Rainer Johnsen

Measurements of the O+ + N2 and O+ + O2 reaction rates from 300°K to 2 eV

Rate coefficients for the O++N2 atom transfer and O++O2 charge transfer reactions are determined at thermal energies between ∼300 and ∼900 K in a heated drift tube–mass spectrometer apparatus. At 300 K the values k (O++N2)(=1.2±0.1) ×10−12 cm3/s and k (O++O2) = (2.1±0.2) ×10−11 cm3/s are obtained, with a ∼50% decrease in the reaction rates upon heating to 700 K. These results are in good agreement with heated flowing afterglow results, but the O++O2 thermal rate coefficients are systematically lower than equivalent Maxwellian rates inferred by conversion of nonthermal drift tube and flow–drift data.

Ion–Molecule Reactions Involving N2+, N+, O2+, and O+ Ions from 300°K to ∼1 eV

Several ion–molecule reactions of ionospheric interest have been studied in a drift‐tube–mass‐spectrometer apparatus for ions of mean energy from thermal energies to ∼ 1 eV. The measured rate coefficients in cubic centimeters/second are N++O2: 5 × 10−10 from 0.039–0.9 eV; N2++O2: 6 × 10−11 at 0.039 eV, decreasing to ∼ 8 × 10−12 at 0.9 eV; O2++NO: 3 × 10−10 from 0.039–1.6 eV; O++CO2: 1 × 10−9 at 0.039 eV, decreasing to 5 × 10−10 cm3/sec at 1.3 eV. The relevance of the first three reactions for the Earths ionosphere and of the last reaction for the Martian atmosphere is discussed briefly.

Ion–Molecule Reactions, He++O2 and He++N2, at Thermal Energies and Above

The rate coefficients for the reactions of He+ ions with N2 and O2 have been measured in a drift tube–mass spectrometer from thermal energies (300°K) to ∼ 0.1 eV. In addition to reaction rate determinations by the usual product‐ion “arrival spectrum” method, a new technique involving “additional residence time” for the parent ion is described which is applicable even when parent and product ions have equal mobilities. The total rate coefficient (i.e., the sum of dissociative and the ordinary charge transfer) for He++N2 reaction was found to be (1.0 + 0.3, − 0.2) × 10−9 cm3/sec at 300°K, decreasing to ∼ 7 × 10−10 cm3/sec at 0.1 eV. The corresponding coefficient for the He++O2 reaction was measured to be (8.5 + 2.5, − 2.0) × 10−10 cm3/sec and to be independent of energy over the same range. The ratio of dissociative charge transfer (with N+ or O+, respectively, as reaction products) to ordinary charge transfer (with N2+ or O2+, respectively, as products) was found to be 0.55 / 0.45 for He++N2 and 0.8 / 0.2 ...

The formation and breakup of NO+⋅N2 clusters in N2 at low temperatures

Drift tube–mass spectrometer apparatus have been used to determine the equilibrium constant K and the forward rate coefficient k+ for the clustering reaction NO++N2+N2?NO+⋅N2+N2. The values of K are 2×10−16, 1.5×10−18, and ?5×10−19 cm3 at 130, 180, and 220 K, respectively, with an estimated uncertainty of ∼50%. The values of k+ are 8×10−30 and ?10−30 cm6/sec at 130 and 220 K, respectively, with an estimated uncertainty of ∼30%. The energy of the NO+–N2 bond deduced from the equilibrium constant data is Eb=0.18±0.02 eV. These results are compared with other measurements, and implications for proposed D‐region reaction paths linking NO+ and H3O+⋅ (H2O)n ions are discussed.

Microwave afterglow measurements of the dissociative recombination of molecular ions with electrons

Abstract Microwave-afterglow measurements of the electron-temperature dependence of dissociative electron—ion recombination coefficients are subject to some recently discovered complications arising from non-uniformities of the microwave heating fields, inelastic collisions of electrons with molecular additives, and effects of vibrational excitation of ions and neutrals. This paper presents the results of recent experimental and theoretical work and examines consequences for earlier experimental data on electron—ion recombination. It appears that, in some cases, the electron temperatures attained by microwave heating were actually lower than had been thought. As a consequence, the inferred dependences on electron temperature of the recombination coefficients were weaker than the true dependences.

