Search

The decay of Δv = −2 band sequence of N2 Second Positive emission system has been investigated in a N2 rf pulsed 500 Hz post discharge in the pressure range 0.1–3 Torr. The intensity of each band head of the sequence has been used to infer the N2(C, v′ = 0–4) relative vibrational distribution. The shape of the distribution varies significantly with the post discharge time and the pressure. A population inversion in the levels 0–1 and 2–3 characterizes the distributions measured at post discharge times higher than 100 μs. The distributions at the various post discharge times have been fitted by a model which takes into account three processes for the (C, v′) excitation: the electron impact from (X, v) states by fast electrons (a), from (A, w) state by slow electrons (b), and the pooling reaction N2(A, w) + N2(A, w′) (c). In the discharge the excitation is predominantly given by process (a). At 1 ms in the post discharge the electronic process (b) still survives, and, together with process (c), provides excitation to C state.

The decay of the N 2 (A 3 Σ + u ) metastable, which has been produced by pulsing a rf discharge fed with a He-N 2 mixture in a strainless steel parallel plate chamber, has been investigated by time-resolved LIF spectroscopy at various pressures and mixture compositions. The nitrogen metastable N 2 (A, v =4), detected by LIF through the (A, 4→B, 8) excitation and (B, 8→A, 5) emission, has a decay time of the order of hundreds of microseconds. In most cases, the decay cannot be fitted by a single-exponential law. A simple numerical model of the decay strongly suggests that in addition to diffusion, the main channel for N 2 (A, 4) quenching is the process N 2 (A, 4)+N 2 (X, v ⩾2)→N 2 (B, v ′)+N 2 (X, v ″).

The CO dissociation of a 5% COHe radiofrequency discharge has been analyzed as a function of CO residence time in discharge, in a reactor cooled at liquid-nitrogen temperature. The gas phase products (CO 2 , O, C 2 ) as well as CO 2 trapped and carbonaceous film deposited on the cooled walls of the reactor, were analyzed by emission spectroscopy and mass spectrometry. The composition of carbonaceous deposit was found to be like C 2 OC 3 O 2 , rather than C-like as generally assumed for COHe discharges. CO 2 and C 2 OC 3 O 2 formation yields increase with residence time, leading to CO loss up to about 80%, at a residence time of a few seconds. The first step of CO dissociation occurs mainly by electron impact, then the hetrogeneous reactions CO+O+wall→(CO 2 ) w and CO+C w →(C 2 O) w effectively enhance the CO loss. In fact an autocatalytic CO dissociation, which arises by the sink of dissociation products on the reactor cooled walls has been measured. The high CO depletion in COHe mixture, as CO residence time is increasing, leads to an increase of electron mean energy which accelerates the electron impact CO dissociation.

Temporal evolution of CO vibrational distribution pumped by vibrationally excited nitrogen molecules has been experimentally determined by side-on infrared emission spectroscopy. Cooled and uncooled radiofrequency N2 discharges have been utilized to pump vibrational energy in N2, which then does excite CO in the post-discharge regime. CO2 formed during the contact time has been detected by a mass-spectrometric technique. The experimental results show the different stages of the relaxation of CO, merely the pumping, the redistribution and the loss of vibrational quanta introduced by N2. The results have been rationalized by means of a theoretical model which qualitatively reproduces the experimental relaxation of CO distributions as well as the production of CO2.

CO—He RF discharge mixtures have been analyzed at various pressures and feed compositions by IR and near UV—VIS emission spectroscopy. The analysis of CO first-overtone emissions has shown the non-Boltzmann character of the ground-state vibrational distributions. The dissociation of CO has been revealed by the presence of various emitting discharge products (CO2, C2, O). The effect on the ground vibrational distributions of the various energy exchange processes (e—V, V—V, V—T) has been discussed. CO vibrational distributions in X 1 Σ+ and A 1∏ states have also been correlated.

Electron energy distribution functions (EDF) under N 2 discharge and post-discharge conditions have been calculated by self-consistently solving the Boltzmann equation, the vibrational master equation (including dissociation) and the kinetics of the most important electronic states of N 2 . The results show that the relaxation of EDF in the post-discharge is strongly linked to the residence time of N 2 in the discharge, which determines the initial conditions. In particular at low residence times the role of metastable states in affecting EDF through superelastic electronic collisions (SEC) overcomes the corresponding one from superelastic vibrational collisions (SVC). On the contrary, SVC dominate at long residence times, when a well-developed vibrational distribution has been built up by the discharge. At intermediate residence times both SVC and SEC affect the relaxation of EDF, which in general presents a non-monotonic behaviour.

The non-equilibrium vibrational kinetics of CO pumped by vibrationally excited N2 has been calculated by solving the vibrational master equations for both N2 and CO molecules, linked by V-V (N2 CO) energy exchange processes. The results have been obtained for different values of the parameters governing the kinetics (in particular gas temperature, initial vibrational content of N2, different mixing ratios N2/CO). Emphasis is also given to dissipative channels present in CO, due to bimolecular reactions involving highly vibrationally excited CO molecules. The results shows the essential features of the temporal evolution of CO and N2 vibrational distribution as well as the strong coupling existing between them.