Molecular ion reaction rates for planetary atmospheres and the interstellar medium

Preface. 1. The molecular universe. 1.1 The Standard Model Big Bang Theory. 1.2 Galaxies, stars and planets. 1.3 Origins of life. 1.4 Other intelligent life. 1.5 Theories of the origin of life. Concepts and calculations. 2. Starlight, galaxies and clusters. 2.1 Simple stellar models-black body radiation. 2.2 2.726 K-cosmic microwave background radiation. 2.3 Stellar classification. 2.4 Constellations. 2.5 Galaxies. 2.6 Cosmology. Concepts and calculations. Problems. 3. Atomic and molecular astronomy. 3.1 Spectroscopy and the structure of matter. 3.2 Line shape. 3.3 Telescopes. 3.4 Atomic spectroscopy. 3.5 Molecular astronomy. 3.6 Molecular masers. 3.7 Detection of hydrogen. 3.8 Diffuse interstellar bands. 3.9 Spectral mapping. Concepts and calculations. Problems. 4. Stellar chemistry. 4.1 Classes of stars. 4.2 Herzprung-Russell diagram. 4.3 Stellar evolution. 4.4 Stellar spectra. 4.5 Exotic stars. 4.6 Cycle of star formation. Concepts and calculations Problems. 5. The interstellar medium. 5.1 Mapping clouds of molecules. 5.2 Molecules in the interstellar and circumstellar medium. 5.3 Physical conditions in the interstellar medium. 5.4 Rates of chemical reactions. 5.5 Chemical reactions in the interstellar medium. 5.6 Photochemistry. 5.7 Charged particle chemistry. 5.8 Polycyclic aromatic hydrocarbons. 5.9 Dust grains. 5.10 Kinetic models of molecular clouds. 5.11 Prebiotic molecules in the interstellar medium. Concepts and calculations. Problems. 6. Meteorite and comet chemistry. 6.1 Formation of the solar system. 6.2 Classification of meteorites. 6.3 Meteorite mineralogy. 6.4 Geological time. 6.5 Chemical analysis of meteorites by L2MS. 6.6 The Murchison meteorite-kerogen. 6.7 Meteorite ALH84001. 6.8 Comet chemistry. 6.9 Structure of a comet. 6.10 Physicochemical conditions in a cometary coma. 6.11 Chemical composition of comets. 6.12 Cometary collisions. 6.13 The Rosetta mission-origin of the solar system. Concepts and calculations. Problems. 7. Planetary chemistry. 7.1 Structure of a star-planet system. 7.2 Surface gravity. 7.3 Formation of the Earth. 7.4 Earth-Moon system. 7.5 Geological time. 7.6 Radiative heating. 7.7 The habitable zone. 7.8 Extrasolar planets. 7.9 Planetary atmospheres. 7.10 Atmospheric photochemistry. 7.11 Biomarkers in the atmosphere. Concepts and calculations. Problems. 8. Prebiotic chemistry. 8.1 Carbon- and water-based life forms. 8.2 Spontaneous chemical reactions. 8.3 Rates of chemical reactions. 8.4 Endogenous production of organic molecules. 8.5 Exogenous delivery of organic molecules. 8.6 Homochirality. 8.7 Surface metabolism-'clay organisms'. 8.8 Geothermal vents-'black smokers'. 8.9 RNA World hypothesis. Concepts and calculations. Problems. 9. Primitive life forms. 9.1 Self-assembly and encapsulation. 9.2 Protocells. 9.3 Universal tree of life. 9.4 Astrobiology. 9.5 Microbial Mars. Concepts and calculations. Problems. 10. Titan. 10.1 Physical properties. 10.2 The atmosphere. 10.3 Temperature-dependent chemistry. 10.4 Energy balance and the greenhouse effect. 10.5 Atmospheric chemistry. 10.6 Astrobiology on Titan. Concepts and calculations. Problems. Glossary of terms and abbreviations. Appendix A: constants and units. Appendix B: astronomical data. Appendix C: thermodynamic properties of selected compounds. Answers to problems. Bibliography. Index.

