Surface deactivation of vibrationally excited N_2 studied using infrared titration combined with quantum cascade laser absorption spectroscopy

摘要

The wall de-excitation probability γN_2 of vibrationally excited nitrogen molecules was determined using infrared (IR) titration with CO, CO_2 and N_2O. Gas mixtures of N_2 with 0.05-0.5% of CO (CO_2 or N_2O) were excited by a pulsed dc discharge at p = 133 Pa in a cylindrical discharge tube. During the afterglow, the vibrational relaxation of titrating molecules was monitored in situ with quantum cascade laser absorption spectroscopy. The value of γN_2 was deduced from measured vibrational relaxation times using a model of vibrational kinetics in N_2. It was found that adsorption of IR tracers on the surface may increase the value of γN_2 by a factor up to two, depending on the molecule and the surface material. It was demonstrated that N_2O is the most inert and reliable tracer and it was used for the determination of γN_2 on silica, Pyrex, TiO_2, Al_2O_3 and anodized aluminum. Pretreatment of the silica surface by low-pressure plasma was found to have a strong effect on the vibrational de-excitation. Values of γN_2 measured after O_2, Ar and N_2 plasma pretreatment of the same silica discharge tube were 5.7×10~(-4), 8.2 × 10~(-4) and 11 × 10~(-4), respectively. This study clearly demonstrates that the presence of adsorbed atoms and molecules on the surface may significantly alter the value of γN_2.