Temperature dependence of binary and ternary recombination of D-3(+) ions with electrons

Flowing and stationary afterglow experiments were performed to study the recombination of D-3(+) ions with electrons at temperatures from 77 to 300 K. A linear dependence of apparent (effective) binary recombination rate coefficients on the pressure of the helium buffer gas was observed. Binary (D-3(+)+e(-)) and ternary (D-3(+)+e(-)+He) recombination rate coefficients were derived. The obtained binary rate coefficient agrees with recent theoretical values for dissociative recombination of D-3(+). We describe the observed ternary process by a mechanism with two rate determining steps. In the first step, a rotationally excited long-lived neutral D-3* is formed in D-3(+)-e(-) collisions. As the second step, the D-3* collides with a helium atom that prevents autoionization of D-3*. We calculate lifetimes of D-3* formed from ortho-, para-, or metastates of D-3(+) and use the lifetimes to calculate ternary recombination rate coefficients

Temperature dependence of binary and ternary recombination of H3+ ions with electron

We study binary and the recently discovered process of ternary He-assisted recombination of H3+ ions with electrons in a low temperature afterglow plasma. The experiments are carried out over a broad range of pressures and temperatures of an afterglow plasma in a helium buffer gas. Binary and He-assisted ternary recombination are observed and the corresponding recombination rate coefficients are extracted for temperatures from 77 K to 330 K. We describe the observed ternary recombination as a two-step mechanism: First, a rotationally-excited long-lived neutral molecule H3* is formed in electron-H3+ collisions. Second, the H3* molecule collides with a helium atom that leads to the formation of a very long-lived Rydberg state with high orbital momentum. We present calculations of the lifetimes of H3* and of the ternary recombination rate coefficients for para and ortho-H3+. The calculations show a large difference between the ternary recombination rate coefficients of ortho- and para-H3+ at temperatures below 300 K. The measured binary and ternary rate coefficients are in reasonable agreement with the calculated values.Comment: 15 page

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

Binary And Ternary Recombination Of H3\u3csup\u3e+\u3c/sup\u3e And D 3\u3csup\u3e+\u3c/sup\u3e Ions With Electrons In Low Temperature Plasma

Measurements of recombination rate coefficients of binary and ternary recombination of H3+ and D3+ ions with electrons in a low temperature plasma are described. The experiments were carried out in the afterglow plasma in helium with a small admixture of Ar and parent gas (H2 or D2). For both ions a linear increase of measured apparent binary recombination rate coefficients (αeff) with increasing helium density was observed: αeff = αBIN + KHe[He]. From the measured dependencies, we have obtained for both ions the binary (αBIN) and the ternary (KHe) rate coefficients and their temperature dependence. For the description of observed ternary recombination a mechanism with two subsequent rate determining steps is proposed. In the first step, in H3++ e- (or D3++ e-) collision, a rotationally excited long-lived Rydberg molecule H3* (or D 3*) is formed. In the following step H 3* (or D3*) collides with a He atom of the buffer gas and this collision prevents autoionization of H3* (or D3*). Lifetimes of the formed H3* (or D3*) and corresponding ternary recombination rate coefficients have been calculated. The theoretical and measured binary and ternary recombination rate coefficients obtained for H3+ and D3+ ions are in good agreement. © 2010 Taylor & Francis

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