In space, nitrogen-rich ice is constantly exposed to ionizing radiation, which triggers chemical reactions and desorption processes allowing a chemical enhancement of interstellar medium (ISM). Here, we present the first part of a series of studies on the effect of cosmic ray bombardment (40 MeV Ni$^{11+}$ ions) on H$_2$O:N$_2$ (1:5) ice at 15 K, employing the PROCODA code as the modelling tool including 28 chemical species and 930 chemical coupled equations (also including desorption). This first part focuses on the reaction rates and chemical equilibrium stage due to radiation processing. Among the results, we characterize the molecular abundances at chemical equilibrium, including experimentally observed and non-observed species (predicted) suggesting some candidates as a target for astronomical observation. The best-fitting models provided the effective rate coefficients, which can be employed in astrochemical models to understand the chemistry of cold space environments. The findings also help to clarify the chemical processes of N-bearing species in the ISM and frozen surfaces of the Solar system, including the moon of giant planets, outer solar system objects, and ices in the interstellar and protostellar medium.

As new facets of reaction chemistry are getting unraveled frequently, obtaining new insights and understanding the possible formation of the simplest ethers CH3OCH3 (dimethyl ether: DME) and CH3SCH3 (dimethyl sulfide: DMS) in DMS/DME chemistry are not well-studied so far and so are the description of the molecular reaction mechanisms by the structural evolution and sequence of bond breaking and bond formation. In this context, in silico studies have been accomplished using the density functional theory (DFT) and meta-hybrid-DFT approaches, and benchmarking compound methods. Theoretical investigations have been carried out along with a collective implementation of the potential energy surface (PES) (i.e., energy profile), rendering the energetics of the reaction, followed by the intrinsic reaction coordinate (IRC) approach. The PES examination acquired from the formation pathways of both species gives the impression that the construction of the DMS molecule appears to be energetically more favorable than that of the DME species. The role of the metal cation (here, the Na cation is chosen) in the chemistry occurring in the proposed reactions is not well-researched so far; herein, it is found that incorporation of an alkali metal cation (Na+) lowers the energy barrier significantly for both organic species and facilitates the molecular reactions. Out of five proposed reactions (without and with the assistance of the Na+ ion), the formation of DMS species for two reactions appeared to be the most feasible among all. The NCI plot and a few selected and important topological parameters analyzed from the quantum theory of atoms in molecules (QTAIM) tool are capable of recognizing the evolution of types, nature, and strength of interactions (H-bonding, NCIs, etc.), recuperating the bonding patterns along with the proceeding of the whole chemical process. Interestingly, as DMS is yet to be detected (or not known experimentally) in the interstellar medium (ISM), understanding theoretically the formation of DMS via possible reaction paths under the suitable/putative conditions in the ISM will be an interesting workout in future work.

view Abstract Citations (31) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Rate of the reaction N2H++CO -> HCO++N2 and its significance for the interstellar chemistry of H2H+. Herbst, E. ; Bohme, D. K. ; Payzant, J. D. ; Schiff, H. I. Abstract The rate constant for the reaction N2H + + CO HCO + + N2 has been determined to be (8.79 l 0.13) x 10-10 cm3 s ' at 297 + 2 K with an estimated accuracy of +25 percent. The implications of this rate constant for the interstellar chemistry of N211 + are discussed. Estimates are made for the frequencies of the first rotational transitions in `5N14NH +, `4N'5NH +, and N2D +. Subject headings:abundances - atomic and molecular processes - interstellar matter - molecules, interstellar - transition probabilities Publication: The Astrophysical Journal Pub Date: November 1975 DOI: 10.1086/153926 Bibcode: 1975ApJ...201..603H Keywords: Interstellar Gas; Ionic Reactions; Molecular Ions; Reaction Kinetics; Carbon Monoxide; Chemical Reactions; Hydrogen Clouds; Microwave Emission; Nitrogen Compounds; Astrophysics full text sources ADS |

