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The experimental determination of the reaction rate coefficients for production and destruction of HCN+ and HNC+ in collision with H2 is presented. A variable-temperature, 22-pole radio frequency ion trap was used to study the reactions in the temperature range 17–250 K. The obtained rate coefficients for the reaction of CN+ and HCN+ with H2 are close to the collisional (Langevin) value, whereas that for the reaction of HNC+ with H2 is quickly decreasing with increasing temperature. The product branching ratios for the reaction of CN+ with H2 are also reported and show a notable decrease of the HNC+ product with respect to the HCN+ product with increasing temperature. These measurements have consequences for current astrochemical models of cyanide chemistry, in particular, for the HCNH+ cation. [ABSTRACT FROM AUTHOR]

The rate coefficients k1 and k2 of the reaction $ \hbox{OD}^- + \hbox{HD} \mathop{\rightleftarrows}\limits_{k_2}^{k_1} \hbox{OH}^- + \hbox{D}_2 $ OD − + HD ⇄ k 2 k 1 ⁡ OH − + D 2 in the forward and reverse direction were measured in the temperature ranges of 15 K–273 K and 95 K–300 K, respectively. The experiments were carried out in a temperature-variable cryogenic linear 22-pole radio-frequency ion trap instrument. The temperature dependence of the equilibrium constant $ { {K_{{\rm eq}12}}}(T) $ K e q 12 (T) obtained from the measured data is in good agreement with the value calculated using the detailed balance principle under the Born-Oppenheimer approximation. The enthalpy and the entropy of the reaction obtained from the linear Van't Hoff equation are $ { {\Delta H}} = (20 \pm 2 \pm 3^{{\rm sys}})\,{\rm meV} $ Δ H = (20 ± 2 ± 3 s y s ) m e V and $ { {\Delta S}} = (5.9 \pm 1.8 \pm 1.4^{{\rm sys}})\,{\rm J}/{\rm mol}\,{\rm K} $ Δ S = (5.9 ± 1.8 ± 1.4 s y s ) J / m o l K , respectively. [ABSTRACT FROM AUTHOR]

Recombination of N2H+ ions with electrons was studied using a stationary afterglow with a cavity ring-down spectrometer. We probed in situ the time evolutions of number densities of different rotational and vibrational states of recombining N2H+ ions and determined the thermal recombination rate coefficients for N2H+ in the temperature range of 80–350 K. The newly calculated vibrational transition moments of N2H+ are used to explain the different values of recombination rate coefficients obtained in some of the previous studies. No statistically significant dependence of the measured recombination rate coefficient on the buffer gas number density was observed. [ABSTRACT FROM AUTHOR]

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.

Detailed measurements employing a combination of a cryogenic flowing afterglow with Langmuir probe (Cryo-FALP II) and a stationary afterglow with near-infrared absorption spectroscopy (SACRDS) show that binary electron recombination of para-H+3 and ortho-H+3 ions occurs with significantly different rate coefficients, pabin and oabin, especially at very lowtemperatures. The measurements cover temperatures from 60Kto 300 K. At the lowest temperature of 60 K, recombination of para-H+3 is at least three times faster than that of ortho-H+3 (pabin = (1.8 ± 0.4) × 10-7 cm³ s-1 vs. oabin = (0+5-0) × 10-8 cm³ s-1). [ABSTRACT FROM AUTHOR]

We studied the reaction of doubly charged carbon C2+ (C iii) with molecular hydrogen, a possible source of the high, unexplained abundances of interstellar CH+. The experiment was carried out using the cryogenic linear 22-pole radio frequency ion trap. The measured reaction rate coefficient amounts to (1.5 ± 0.2) × 10−10 cm3 s−1, nearly independently of the covered temperature range from 15 to 300 K. In the product distribution study, the C+ ion was identified as the dominant product of the reaction. For the CH+ production, we determine an upper limit for the reaction rate coefficient at 2 × 10−12 cm3 s−1. [ABSTRACT FROM AUTHOR]

