Radiative Electron Attachment Research Articles

Revisiting a Chemical Route to the Formation of CN− in the Interstellar Medium

We discuss the HCN + H− reaction as a path to the formation of CN−, the smallest cyanopolyyne anion observed in several interstellar environments. We first obtain the new ab initio reactive potential energy surface using a full 5D representation, where only the C–N bond is kept fixed, and discuss the neural network procedure employed to yield an accurate fit for the dynamics. The reaction is then investigated by using a quasi-classical trajectory approach to scan the low-temperature range of the dark molecular clouds where the anion has been sighted. Calculations are extended to room temperature to make a successful comparison with existing experimental data. We further present reduced dimensionality modeling of the reaction as a 2D process within a variational-transition state treatment with the inclusion of long-range forces. The dominant role of such forces in producing large reaction rate coefficients is discussed for both treatments, which yield very similar sizes and behavior of such coefficients from 50 to 300 K. The implications of our results for the interstellar medium formation of the CN− species via this chemical route are discussed, suggesting its greater significance over the radiative electron attachment paths, whose rate coefficients were found by recent calculations to be orders of magnitude smaller.

Abundance and excitation of molecular anions in interstellar clouds

We present new observations of molecular anions with the Yebes 40 m and IRAM 30 m telescopes toward the cold, dense clouds TMC-1 CP, Lupus-1A, L1527, L483, L1495B, and L1544. We report the first detections of C3N− and C5N− in Lupus-1A as well as C4H− and C6H− in L483. In addition, we detected new lines of C6H− toward the six targeted sources, of C4H− toward TMC-1 CP, Lupus-1A, and L1527, and of C8H− and C3N− in TMC-1 CP. Excitation calculations using recently computed collision rate coefficients indicate that the lines of anions accessible to radiotelescopes run from subthermally excited to thermalized as the size of the anion increases, with the degree of departure from thermalization depending on the H2 volume density and the line frequency. We noticed that the collision rate coefficients available for the radical C6H are not sufficient to explain various observational facts, thereby calling for the collision data for this species to be revisited. The observations presented here, together with observational data from the literature, have been used to model the excitation of interstellar anions and to constrain their abundances. In general, the anion-to-neutral ratios derived here agree with the literature values, when available, within 50% (by a factor of two at most), except for the C4H−/C4H ratio, which shows higher differences due to a revision of the dipole moment of C4H. From the set of anion-to-neutral abundance ratios derived two conclusions can be drawn. First, the C6H−/C6H ratio shows a tentative trend whereby it increases with increasing H2 density, as we would expect on the basis of theoretical grounds. Second, the assertion that the higher the molecular size, the higher the anion-to-neutral ratio is incontestable; furthermore, this supports a formation mechanism based on radiative electron attachment. Nonetheless, the calculated rate coefficients for electron attachment to the medium size species C4H and C3N are probably too high and too low, respectively, by more than one order of magnitude.

Detectable interstellar anions: Examining the key factors.

Electron attachment energy (eAE) and the dipole moment of the neutral molecules are related to the radiative electron attachment (REA) process, which is an important formation route of interstellar anions under various astrophysical environments. Electron attachment energies (eAEs) have been calculated for over 80 carbon chain neutral molecules; Cn, CnX (X=N, O, S, H, Si, P), HCnN using ab initio quantum chemical method. Dipole moments of these neutral species and those of their corresponding anions are also theoretically estimated. The anions are found to be significantly more stable than their corresponding neutral analogues in most of the carbon chains considered. The high eAEs and the large dipole moment of most of the neutral species suggest a high detectability of their corresponding anions in the interstellar medium if they are abundant. Members of the C2nO-, C2nS-, C2n1Si, HC2nN-, CnP-, and C2n chains with increasing n are good candidates for future astronomical detection including the higher members of the C2nH- and C2n-1N- groups whose lower members have been observed.