A novel ethynyl addition mechanism (EAM) has been established computationally as a practicable alternative to high-temperature hydrogen-abstraction-C2H2-addition (HACA) sequences to form polycyclic aromatic hydrocarbon (PAH) -like species under low-temperature conditions in the interstellar medium and in hydrocarbon-rich atmospheres of planets and their moons. Initiated by an addition of the ethynyl radical (C2H) to the ortho-carbon atom of the phenylacetylene (C6H5C2H) molecule, the reactive intermediate loses rapidly a hydrogen atom, forming 1,2-diethynylbenzene. The latter can react with a second ethynyl molecule via addition to a carbon atom of one of the ethynyl side chains. A consecutive ring closure of the intermediate leads to an ethynyl-substituted naphthalene core. Under single-collision conditions as present in the interstellar medium, this core loses a hydrogen atom to form ethynyl-substituted 1,2-didehydronaphthalene. However, under higher pressures as present, for example, in the atmosphere of Saturn's moon Titan, three-body reactions can lead to a stabilization of this naphthalene-core intermediate. We anticipate this mechanism to be of great importance to form PAH-like structures in the interstellar medium and also in hydrocarbon-rich, low-temperature atmospheres of planets and their moons such as Titan. If the final ethynyl addition to 1,2-diethynylbenzene is substituted by a barrierless addition of a cyano (CN) radical, this newly proposed mechanism can even lead to the formation of cyano-substituted naphthalene cores in the interstellar medium and in planetary atmospheres.

In the interstellar medium (ISM) ion–molecule reactions play a key role in forming complex molecules. Since 2006, after the radioastronomical discovery of the first of by now six interstellar anions, interest has grown in understanding the formation and destruction pathways of negative ions in the ISM. Experiments have focused on reactions and photodetachment of the identified negatively charged ions. Hints were found that the reactions of CnH(–) with H2 may proceed with a low (<10(–13) cm(3) s(–1)), but finite rate [Eichelberger, B.; et al. Astrophys. J. 2007, 667, 1283]. Because of the high abundance of molecular hydrogen in the ISM, a precise knowledge of the reaction rate is needed for a better understanding of the low-temperature chemistry in the ISM. A suitable tool to analyze rare reactions is the 22-pole radiofrequency ion trap. Here, we report on reaction rates for Cn(–) and CnH(–) (n = 2, 4, 6) with buffer gas temperatures of H2 at 12 and 300 K. Our experiments show the absence of these reactions with an upper limit to the rate coefficients between 4 × 10(–16) and 5 × 10(–15) cm(3) s(–1), except for the case of C2(–), which does react with a finite rate with H2 at low temperatures. For the cases of C2H(–) and C4H(–), the experimental results were confirmed with quantum chemical calculations. In addition, the possible influence of a residual reactivity on the abundance of C4H(–) and C6H(–) in the ISM were estimated on the basis of a gas-phase chemical model based on the KIDA database. We found that the simulated ion abundances are already unaffected if reaction rate coefficients with H2 were below 10(–14) cm(3) s(–1).

