Cross sections for the reactions of Ar+ with H2, D2, and HD to form ArH+ and ArD+ are measured using a new guided ion beam tandem mass spectrometer which affords an experimental energy range from 0.05 to 500 eV laboratory. The apparatus and experimental techniques are described in detail. Cross sections for H2 and D2 are found to be nearly identical over this entire energy range when compared at the same barycentric energy. The total HD cross section is the same as H2 and D2 at low energies, but differs significantly above 4 eV c.m., where product dissociation becomes important. The intramolecular isotope effect for reaction with HD exhibits a reversal at low energy, favoring the deuteride product below ∼0.14 eV c.m., and surprising nonmonotonic behavior at energies above 5 eV c.m. In all these systems, a new feature at higher energies is observed. This is interpreted as the onset of a product channel having an energy barrier of 8±1 eV. The room temperature rate constant derived from the data for the reac...

Translational energy dependence of Ar++XY→ArX++Y (XY=H2,D2,HD) from thermal to 30 eV c.m.

We present results from high-resolution three-dimensional simulations of turbulent interstellar gas that self-consistently follow its coupled thermal, chemical and dynamical evolution, with a particular focus on the formation and destruction of H 2 and CO. We quantify the formation time-scales for H 2 and CO in physical conditions corresponding to those found in nearby giant molecular clouds, and show that both species form rapidly, with chemical time-scales that are comparable to the dynamical time-scale of the gas. We also investigate the spatial distributions of H 2 and CO, and how they relate to the underlying gas distribution. We show that H 2 is a good tracer of the gas distribution, but that the relationship between CO abundance and gas density is more complex. The CO abundance is not well-correlated with either the gas number density n or the visual extinction Av : both have a large influence on the CO abundance, but the inhomogeneous nature of the density field produced by the turbulence means that n and A v are only poorly correlated. There is a large scatter in A v , and hence CO abundance, for gas with any particular density, and similarly a large scatter in density and CO abundance for gas with any particular visual extinction. This will have important consequences for the interpretation of the CO emission observed from real molecular clouds. Finally, we also examine the temperature structure of the simulated gas. We show that the molecular gas is not isothermal. Most of it has a temperature in the range of 10-20 K, but there is also a significant fraction of warmer gas, located in low-extinction regions where photoelectric heating remains effective.

Modelling CO formation in the turbulent interstellar medium

Protonated molecular hydrogen, H3+, is the simplest polyatomic molecule It is the most abundantly produced interstellar molecule, next only to H2, although its steady state concentration is low because of its extremely high chemical reactivity H3+ is a strong acid (proton donor) and initiates chains of ion-molecule reactions in interstellar space thus leading to formation of complex molecules Here, I summarize the understandings on this fundamental species in interstellar space obtained from our infrared observations since its discovery in 1996 and discuss the recent observations and analyses of H3+ in the Central Molecular Zone near the Galatic center that led to a revelation of a vast amount of warm and diffuse gas existing in the region

https://www.pnas.org/content/pnas/103/33/12235.full.pdf

Interstellar H(3)(

Publisher Summary This chapter discusses several experimental studies conducted on cluster ions. Cluster ions comprise a nonrigid assembly of components having properties between those of large gas phase molecules and the bulk condensed state. Research on the properties of cluster ions is valuable in obtaining a complete understanding of the forces between ions and neutral atoms or molecules, where, for example, data on energetics provide a direct measure of the depth of the potential well of interaction between the ion and the collection of neutral molecules. There are three types of experimental approaches to produce and study these cluster ions depending on different degrees of physical isolation, that is, clusters moving freely in space (gas phase studies), clusters supported upon a substrate surface, and clusters existing in solids and liquids. Moreover, the use of cluster ions for fuel injection into fusion reactors and cluster ions are also present in other high-temperature systems such as those designed as sources of energy employing magnets and hydrodynamics.