Radiative Electron Attachment and Photodetachment Rate Constants for Linear Carbon Chains

Since their recent detection in interstellar clouds and circumstellar envelopes, polyyne and cyanopolyyne anions have raised the question of their possible mechanisms of formation, destruction, and excitation. These anions are observed in the same regions as the corresponding neutral species, with anion-to-neutral abundance ratios of a few percent. It is believed that the abundance ratios are controlled mainly by the radiative attachment processes. We present a quantum study of the radiative electron attachment and photodetachment rate constants for selected linear carbon-chain anions, namely the already detected interstellar anions as well as other potential candidates. The rate constants are calculated within the rigid molecule approximation, with the attached or ejected electron described by a plane wave. A qualitative agreement is obtained with the previous accurate quantum results. For the radiative electron attachment process, the discrepancies between the quantum rate constants calculated here and ...

Anionic Carbon Chain Growth in Reactions of , , , C2H−, C4H−, and C6H− with C2H2

Abstract The observed abundances of interstellar carbon chain anions are not yet fully understood in recent models of astrochemical reaction networks. The major formation route is assumed to be the chain growth of neutral carbon chains followed by the formation of anions via radiative electron attachment. Besides this, chemical reactions with ions and, in a single study, the anionic chain growth reaction C2n H− + C2H2 → C2n+2H− + H2 have been proposed to influence molecular abundances. In the present work, the title reactions have been investigated experimentally by velocity map imaging in a crossed beam setup. All reactions take place at well-defined collision energies in the range from 0.3 to 3.0 eV. The dominant products are C4H−, C6H−, and C8H− in the reactions with n = 1, 2, 3, respectively. Preferentially at low energies, , , and are also formed. The reactions of C2H−, C4H−, and C6H− with acetylene are slower than those with , , and , respectively. Increasing collision energy and reactant ion chain length appears to enhance the reaction rates of the hydrogenated species relative to the latter ions, forming both and . These results are discussed with the aim to clarify their possible relevance in interstellar environments.

Single-center approach for photodetachment and radiative electron attachment: Comparison with other theoretical approaches and with experimental photodetachment data

A single-center method for calculating photodetachment cross section for anions and radiative electron attachment cross section for neutral molecules by microreversibility is presented. It uses the integral equation method to calculate the ejected electron's continuum wave function while the single-electron bound function of the anion is described by the Dyson orbital. It is compared with related theoretical approaches and benchmarked to the experimental photodetachment cross sections of ${{\mathrm{O}}_{2}}^{\ensuremath{-}}$, ${\mathrm{OH}}^{\ensuremath{-}}$, and ${\mathrm{CN}}^{\ensuremath{-}}$. The use of the plane-wave approximation of the ejected electron wave function combined with the Hartree-Fock frozen-core approximation of the Dyson orbital is also considered and its results are compared with those of our methods and with experiment. A good agreement between the calculated photodetachment cross sections and the experimental data is obtained for ${{\mathrm{O}}_{2}}^{\ensuremath{-}}$ and ${\mathrm{CN}}^{\ensuremath{-}}$ when using the three methods. For ${\mathrm{OH}}^{\ensuremath{-}}$, the calculated scattering-wave electron photodetachment cross sections agree well with two most recent sets of experimental data among the three available while the plane-wave results disagree with all the experimental and theoretical data. The different approaches to calculate the Dyson orbital are also discussed as well as the convergence of the calculations with respect to the choice of the one-electron basis set. The approximation of Dyson orbitals by Kohn-Sham orbitals appears to overestimate the photodetachment cross section.

On the gas-phase formation of the HCO- anion: accurate quantum study of the H- + CO radiative association and HCO radiative electron attachment.

The hydrogen anion has never been observed in the interstellar medium, but it is most likely present in some interstellar regions. Since direct detection appears especially difficult, improving the knowledge of the astrochemical processes involving this anion should be valuable in defining a way of indirect detection. We present the first study of the radiative association of H- and CO to form the HCO- anion within a quantum time-independent approach. We use a state-of-the-art potential energy surface which has been calculated for the present study. The calculated radiative association rate coefficient is monotonically decreasing from 6 × 10-16 to 5 × 10-19 cm3 per molecule per s across the 0.01-1000 K temperature range. At the typical temperature of the cold interstellar medium, ∼10 K, the radiative association rate is ∼2 × 10-17 cm3 per molecule per s. On the other hand, the plane wave approximation is used to calculate the HCO radiative electron attachment rate coefficient. It is found to be almost constant and also equal to 2 × 10-17 cm3 per molecule per s. Setting aside the question of the abundances of the reactants of both processes, these results demonstrate that among the two gas-phase modes of production of the HCO- anion in cold interstellar medium considered in this study, the H- + CO radiative association is dominating below 10 K while the radiative electron attachment rate is larger above 10 K.