A recurring theme throughout this thesis is the analysis of Fourier transform infrared (FTIR) spectra of molecules that exist in planetary atmospheres and interstellar media. FTIR spectroscopy enables the accurate characterisation and identification of molecules based on the shapes and positions of vibrational bands. Unless stated otherwise, all of the experimental work has been performed at the Australian Synchrotron THz/far-IR beamline utilising a synchrotron source which is several magnitudes brighter than conventional thermal sources. This is crucial for the far-IR region, as there are no efficient thermal emitters of far-IR radiation. Furthermore, spectroscopy within this region is most applicable to monitoring atmospheric and interstellar molecules. Chapter 1 introduces some of the key concepts associated with FTIR spectroscopy as well as the instrumentation used for the experiments. Quantum theory is also introduced where needed as part of the concepts that are discussed. Chapter 2 focuses on the spectra of molecules recorded using high resolution FTIR spectroscopy and the fundamental theory is introduced in Chapter 2.1 as it is a prerequisite to understanding the rotational Hamiltonian, transition assignment and fitting processes used throughout Chapters 2.2 - 2.5. Chapter 2.2 describes an ongoing analysis of the first high resolution FTIR spectrum of propynethial which is predicted to exist in interstellar media. The v5 band near 1100 cm-1 is the focus of this work as it was predicted to be the most intense band based on B3LYP/cc-pVTZ calculations. Due to inaccurate ground state rotational and centrifugal distortion constants, it was not possible to accurately assign the ro-vibrational transitions of the v5 band. Thus, it was necessary to record millimetre-wave spectra in order to assign transitions with a wider range of J″ and Ka″ quantum number so that more accurate ground state constants could be determined before re-assigning the v5 infrared transitions. This work will hopefully lead to the identification of propynethial in the interstellar media. Chapter 2.3 contains the first published paper on the analysis of the far-IR bands of 1,1-difluoroethane. This molecule is relevant to atmospheric applications due to its high global warming potential and will help facilitate the analysis of higher energy vibrational bands that are present within the greenhouse window. Chapter 2.4 contains a publication on one of the far-IR bands of 1,1,1,2-tetrafluoroethane. Also known as R134a, this molecule is commonly used in industry and needs to be continuously monitored due to its high global warming potential and long atmospheric lifetime. iv Chapter 2.5 is a submitted paper on the analysis of the four lowest IR-active fundamental bands of trans-d2-ethylene. The work completed here is part of a chemical-education collaboration between Dr. C. Thompson (Monash University) and Prof. T. L. Tan (Nanyang Technological University). Chapter 3 progresses into the analysis of low resolution particulate ices that were formed in a collisional cooling cell that has been installed onto the Australian Synchrotron THz/far-IR beamline. These molecular ices are important to interstellar chemistry, as they can provide reaction sites for building more complex molecules from smaller molecules; and atmospheric chemistry by having a large influence on the radiative forcing of planetary atmospheres. Some key aspects of condensed phase spectroscopy are introduced in Chapter 3.1 Chapter 3.2 is an accepted paper detailing the analysis of the mid-IR spectra of isotope mixed H2O crystalline ice particles. The effect of isotopic mixing using D2O is explored over a range of temperatures and concentrations, giving new insights to the behaviour of inter- and intramolecular bonding. Chapter 3.3 contains a manuscript that reports the mid- and far-IR spectra of particulate crystalline ethylene. Ethylene is one of the most abundant species found in Titan’s atmosphere (largest moon orbiting Saturn) and is formed from the photodissociation of methane. An accurate characterisation of these ethylene particles in situ will provide a more direct relationship with what is observed from passing satellites and help profile the temperature of Titan’s atmosphere at different altitudes. Chapter 4 describes the application of a chemometric technique: band target entropy minimisation (BTEM) to gas phase Fourier transform microwave (FTMW) and FTIR spectra. Chemometrics is a widely used technique to help extract additional information from chemical, physical and biological systems that may not be readily discernible through conventional methods. The preliminary results from the analysis of FTMW and FTIR spectra, using BTEM, highlights both the potential application of chemometrics to gas phase spectroscopy as well as its shortcomings. Chapter 5 introduces preliminary data on the application of the EFC cell to transient spectroscopy. Many of the gaseous molecules that are found in interstellar media often exist as short-lived species under laboratory conditions. This work is aimed at increasing the lifetime of transient molecules so that some of the experimental burdens can be reduced when performing high resolution spectroscopy.

The crossed molecular beams reaction of atomic carbon C(3P with hydro- j) gen sulÐde, H allene, the vinyl radical, and deutero- 2S, H2CCCH2, C2H3 , acetylene, C have been studied at di erent collision energies up to 42.2 2HD, kJ mol~1 and combined with high level ab initio calculations. All reactions are barrier-less and are dominated by a carbonÈhydrogen exchange to form\nthioformyl (HCS), butatrienyl (HCCCCH isomer(s), and deuteriated 2), C3Htricarbon hydride(s). This carbonÈhydrogen repla2cement channel represents a one-step alternative reaction pathway to competing ionÈmolecule reactions to form complex, carbon-bearing molecules in the interstellar medium as well as in the outÑow of carbon stars.

Certain important molecules of the interstellar medium (ISM), such as H2 and CO2, are believed to have been formed on surfaces of dust grains. We describe experimental methods that we used to study the formation of H2 and CO2 on dust grain analogues in conditions approximating the ones found in key interstellar environments. By using state‐of‐the‐art surface science techniques we obtained information on the efficiency of the molecular formation reactions, the reaction kinetics, and the reaction dynamics. Selected results are presented on the formation of molecular hydrogen on surfaces of silicates, amorphous carbon, and amorphous ice and on the synthesis of carbon dioxide. We then briefly show how these results have been applied to the quantitative determination of processes occurring in the ISM.

