Isomer-specific kinetics of the C ⁺ + H ₂ O reaction at the temperature of interstellar clouds

Abstract

The reaction C+ + H2O → HCO+/HOC+ + H is one of the most important astrophysical sources of HOC+ ions, considered a marker for interstellar molecular clouds exposed to intense ultraviolet or x-ray radiation. Despite much study, there is no consensus on rate constants for formation of the formyl ion isomers in this reaction. This is largely due to difficulties in laboratory study of ion-molecule reactions under relevant conditions. Here, we use a novel experimental platform combining a cryogenic buffer-gas beam with an integrated, laser-cooled ion trap and high-resolution time-of-flight mass spectrometer to probe this reaction at the temperature of cold interstellar clouds. We report a reaction rate constant of k = 7.7(6) × 10−9 cm3 s−1 and a branching ratio of formation η = HOC+/HCO+ = 2.1(4). Theoretical calculations suggest that this branching ratio is due to the predominant formation of HOC+ followed by isomerization of products with internal energy over the isomerization barrier.

Inverse kinetic isotope effects in the charge transfer reactions of ammonia with rare gas ions

“…Systematic uncertainties in pressure gauge readings are difficult to quantify and are not included in our reported uncertainties. 21,22 Note that this systematic uncertainty does not affect the relative reaction rate coefficients recorded for each reaction system, as we are using the same methodology and ion gauge for each set of experimental measurements. (See the Supporting Material for more details.)…”

Theoretical studies on the kinetics and dynamics of the BeH+ + H2O reaction: comparison with the experiment

“…[1][2][3][4][5][6][7][8][9][10] Much effort has been made to study their energetics, kinetics and dynamics in the past several decades. [11][12][13][14][15][16][17] The long-range attractive interaction between ion charge and other neutral atoms or molecules was recognized to usually give rise to large reactive scattering cross-sections at low or ultralow temperatures (or collision energies). 4,18 If the colliders are captured by a deep potential well and form a long-lived intermediate, the reaction often behaves as statistical due to significant energy randomization, which can be reliably described by capture theories, 19,20 such as the Langevin capture theory.…”

Rotational Cooling Effect on the Rate Constant in the CH3F + Ca+ Reaction at Low Collision Energies

“…Recently, measurements have been carried out with combined use of lowtemperature polar molecular beam generation and ion trapping: The rate constants and branching ratios of C + + H 2 O → HOC + /HCO + + H were measured at a reaction temperature of about 20 K by using sympathetically cooled C + ions and buffer gas-cooled H 2 O molecular beams. 23 Very recently, Zhelyazkova et al 24,25 have utilized a mergedsupersonic beam of atoms (or molecules) in Rydberg states and a Rydberg Stark decelerator/deflector to obtain a cold ion source. They have succeeded in studying the reactions between ions (He + , H 2 + ) and polar molecules (CH 3 F, ND 3 , NH 3 ) at temperatures ranging from 0 to several tens of kelvin, and further in measuring the branching ratios of the products.…”