Recombination of D3+ ions with electrons at low temperatures (80-130 K) was studied using spectroscopic determination of D3+ ions density in afterglow plasmas. The use of cavity ring-down absorption spectroscopy enabled an in situ determination of the abundances of the ions in plasma and the translational and the rotational temperatures of the recombining ions. Two near infrared transitions at (5792.70 ± 0.01) cm-1 and at (5793.90 ± 0.01) cm-1 were used to probe the number densities of the lowest ortho state and of one higher lying rotational state of the vibrational ground state of D3+ ion. The results show that D3+ recombination with electrons consists of the binary and the third-body (helium) assisted process. The obtained binary recombination rate coefficients are in agreement with a recent theoretical prediction for electron-ion plasma in thermodynamic equilibrium with αbin(80 K) = (9.2 ± 2.0) × 10-8 cm3 s-1. The measured helium assisted ternary rate coefficients KHe are in agreement with our previously measured flowing afterglow data giving a value of KHe(80 K) = (1.2 ± 0.3) × 10-25 cm6 s-1. [ABSTRACT FROM AUTHOR]

Measurements in H3+ afterglow plasmas with spectroscopically determined relative abundances of H3+ ions in the para-nuclear and ortho-nuclear spin states provide clear evidence that at low temperatures (77-200 K) para-H3+ ions recombine significantly faster with electrons than ions in the ortho state, in agreement with a recent theoretical prediction. The cavity ring-down absorption spectroscopy used here provides an in situ determination of the para/ortho abundance ratio and yields additional information on the translational and rotational temperatures of the recombining ions. The results show that H3+ recombination with electrons occurs by both binary recombination and third-body (helium) assisted recombination, and that both the two-body and three-body rate coefficients depend on the nuclear spin states. Electron-stabilized (collisional-radiative) recombination appears to make only a small contribution. [ABSTRACT FROM AUTHOR]

We present studies on the thermalisation of H3+ ions in a cold He/Ar/H2 plasma at temperatures 30–70 K. We show that we are able to generate a rotationally thermalised H3+ ensemble with a population of rotational and nuclear spin states corresponding to a particular ion translational temperature. By varying the para-H2 fraction used in the experiment we are able to produce para-H3+ ions with fractional populations higher than those corresponding to thermodynamic values. At 35 K, only the lowest rotational states of para and ortho H3+ are populated. This is the first step towards experimental studies of electron–molecular ion recombination processes with precisely specified quantum states at astrophysically relevant temperatures. [ABSTRACT FROM AUTHOR]

The recombination of deuterated trihydrogen cations with electrons has been studied in afterglow plasmas containing mixtures of helium, argon, hydrogen and deuterium. By monitoring the fractional abundances of H3+, H2D+, HD2+ and D3+ as a function of the [D2]/[H2] ratio using infrared absorption observed in a cavity ring down absorption spectrometer (CRDS), it was possible to deduce effective recombination rate coefficients for H2D+ and HD2+ ions at a temperature of 80 K. From pressure dependences of the measured effective recombination rate coefficients the binary and the ternary recombination rate coefficients for both ions have been determined. The inferred binary and ternary recombination rate coefficients are: αbinH2D(80 K) = (7.1 ± 4.2) × 10−8 cm3 s−1, αbinHD2(80 K) = (8.7 ± 2.5) × 10−8 cm3 s−1, KH2D(80 K) = (1.1 ± 0.6) × 10−25 cm6 s−1 and KHD2(80 K) = (1.5 ± 0.4) × 10−25 cm6 s−1. [ABSTRACT FROM AUTHOR]

Stationary afterglow (SA) experiments with cavity ring down absorption spectrometer (cw-CRDS) have been used to study recombination of H+3 ions with electrons. To characterize the plasma during the afterglow we monitored the time evolution of density of He2 excited dimers (a³Σ+u) 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 H+3 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 H+3 and D+3 dominated plasma. [ABSTRACT FROM AUTHOR]