Study of two-body and three-body channels for the reaction of metastable helium atoms with selected atomic and molecular species

Abstract

This work reports the measurements of rate coefficients for excitation transfer reactions of metastable He(2 3S) atoms, produced in 0.3–4 atm of helium, with various reactants presenting a wide range of characteristics. In all cases studied, three-body reaction channels were identified with most probable values of rate coefficients lying in the range from 0.2 to 6.7×10−31 cm6 s−1, for Ne and N2O, respectively. These are generally more than one order of magnitude smaller than previously reported. The interpretation of this discrepancy in terms of a more reactive intermediate He2(Σ*) complex in dynamic equilibrium with the He(2 3S) population seems to be confirmed, and is analyzed in some detail.

The role of helium metastable states in radio-frequency driven helium–oxygen atmospheric pressure plasma jets: measurement and numerical simulation

“…exp(t /τ ) dt including the effective excitation rate, and τ = (k ion n O 2 ) −1 the effective He* lifetime due to the loss through Penning ionization. The effective lifetime τ ≈ 35 ns, calculated on the basis of the rate coefficient k ion = 2.54×10 −16 m 3 s −1 from [32], is shorter than the rf period, but longer than the width of the excitation pulses (≈15 ns). This explains the modulation of the He* density,…”

“…The formal solution of the rate equation for periodic boundary conditions, n He * (t) = n He * (t + T ), can be expressed as [32], is shorter than the rf period, but longer than the width of the excitation pulses (≈15 ns). This explains the modulation of the He* density, [34] as well as the temporal shift (≈10 ns) and broadening of the corresponding spatio-temporal structures with respect to the excitation pulses.…”

Spatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges in helium with nitrogen admixtures

“…This is caused by the decrease in the effective lifetime of helium metastable states participating in Penning ionization (7) of nitrogen admixtures, as already discussed in the previous section. With consideration of the rate coefficient k PI = 7 × 10 −11 cm 3 s −1 for Penning ionization of N 2 [41][42][43][44], and the total pressure of 500 mbar, the effective lifetime of He m decreases from about 10 µs in helium (assumption of about 100 ppm nitrogen impurities) to less than 1 µs in helium with 2000 ppm nitrogen admixture. Moreover, compared to the discharge in helium, the optical emission in (e) and (f) reveals a faster propagation of the ionization front as well as a smaller extent of the cathode fall region for the first intensive breakdown.…”

He(23 S 1) and He2(a 3Σu +) metastables densities measured in an RF-driven helium plasma using broadband absorption spectroscopy

“…Collins et al employed optical absorption spectroscopy to measure the quenching rate coefficients of He(2 3 S) in bi-and termolecular reactions with added molecules (O 2 /N 2 /H 2 O) in atmospheric pressure afterglow of helium excited by intense electron beam discharges [33,34]. Pouvesle et al have updated many of these reaction rate coefficients for both He(2 3 S) and He 2 (a 3 Σ u + ) metastable states at atmospheric pressure using laser spectroscopy and time-resolved emission of the first negative system of nitrogen, N 2 + (B-X), produced by Penning ionization by metastables, and extended these rate coefficients to include reactions with water as an impurity [35][36][37][38].…”