Experimental studies on cluster ions

Energetic particle precipitation (EPP) and ion chemistry affect the neutral composition of the polar middle atmosphere. For example, production of odd nitrogen and odd hydrogen during strong events can decrease ozone by tens of percent. However, the standard ion chemistry parameterization used in atmospheric models neglects the effects on some important species, such as nitric acid. We present WACCM-D, a variant of the Whole Atmosphere Community Climate Model, which includes a set of lower ionosphere (D-region) chemistry: 307 reactions of 20 positive ions and 21 negative ions. We consider realistic ionization scenarios and compare the WACCM-D results to those from the Sodankyla Ion and Neutral Chemistry (SIC), a state-of-the-art 1-D model of the D-region chemistry. We show that WACCM-D produces well the main characteristics of the D-region ionosphere, as well as the overall proportion of important ion groups, in agreement with SIC. Comparison of ion concentrations shows that the WACCM-D bias is typically within ±10% or less below 70 km. At 70–90 km, when strong altitude gradients in ionization rates and/or ion concentrations exist, the bias can be larger for some groups but is still within tens of percent. Based on the good agreement overall and the fact that part of the differences are caused by different model setups, WACCM-D provides a state-of-the-art global representation of D-region ion chemistry and is therefore expected to improve EPP modeling considerably. These improvements are demonstrated in a companion paper by Andersson et al.

/pdf/waccm-d-whole-atmosphere-community-climate-model-with-d-2ahtkksljv.pdf

WACCM-D—Whole Atmosphere Community Climate Model with D-region ion chemistry

A drift chamber mass spectrometer apparatus was used to determine products and rate coefficients at 300 K for reactions of CO2+, CO2CO2+ and H2O+ with H2, CH4, SO2, O2, C2H2, C2H4, COS, CS2, NH3, NO2 and NO. Charge transfer is the predominant reaction channel in most cases. Reaction efficiencies are discussed by comparison with rate coefficients calculated from the capture mechanism according to averaged dipole orientation theory.

Reactions of CO2+, CO2CO2+ and H2O+ ions with various neutral molecules

Ion–molecule reactions in argon– hydrogen mixtures were studied as a function of total and partial pressures, using a drift chamber mass spectrometer. The rate coefficient of the title reaction was found to depend on pressure, decreasing as the total pressure is increased from a value of 3.5×10−10 at low pressure to 3.5×10−11 cm3  molecule −1 s−1 at pressures greater than 0.8 Torr. This behavior is explained by the presence of vibrationally excited ArH+ ions formed in the reaction Ar+ + H2, and their deactivation by collisions with argon atoms. The thermalized ArH+ ions react with hydrogen more slowly than the excited ArH+ ions first formed. Rate coefficients for these processes and for several other ion reactions occurring in argon–hydrogen mixtures are given.

A drift chamber study of the reaction ArH++H2 →H+3 + Ar and related reactions

The mechanism of formation of the ions H3O+H2O and H3O+(H2O)2 in ionized oxygen, starting from O2+/O4+ has been investigated with a selected ion‐drift chamber mass spectrometer, using lower water vapor pressures than previous studies. With these conditions, the H3O+ ion was not observed indicating that the reaction O2+H2O+H2O→H3O++OH+O2 is endothermic; the ion H3O+(H2O)2 was found to arise mainly from the sequence O2+H2O→O2(H2O)2+→(H2O)3+→H3O+ (H2O)2 rather than by attachment of H2O to H3O+H2O. For the intermediate ion O2(H2O)2+ one must consider the possibility that two different structures exist whose reactions with water vapor give different products.

Asit B. Rakshit

Papers

Reactions of CO2+, CO2CO2+ and H2O+ ions with various neutral molecules

A drift chamber study of the reaction ArH++H2 →H+3 + Ar and related reactions

A drift chamber study of the formation of water cluster ions in oxygen

Reactions of CO2+ and CO2 · CO2 with ethylene. Heat of formation of CO2 · CO3+

Formation and reactions of O2+·CO2, O2+·H2O and O2+(CO2)2 ions