As an important class of carbon reservoirs in the interstellar medium (ISM), polycyclic aromatic hydrocarbon (PAH) molecules play an important role in the evolutionary network of prebiotic molecules. Here, the experimental evidence of astronomically relevant amino-acid derivatives – PAH/amino-acid clusters – is provided, and we study their ion–molecular collision reactions in the gas phase. With the initial molecular precursors dicoronylene (DC, C48H20)/alanine (Ala, C3H7NO2) and DC/isoleucine (Ile, C6H13NO2), the experiments indicate that PAH–amino-acid cluster cations (e.g. (Ala)C48H$_{(0-19)}\, ^+$ and (Ile)C48H$_{(0-19)}\, ^+$) and graphene–amino-acid cluster cations (e.g. (Ala)nC$_{48}\, ^+$ and (Ile)nC$_{48}\, ^+$, n = 1, 2, 3, 4) are efficiently formed in a strong interstellar radiation field. In addition, the structure of clusters and the binding energy of their formation pathways are studied by a quantum chemistry calculation method: gas-phase reactions (ion–molecule reactions) between PAH cations with amino acids readily occur (exothermic energy around 2.0–4.7 eV), and these newly formed clusters have a complex molecular configuration (C–O and C–N bond type). These laboratory studies provide a cluster growth pathway (through an ion–molecule reaction) towards the formation of amino-acid derivatives in a bottom-up process and insight into their chemical-evolution behaviour, opening up aromatic-based chemistry that is available to the species (dehydrogenated PAHs or graphene molecules) that formed from the photofragmentation process of PAHs in interstellar environments. When conditions are suitable (e.g. have similar molecular abundance spatial distributions in the ISM), amino-acid derivatives can form efficiently, and newly built large PAH/amino-acid clusters may be widespread in space.

As one class of important carbon reservoirs in interstellar clouds, large polycyclic aromatic hydrocarbons (PAHs) and their derivative species play an important role in the formation and evolution of interstellar carbonaceous compounds. To understand these chemical routes, the gas-phase ion–molecular collision reaction between large, astronomically relevant PAH (dicoronylene, DC, C48H20) cations and smaller neutral superhydrogenated PAHs (2, 3–benzofluorene, C17H12) are investigated. Series of large DC/2, 3–benzofluorene cluster cations (e.g., [(C17H12)6C48H14]+, 236 atoms, and [(C17H12)5C48]+, 193 atoms) are efficiently formed by gas-phase condensation under laser irradiation conditions. With theoretical calculations, the structure of newly formed DC/2, 3-benzofluorene cluster cations and the bonding energy for these formation reactions are obtained. Moreover, the IR spectra of DC/2, 3-benzofluorene cluster cations are also calculated. The gas-phase reactions between large PAH species occur relatively easily, resulting in a very large number of reactions and very complex molecular clusters. The adduct processes and the formed molecular structure relatively depend on the carbon reaction sites. The carbon edge sites have different chemical reactivity, which may affect the abundance of these relevant interstellar substances. Furthermore, intermolecular hydrogen transfer plays an important role in cluster formation processes, which can lead the newly formed clusters to become more stable. We infer that small superhydrogenated PAH molecules (e.g., 2, 3-benzofluorene) can effectively aggregate on the large PAH molecules (e.g., dehydrogenated DC cations or carbon clusters) in the gas phase, which provides proposed chemical-evolution routes (ion–molecular reaction pathways) for the formation of the nanometer-sized dust grains in a bottom-up process (in building block pathways) in the interstellar medium.