Formation of Anionic C, N-bearing Chains in the Interstellar Medium via Reactions of H− with HCxN for Odd-valued x from 1 to 7

Abstract We investigate the relative efficiencies of low-temperature chemical reactions in the interstellar medium with H− anion reacting in the gas phase with cyanopolyyne neutral molecules, leading to the formation of anionic linear chains of different lengths and of H2. All the reactions turn out to be without barriers, highly exothermic reactions that provide a chemical route to the formation of anionic chains of the same length. Some of the anions have been observed in the dark molecular clouds and in the diffuse interstellar envelopes. Quantum calculations are carried out for the corresponding reactive potential energy surfaces for all the odd-numbered members of the series (x = 1, 3, 5, 7). We employ the minimum energy paths to obtain the relevant transition state configurations and use the latter within the variational transition state model to obtain the chemical rates. The present results indicate that at typical temperatures around 100 K, a set of significantly larger rate values exists for x = 3 and x = 5, while the rate values are smaller for CN− and . At those temperatures, however, all the rates turn out to be larger than the estimates in the current literature for the radiative electron attachment (REA) rates, thus indicating the greater importance of the present chemical path with respect to REA processes at those temperatures. The physical reasons for our findings are discussed in detail and linked with the existing observational findings.

Formation and stabilization of C6− by radiative electron attachment

Radiative electron attachment (REA) plays an important role in forming molecular anions in various astrophysical environments. In this work, we determined the rate coefficient for the formation of C6− by REA based on a detailed balance approach. C6− ions are stored in an electrostatic ion beam trap and are photoexcited above their adiabatic detachment energy (4.18 eV). Due to fast internal conversion and intramolecular vibrational redistribution, photoexcitation leads to the formation of temporary negative ions (TNIs), the same as those one formed by the electron attachment. Absolute vibrational autodetachment and recurrent (or Poincaré) fluorescence (RF) rate coefficients have already been reported [V. Chandrasekaran et al., J. Phys. Chem. Lett. 5, 4078 (2014)]. Knowing the branching ratios of the various competing rate coefficients is decisive to the understanding of the formation probability of anions via REA. The radiative stabilization rate of C6−, shown to be dominated by RF, was determined to be 5 × 104 s−1 at the electron detachment energy, i.e., at least a factor of 100 faster than the stabilization by infrared transitions. The RF is found to very effectively stabilize the TNI formed by electron attachment. Using detailed balance to link the measured delayed detachment rate to the rate of electron attachment, we estimate the REA rate leading to the formation of C6− to be 3 × 10−7 cm3 s−1 at 300 K in agreement with theory (1.7 × 10−7 cm3 s−1 [R. Terzieva and E. Herbst, Int. J. Mass Spectrom. 201, 135 (2000)]). Such a high rate for REA to C6 indicates that REA may play a prominent role in the formation of anions in the interstellar medium.

Study of the radiative electron attachment and photodetachment processes for the C2H/C2H- and C4H/C4H- molecules

The photodetachment cross sections of the C2H− and C4H− molecular anions in their ground electronic and vibrational states are determined using the R-matrix approach. The obtained results are compared with available experimental data of absolute photodetachment cross sections. In addition, cross sections for the inverse process, i.e., radiative electron attachment, have also been determined and are used to obtain the corresponding thermally-averaged rate coefficients. At a temperature of 300 K, the rate coefficients are 2.3 × 10-16 and 18 × 10-16 cm3/s for C2H− and C4H−, respectively.

