Surface vibrational relaxation of N$_{2}$ studied by infrared titration with time resolved Quantum Cascade Laser diagnostics

Relaxation of vibrationally excited nitrogen molecules on reactor walls is the most efficient N$_{2}$(v) loss mechanism in laboratory plasmas at pressures up to few tens of mbars. In the present study a new method for determination of the de-excitation probability $\gamma _{{\rm N}2}$ of vibrationally excited N$_{2}$ on different surfaces has been developed. A short dc discharge pulse was applied to a mixture containing 0.05-1{\%} of CO$_{2}$, N$_{2}$O or CO in N$_{2}$ at 1.3 mbar. Due to a very efficient vibrational coupling between N$_{2}$(v) and CO$_{2}$ (N$_{2}$O, CO), the vibrational excitation of these titrating molecules is an image of the vibrational excitation of N$_{2}$. In the afterglow, the vibrational relaxation was monitored \textit{in-situ} using quantum cascade laser absorption spectroscopy. The measurements were performed in a single discharge pulse without signal accumulation. Experimental results were interpreted in terms of a numerical model of non-equilibrium vibrational kinetics. The value of $\gamma _{{\rm N}2}$ was determined from the best agreement between the measured and calculated relaxation times. Using new technique the relaxation probability of N$_{2}$(v) was measured for SiO$_{2}$, TiO$_{2}$, Al$_{2}$O$_{3,}$ Pyrex and anodized aluminum.