Planetary systems are angular momentum reservoirs generated during star formation. Solutions to three of the most important problems in contemporary astrophysics are needed to understand the entire process of planetary system formation: The physics of the ISM. Stars form from dense molecular clouds that contain ∼30% of the total interstellar medium (ISM) mass. The structure, properties and lifetimes of molecular clouds are determined by the overall dynamics and evolution of a very complex system — the ISM. Understanding the physics of the ISM is of prime importance not only for Galactic but also for extragalactic and cosmological studies. Most of the ISM volume (∼65%) is filled with diffuse gas at temperatures between 3000 and 300 000 K, representing about 50% of the ISM mass. The physics of accretion and outflow. Powerful outflows are known to regulate angular momentum transport during star formation, the so-called accretion—outflow engine. Elementary physical considerations show that, to be efficient, the acceleration region for the outflows must be located close to the star (within 1AU) where the gravitational field is strong. According to recent numerical simulations, this is also the region where terrestrial planets could form after 1 Myr. One should keep in mind that today the only evidence for life in the Universe comes from a planet located in this inner disk region (at 1AU) from its parent star. The temperature of the accretion—outflow engine is between 3000 and 107 K. After 1 Myr, during the classical T Tauri stage, extinction is small and the engine becomes naked and can be observed at ultraviolet wavelengths. The physics of planet formation. Observations of volatiles released by dust, planetesimals and comets provide an extremely powerful tool for determining the relative abundances of the vaporizing species and for studying the photochemical and physical processes acting in the inner parts of young planetary systems. This region is illuminated by the strong UV radiation field produced by the star and the accretion—outflow engine. Absorption spectroscopy provides the most sensitive tool for determining the properties of the circumstellar gas as well as the characteristics of the atmospheres of the inner planets transiting the stellar disk. UV radiation also pumps the electronic transitions of the most abundant molecules (H2, CO, etc.) that are observed in the UV.Here we argue that access to the UV spectral range is essential for making progress in this field, since the resonance lines of the most abundant atoms and ions at temperatures between 3000 and 300 000 K, together with the electronic transitions of the most abundant molecules (H2, CO, OH, CS, S2, CO+ 2 , C2,O2,O3, etc.) are at UV wavelengths. A powerful UV-optical instrument would provide an efficient mean for measuring the abundance of ozone in the atmosphere of the thousands of transiting planets expected to be detected by the next space missions (GAIA, Corot, Kepler, etc.). Thus, a follow-up UV mission would be optimal for identifying Earth-like candidates.

It is pointed out that the interstellar gas which passes through the solar system may affect the interplanetary gas, planetary atmospheres, and the orbits of satellites. This paper discusses the interaction of the interplanetary and interstellar gases via atomic collisions. Using the results of zodiacal light observations and far ultraviolet rocket measurements, interplanetary densities near the earth are adopted as n e 30, n 11 ~ 0.2. The fraction of interstellar gas of density n l which is captured as it passes through the solar system is expressed in terms of a fractional-capture parameter (f m ) av. If n 1 (f m)av is of order 10−3 to 10−4 cm−3 then the interstellar gas contributes appreciably to the mass and energy of the interplanetary gas. A brief discussion of atomic scattering cross sections indicates that n 1 (f m ) av is of the required order of magnitude.

view Abstract Citations (23) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Calculations of the lower electronic states of CH3+: a postulated intermediate in interstellar reactions. Blint, R. J. ; Marshall, R. F. ; Watson, W. D. Abstract Molecular-structure calculations are presented to estimate the photodissociation rate for CH3 + in the interstellar gas. For the average interstellar radiation field, we find that CH3 + probably has a lifetime against photodissociation greater than 1011 s. Thus, in the proposed formation sequence for interstellar CH + that is initiated by a fast C + + H2 radiative association, the postulated (though possibly not essential) rapid photodissociation of CH3 + is unlikely to occur. Restricted Hartree-Fock wave functions for the ground and lowest three singlet states of CH3 + are calculated at various molecular geometries. The energies of the states, the force constants for the ground state, and the oscillator strength of the first excitation are determined from these wave functions. In addition, zero-point energies of CH3 + and CH2D + are calculated for application in proposed reaction schemes that cause deuterium enhancement in interstellar molecules. Subject headings: interstellar: molecules - molecular processes Publication: The Astrophysical Journal Pub Date: June 1976 DOI: 10.1086/154420 Bibcode: 1976ApJ...206..627B Keywords: Deuterium; Electron States; Interstellar Gas; Ionic Reactions; Molecular Structure; Free Radicals; Hartree Approximation; Methyl Compounds; Molecular Ions; Photodissociation; Reaction Kinetics; Astrophysics full text sources ADS |

Molecular ions are key species in the chemistry of the interstellar medium (ISM). Given the low temperatures and number densities typically occurring in the ISM, one of the few available mechanisms to form more complex molecules is through barrierless exothermic reactions, as it is the case for many ion-molecule reactions. Ions are highly reactive species but they can be formed efficiently in the ISM by cosmic-ray or ultraviolet ionization and can survive for relatively long times due to the few collisions they suffer. On earth, molecular ions are “exotic” species much more difficult to produce in appreciable quantities. Electrical discharges in low pressure gases form cold plasmas which can be used to produce molecular ions in abundances high enough to enable their spectroscopic study.