Modelling the role of electron attachment rates on column density ratios for CnH−/CnH () in dense molecular clouds

The fairly recent detection of a variety of anions in the interstellar molecular clouds have underlined the importance of realistically modelling the processes governing their abundance. To pursue this task, our earlier calculations for the radiative electron attachment (REA) rates for C4H−, C6H−, and C8H− are employed in the present work, within a broad network of other concurrent reactions, to generate the corresponding column density ratios of anion/neutral (A/N) relative abundances. The latter are then compared with those obtained in recent years from observational measurements. The calculations involved the time-dependent solutions of a large network of chemical processes over an extended time interval and included a series of runs in which the values of REA rates were repeatedly scaled over several orders of magnitude. Macroscopic parameters for the Clouds’ modelling were also varied to cover a broad range of physical environments. It was found that, within the range and quality of the processes included in the present network,and selected from state-of-the-art astrophysical databases, the REA values required to match the observed A/N ratios needed to be reduced by orders of magnitude for C4H− case, while the same rates for C6H− and C8H− only needed to be scaled by much smaller factors. The results suggest that the generally proposed formation of interstellar anions by REA mechanism is overestimated by current models for the C4H− case, for which is likely to be an inefficient path to formation. This path is thus providing a rather marginal contribution to the observed abundances of C4H−, the latter being more likely to originate from other chemical processes in the network, as we discuss in some detail in the present work. Possible physical reasons for the much smaller differences against observations found instead for the values of the (A/N) ratios in two other, longer members of the series are put forward and analysed within the evolutionary modelling discussed in the present work.

EXPLORING A CHEMICAL ROUTE FOR THE FORMATION OF STABLE ANIONS OF POLYYNES [CnH− (n = 2, 4)] IN MOLECULAR CLOUDS

ABSTRACT Using quantum chemical methods, we investigate the possible outcomes of reactions with acetylene and diacetylene molecules. We find both reactions to be exothermic reactions without barriers, yielding stable anions of the corresponding polyynes: and . We show in this work that the computed chemical rates in the case of the formation of the anion would be larger than those existing for the direct radiative electron attachment (REA) process, the main mechanism generally suggested for their formation. In the case of the anion, however, the present chemical rates of formation at low T are even lower than those known for its REA process, both mechanisms being inefficient for its formation under astrochemical conditions. The present results are discussed in view of their consequences on the issue of the possible presence of such anions in the ISM environments. They clearly indicate the present chemical route to formation to be inefficient at the expected temperatures of a dark molecular cloud, whereas this is found not to be the case for the , in line with the available experimental findings.

Formation of CN^{-}, C_{3}N^{-}, and C_{5}N^{-} Molecules by Radiative Electron Attachment and their Destruction by Photodetachment.

The existence of negative ions in interstellar clouds has been associated for several decades with the process of radiative electron attachment. In this Letter, we report compelling evidence supporting the fact that the radiative attachment of a low-energy electron is inefficient to form the carbon chain anions CN^{-}, C_{3}N^{-}, and C_{5}N^{-} detected in interstellar clouds. The validity of the approach is confirmed by good agreement with experimental data obtained for the inverse photodetachment process, which represents the major cause of anion destruction in interstellar space. As a consequence, we suggest alternative models that could explain the formation of anions.

Theoretical study of radiative electron attachment to CN, C2H, and C4H radicals.

A first-principle theoretical approach to study the process of radiative electron attachment is developed and applied to the negative molecular ions CN(-), C4H(-), and C2H(-). Among these anions, the first two have already been observed in the interstellar space. Cross sections and rate coefficients for formation of these ions by direct radiative electron attachment to the corresponding neutral radicals are calculated. For the CN molecule, we also considered the indirect pathway, in which the electron is initially captured through non-Born-Oppenheimer coupling into a vibrationally resonant excited state of the anion, which then stabilizes by radiative decay. We have shown that the contribution of the indirect pathway to the formation of CN(-) is negligible in comparison to the direct mechanism. The obtained rate coefficients for the direct mechanism at 30 K are 7 × 10(-16) cm(3)/s for CN(-), 7 × 10(-17) cm(3)/s for C2H(-), and 2 × 10(-16) cm(3)/s for C4H(-). These rates weakly depend on temperature between 10 K and 100 K. The validity of our calculations is verified by comparing the present theoretical results with data from recent photodetachment experiments.