Three‐body association reactions of He+, Ne+, and Ar+ ions in their parent gases from 78 to 300 K

Three‐body association coefficients for the conversion of atomic He+, Ne+, and Ar+ ions to molecular ions have been determined using drift‐tube mass‐spectrometer techniques at gas temperatures from 78 to 300 K. The 300 K results for the conversion of He+ to He2+ agree well with those obtained by other investigators using various techniques; however at 78 K, our results disagree strongly with one of the previous determinations. In the cases of conversion of Ne+ to Ne2+ and Ar+ to Ar2+, the rate coefficients differ substantially for ground state ions in their 2P3/2 and 2P1/2 fine structure states; also, the temperature dependence of this effect is distinctly different for the two gases. It does not appear that available theories give an adequately quantitative description of these association processes.

Three‐body association reactions of H+ and H3+ ions in hydrogen from 135 to 300 K

Rate coefficients for the reactions H++2H2?h3++H2 [Reaction (1)] and H3++2H2?H5++H2 [Reaction (2)] have been determined at several temperatures in a drift‐tube mass spectrometer apparatus. Reaction (1) was found to have forward rate coefficients of (3.0±0.3) and (4.3±0.4) ×10−29 cm6/s at a temperature of 300 K and at temperatures between 135 and 160 K, respectively. Reaction (2) was found to have forward rate coefficients of (12±2), (9.5±3), (6.2±2.0), and (4.6±1.5) ×10−30 cm6/s at 156, 195, 203, and 210 K, respectively. Equilibrium constants for Reaction (2) were determined in the temperature range 176–320 K. The temperature dependence of the equilibrium constant indicates a dissociation energy of (0.35±0.03) eV for breakup of H5+ into H3+ and H2.

Electron-ion recombination rate coefficient measurements in a flowing afterglow plasma

Abstract The flowing-afterglow technique in conjunction with computer modelling of the flowing plasma has been used to determine accurate dissociative-recombination rate coefficients α for the ions O2+, HCO+, CH5+, C2H5+, H3O+, CO2+, HCO2+, HN2O+, and N2O+ at 295 K. We find that the simple form of data analysis that was employed in earlier experiments was adequate and we largely confim earlier results. In the case of HCO+ ions, published coefficients range from 1.1 × 10−7 to 2.8 × 10−7 cm3/s, while our measurements give a value of 1.9 × 10−7 cm3/s.

Measurements of ion‐molecule reactions of He+, H+, and HeH+ with H2 and D2

A drift tube‐mass spectrometer apparatus has been used to determine the rate coefficient, energy dependence, and product ions of the reaction He+ + H2. The total rate coefficient at 300 K is (1.1±0.1)×10−13 cm3/sec. The reaction proceeds principally (80%) by dissociative charge transfer to produce H+, with the small remainder going by charge transfer to produce H2+ and by atom rearrangement to produce HeH+. The rate coefficient increases slowly with increasing ion mean energy, reaching a value of 2.8×10−13 cm3/sec at 0.18 eV. The corresponding reaction with deuterium, He+ + D2, exhibits a value (5±1)×10−14 cm3/sec at 300 K. The reaction rates for conversion of H+ and HeH+ to H3+ on collisions with H2 molecules are found to agree well with results of previous investigations.

Recombination of electrons with H3+ and H5+ ions

Abstract The dissociative recombination coefficients α for capture of electrons by H3+ and H5+ ions have been determined as a function of electron temperature Te using a microwave afterglow-mass spectrometer apparatus. At ion and neutral temperatures Tu+ = Tn = 240 K, the coefficient α (H3+) is found to vary slowly with Te at first, decreasing from 1.6 × 10−7 cm3/s at Te = 240 K to 1.2 × 10−7 cm3/s at Te = 500 K, thereafter falling as Te−1 over the range 500 K ⩽ Te, ⩽ 3000 K. These results, which have a ± 20% uncertainty, agree satisfactorily over the common energy range (0.03–0.36 eV) with the recombination cross sections determined in merged beam measurements by Auerbach et al. At T+ = Tn = 128 K, the coefficient α(H5+) is found to be (1.8 ± 0.3) × 10−6 [Te(K)/300]−0.69 cm3/s over the range 128 K ⩽ Te ⩽ 3000 K, with a more rapid decrease, as Te−1, between 3000 K and 5500 K. The implications of these results for modelling planetary atmospheres and interstellar clouds are briefly touched on.