the afterglow, the vibrational relaxation of CO2 was monitored in situ using quantum cascade laser absorption spectroscopy. The experimental results were interpreted in terms of a numerical model of non-equilibrium vibrational kinetics in CO2-N-2 mixtures. Heterogeneous relaxation was the main quenching process of N-2(v) under the conditions of this study, which allowed determination of the value of gamma(N2) from the best agreement between the experiment and the model. The new method is suitable for gamma(N2) determination in a single plasma pulse with the discharge tube surface pretreated by a low-pressure plasma. The relaxation probability of the first vibrational level of nitrogen gamma(1) = (1.1 +/- 0.15) x 10(-3) found for Pyrex and silica is in reasonable agreement with the literature data. Using the new technique the N-2(v = 1) quenching probability was measured on TiO2 surface, gamma(1) = (9 +/- 1) x 10(-3). A linear enhancement of the N-2(v) wall deactivation probability with an increase in the admixture of CO2 was observed for all studied materials. In order to explain this effect, a vibrational energy transfer mechanism between N-2(v) and adsorbed CO2 is proposed.