Organic chemistry in Titan׳s upper atmosphere and its astrobiological consequences: I. Views towards Cassini plasma spectrometer (CAPS) and ion neutral mass spectrometer (INMS) experiments in space

The discovery of carbocations and carbanions by Ion Neutral Mass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS) instruments onboard the Cassini spacecraft in Titan׳s upper atmosphere is truly amazing for astrochemists and astrobiologists. In this paper we identify the reaction mechanisms for the growth of the complex macromolecules observed by the CAPS Ion Beam Spectrometer (IBS) and Electron Spectrometer (ELS). This identification is based on a recently published paper (Ali et al., 2013. Planet. Space Sci. 87, 96) which emphasizes the role of Olah׳s nonclassical carbonium ion chemistry in the synthesis of the organic molecules observed in Titan׳s thermosphere and ionosphere by INMS. The main conclusion of that work was the demonstration of the presence of the cyclopropenyl cation – the simplest Huckel׳s aromatic molecule – and its cyclic methyl derivatives in Titan׳s atmosphere at high altitudes. In this study, we present the transition from simple aromatic molecules to the complex ortho-bridged bi- and tri-cyclic hydrocarbons, e.g., CH2+ mono-substituted naphthalene and phenanthrene, as well as the ortho- and peri-bridged tri-cyclic aromatic ring, e.g., perinaphthenyl cation. These rings could further grow into tetra-cyclic and the higher order ring polymers in Titan׳s upper atmosphere. Contrary to the pre-Cassini observations, the nitrogen chemistry of Titan׳s upper atmosphere is found to be extremely rich. A variety of N-containing hydrocarbons including the N-heterocycles where a CH group in the polycyclic rings mentioned above is replaced by an N atom, e.g., CH2+ substituted derivative of quinoline (benzopyridine), are found to be dominant in Titan׳s upper atmosphere. The mechanisms for the formation of complex molecular anions are discussed as well. It is proposed that many closed-shell complex carbocations after their formation first, in Titan׳s upper atmosphere, undergo the kinetics of electron recombination to form open-shell neutral radicals. These radical species subsequently might form carbanions via radiative electron attachment at low temperatures with thermal electrons. The classic example is the perinaphthenyl anion in Titan׳s upper atmosphere. Therefore, future astronomical observations of selected carbocations and corresponding carbanions are required to settle the key issue of molecular anion chemistry on Titan. Other than earth, Titan is the only planetary body in our solar system that is known to have reservoirs of permanent liquids on its surface. The synthesis of complex biomolecules either by organic catalysis of precipitated solutes “on hydrocarbon solvent” on Titan or through the solvation process indeed started in its upper atmosphere. The most notable examples in Titan׳s prebiotic atmospheric chemistry are conjugated and aromatic polycyclic molecules, N-heterocycles including the presence of imino >CN–H functional group in the carbonium chemistry. Our major conclusion in this paper is that the synthesis of organic compounds in Titan׳s upper atmosphere is a direct consequence of the chemistry of carbocations involving the ion–molecule reactions. The observations of complexity in the organic chemistry on Titan from the Cassini–Huygens mission clearly indicate that Titan is so far the only planetary object in our solar system that will most likely provide an answer to the question of the synthesis of complex biomolecules on the primitive earth and the origin of life.

Publisher's Note: Theory of radiative electron attachment to molecules: Benchmark study of CN−[Phys. Rev. A88, 052710 (2013)

Received 3 December 2013DOI:https://doi.org/10.1103/PhysRevA.88.069904©2013 American Physical Society

Theory of radiative electron attachment to molecules: Benchmark study of CN−

We have developed an approach based on first principles to study the process of radiative electron attachment (REA) to linear molecules of astrophysical interest in collisions between the molecules and electrons at energies below 1 eV. The approach is based on accurate ab initio calculations of electronic bound and continuum states of the negative ion. The electronic continuum states are obtained with the complex-Kohn variational method. The benchmark calculation for the REA to the simplest negative ion CN${}^{\ensuremath{-}}$, which was recently observed in the interstellar medium, has produced a relatively low rate coefficient $\ensuremath{\alpha}(T)=7.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ cm${}^{3}/$s at $T=30$ K. Moreover, our results are shown to agree well with microscopic reversibility applied on a recent photodetachment experiment on CN${}^{\ensuremath{-}}$. Finally, the study confirms a previous assessment that the CN${}^{\ensuremath{-}}$ ion is unlikely be formed by REA in the interstellar medium.

