H/D exchange in reactions of OH− with D2 and of OD− with H2 at low temperatures

Using a cryogenic linear 22-pole rf ion trap, rate coefficients for H/D exchange reactions of OH− with D2 (1) and OD− with H2 (2) have been measured at temperatures between 11 K and 300 K with normal hydrogen. Below 60 K, we obtained k1 = 5.5 × 10−10 cm3 s−1 for the exoergic reaction (1). Upon increasing the temperature above 60 K, the data decrease with a power law, k1(T) [similar] T−2.7, reaching ≈1 × 10−10 cm3 s−1 at 200 K. This observation is tentatively explained with a decrease of the lifetime of the intermediate complex as well as with the assumption that scrambling of the three hydrogen atoms is restricted by the topology of the potential energy surface. The rate coefficient for the endoergic reaction (2) increases with temperature from 12 K up to 300 K, following the Arrhenius equation, k2 = 7.5 × 10−11 exp(−92 K/T) cm3 s−1 over two orders of magnitude. The fitted activation energy, EA-Exp = 7.9 meV, is in perfect accordance with the endothermicity of 24.0 meV, if one accounts for the thermal population of the rotational states of both reactants. The low mean activation energy in comparison with the enthalpy change in the reaction is mainly due to the rotational energy of 14.7 meV contributed by ortho-H2 (J = 1). Nonetheless, one should not ignore the reactivity of pure para-H2 because, according to our model, it already reaches 43% of that of ortho-H2 at 100 K.Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich

Monitoring the removal of excited particles in He/Ar/H

Stationary afterglow (SA) experiments with cavity ring down absorption spectrometer (cw-CRDS) have been used to study recombination of H3+ ions with electrons. To characterize the plasma during the afterglow we monitored the time evolution of density of He2 excited dimers (a3Ʃu+) in plasmas in pure helium and in helium with small admixture of Ar and H2. By monitoring the plasma decay and its dependence on [H2] and [Ar] in the afterglow in pure He and in He/Ar/H2 mixture we estimated the rate of plasma thermalization in the temperature range of 80–300 K. The inferred deexcitation rate coefficients for reaction of helium metastable atoms with H2 were (0.9 ± 0.3) × 10−10 cm3 s−1, (1.9 ± 0.2) × 10−10 cm3 s−1 and (2.7 ± 0.2) × 10−10 cm3 s−1 at 80 K, 140 K and 300 K, respectively. The effective recombination rate coefficients for H3+ were evaluated from the decay of the electron number density. We propose the lower estimate for the saturation of the effective recombination rate coefficient in H3+ and D3+ dominated plasma

State selective study of H

Rate coefficients for nuclear spin state-specific recombination of H3+ ions with thermal electrons were measured using FALP and SA techniques at temperatures 77–300 K. For this purpose H2 gas with both thermal and enriched population of the para nuclear spin configuration was used. Measurements have shown that at 77 K para-H3+ exhibits five times higher binary recombination rate coefficient than ortho-H3+: (1.5 ± 0.4) × 10−7 vs. (3 ± 2) × 10−8 cm3s−1

Electron-ion recombination in low temperature hydrogen/deuterium plasma

The stationary afterglow with cavity ring down spectrometer (SA-CRDS) was used to study the recombination of H3+, H2D+, HD2+ and D3+ ions with electrons in low temperature (77–300 K) plasmas in He/Ar/H2/D2 gas mixtures. By measuring effective recombination rate coefficients (αeff) in plasma with mixtures of ions and their dependences on temperature and partial densities of He, H2 and D2, αeff (T, [He],[H2],[D2]), we determined binary (αbinH3, αbinH2D, αbinHD2, αbinD3) and ternary (KH3, KH2D, KHD2, KD3) recombination rate coefficients for H3+, H2D+, HD2+ and D3+ ions. For all four ions we observed very efficient He assisted ternary recombination which is comparable with binary recombination already at [He] =1 × 1017 cm−3. The removal of excited particles in afterglow plasma was monitored to obtain the plasma thermalisation rate at given experimental conditions. The inferred deexcitation rates for reaction of helium metastable atoms with D2 are kD2 (300 K)=(2.1 ± 0.3) × 10−10 cm3 s−1 and kD2 (140 K)=(1.3 ± 0.3) × 10−10 cm3 s−1

State selective study of H 3

Rate coefficients for nuclear spin state-specific recombination of H3+ ions with thermal electrons were measured using FALP and SA techniques at temperatures 77–300 K. For this purpose H2 gas with both thermal and enriched population of the para nuclear spin configuration was used. Measurements have shown that at 77 K para-H3+ exhibits five times higher binary recombination rate coefficient than ortho-H3+: (1.5 ± 0.4) × 10−7 vs. (3 ± 2) × 10−8 cm3s−1