ON THE UBIQUITY OF MOLECULAR ANIONS IN THE DENSE INTERSTELLAR MEDIUM

Results are presented from a survey for molecular anions in seven nearby Galactic star-forming cores and molecular clouds. The hydrocarbon anion C6H- is detected in all seven target sources, including four sources where no anions have been previously detected: L1172, L1389, L1495B and TMC-1C. The C6H-/C6H column density ratio is greater than about 1.0% in every source, with a mean value of 3.0% (and standard deviation 0.92%). Combined with previous detections, our results show that anions are ubiquitous in dense clouds wherever C6H is present. The C6H-/C6H ratio is found to show a positive correlation with molecular hydrogen number density, and with the apparent age of the cloud. We also report the first detection of C4H- in TMC-1 (at 4.8-sigma confidence), and derive an anion-to-neutral ratio C4H-/C4H = (1.2 +- 0.4) x 10^-5 (= 0.0012 +- 0.0004%). Such a low value compared with C6H- highlights the need for a revised radiative electron attachment rate for C4H. Chemical model calculations show that the observed C4H- could be produced as a result of reactions of oxygen atoms with C5H- and C6H-.

Negative ion chemistry in the coma of comet 1P/Halley

AbstractNegative ions (anions) were identified in the coma of comet 1P/Halley during in situ Electron Electrostatic Analyzer measurements performed by the Giotto spacecraft in 1986. These anions were detected with masses in the range 7–110 amu, but with insufficient mass resolution to permit unambiguous identification. We present details of a new chemical‐hydrodynamic model for the coma of comet Halley that includes—for the first time—atomic and molecular anions, in addition to a comprehensive hydrocarbon chemistry. Anion number densities are calculated as a function of radius in the coma, and compared with the Giotto results. Important anion production mechanisms are found to include radiative electron attachment, polar photodissociation, dissociative electron attachment, and proton transfer. The polyyne anions C4H−and C6H−are found to be likely candidates to explain the Giotto anion mass spectrum in the range 49–73 amu. TheCN−anion probably makes a significant contribution to the mass spectrum at 26 amu. Larger carbon‐chain anions such as C8H−can explain the peak near 100 amu provided there is a source of large carbon‐chain‐bearing molecules from the cometary nucleus.

DENSITY-ENHANCED GAS AND DUST SHELLS IN A NEW CHEMICAL MODEL FOR IRC+10216

A new chemical model is presented for the carbon-rich circumstellar envelope (CSE) of the asymptotic giant branch star IRC+10216. The model includes shells of matter with densities that are enhanced relative to the surrounding circumstellar medium. The chemical model uses an updated reaction network including reactions from the RATE06 database and a more detailed anion chemistry. In particular, new mechanisms are considered for the formation of CN{sup -}, C{sub 3}N{sup -}, and C{sub 2}H{sup -}, and for the reactions of hydrocarbon anions with atomic nitrogen and with the most abundant cations in the CSE. New reactions involving H{sup -} are included which result in the production of significant amounts of C{sub 2}H{sup -} and CN{sup -} in the inner envelope. The calculated radial molecular abundance profiles for the hydrocarbons C{sub 2}H, C{sub 4}H, and C{sub 6}H and the cyanopolyynes HC{sub 3}N and HC{sub 5}N show narrow peaks which are in better agreement with observations than previous models. Thus, the narrow rings observed in molecular microwave emission surrounding IRC+10216 are interpreted as arising in regions of the envelope where the gas and dust densities are greater than the surrounding circumstellar medium. Our models show that CN{sup -} and C{sub 2}H{sup -}more » may be detectable in IRC+10216 despite the very low theorized radiative electron attachment rates of their parent neutral species. We also show that magnesium isocyanide (MgNC) can be formed in the outer envelope through radiative association involving Mg{sup +} and the cyanopolyyne species.« less