Radiative Association Rate Coefficients Research Articles

The magnesium paradigm in IRC +10216: Discovery of MgC4H+, MgC3N+, MgC6H+, and MgC5N+

We found four series of harmonically related lines in IRC +10216 with the Yebes 40 m and IRAM 30 m telescopes. The first series corresponds to a molecule with a rotational constant, B, of 1448.5994±0.0013 MHz and a distortion constant, D, of 63.45±1.15 Hz and covers upper quantum numbers from Ju = 11 up to 33 (B1449). The second series is fitted with B = 1446.9380±0.0098 MHz and D = 91±23 Hz and covers upper quantum numbers from Ju = 11 up to 17 (B1447). The third series is fitted with B = 598.7495±0.0011 MHz and D = 6.13±0.43 Hz and covers quantum numbers from Ju = 26 up to 41 (B599). Finally, the frequencies of the last series of lines can be reproduced with B = 594.3176±0.0026 MHz and D = 4.92±1.16 Hz (B594). The large values of D point toward four metal-bearing carriers. After exploring all plausible candidates containing Na, Al, Mg, and other metals, our ab initio calculations indicate that the cations MgC4H+, MgC3N+, MgC6H+, and MgC5N+ must be the carriers of B1449, B1447, B599, and B594, respectively. These cations could be formed by the radiative association of Mg+ with C4H, C3N, C6H, and C5N, respectively. We calculated the radiative association rate coefficient of Mg+ with C4H, C3N, C6H, and C5N and incorporated them in our chemical model. The results confirm that the Mg-bearing cations can be formed through these radiative association reactions in the outer layers of IRC +10216. This is the first time that cationic metal-bearing species have been found in space. These results provide a new paradigm on the reactivity of ionized metals with abundant radicals and open the door for further characterization of similar species in metal-rich astrophysical environments.

Non-adiabatic dynamics in collisions of sodium and chlorine atoms and their ions.

Collisions of sodium and chlorine atoms and of their ions are studied within the diabatic two-state picture at energies below and above the ionic threshold with focus on the processes of radiative association, chemiionisation, and mutual neutralisation. The radiative-association cross sections as functions of collision energy are calculated up to 4.6 eV in the case of neutral atoms and up to 3.12 eV in the case of ions. The non-radiative charge-exchange cross sections as functions of collision energy are calculated up to 12 eV for chemiionisation and up to 10.52 eV for mutual neutralisation. The corresponding radiative-association rate coefficients are then determined up to 5300 K for the radiative association of neutral atoms and non-radiative charge-exchange and up to 3615 K for the radiative association of ions. Contribution of many Fano-Feshbach-type resonances is included to the rate coefficient of neutral-atom radiative association. The chemiionisation rate coefficients were calculated from 1000 K to 5300 K. The process of mutual neutralisation exhibits the largest cross sections and also the largest rate coefficients with values around 10-9 cm3 s-1 at all calculated temperatures, 120-5300 K.

Comprehensive Chemistry of HeH+ in the Early Universe*

Abstract The recent detection of the helium hydride ion (HeH+) in the planetary nebula NGC 7027 has inspired studies revisiting the dominant processes for its formation and destruction. Because radiative association is the primary formation mechanism for the helium hydride ion at all redshifts, and many early universe chemistry models rely on accurate results for calculations of destruction rates, we explicitly computed rovibrationally distinguished partial cross sections and rate coefficients of helium hydride formation via radiative association, and modeled the abundance of HeH+ in the early universe using recently available rate coefficients for all relevant formation and destruction reactions. The rate coefficients for the radiative association formation reaction of HeH+ and radiative charge transfer in He+ + H collisions were obtained using two distinct methods. Our newly calculated rate coefficients along with other recently reported He chemistry reactions and updated cosmological parameters were incorporated into a new calculation of the abundance of HeH+ for redshifts between z = 10 and 4000. We find that the abundance of the helium hydride ion is at least three orders of magnitude larger than previous predictions for redshifts near z = 20. Rovibrationally distinguished radiative association rate coefficients are given as a function of temperature, and the significance of stimulated radiative association for redshifts z > 400 is also shown.

Formation of CO+ by radiative association II

ABSTRACT Radiative association of an oxygen atom with a carbon cation is investigated using quantal and semiclassical methods. The total rate coefficient for spontaneous radiative association of O(2s22p4, 3P) with C+(2s22p, 2P) on the doublet manifold is determined from the corresponding cross-sections. The cross-sections for the ${\rm 1}^2\, \Sigma ^-\rightarrow {\rm A}^2\Pi$, ${\rm 2}^2\, \Sigma ^-\rightarrow {\rm A}^2\Pi$, and ${\rm C}^2\, \Delta \rightarrow {\rm A}^2\Pi$ continuum-bound processes are calculated either semiclassically, in combination with the Breit–Wigner approach, or fully quantum mechanically. In the temperature range 10–10 000 K, our recommended total rate coefficient, obtained from these calculations and the data of Zámečníková et al. (2019), slowly increases from 7.5 × 10−18 cm3s−1 to 2.1 × 10−17 cm3s−1. Corresponding aspects of the CO+ and CO formations in SN 1987A are discussed.

Formation of CO+ by radiative association

ABSTRACT We theoretically estimate formation rate coefficients for CO+ through the radiative association of C+(2P) with O(3P). In 1989, Petuchowski et al. claimed radiative association to be the most important route for CO+ formation in SN 1987A. In 1990, Dalgarno, Du and You challenged this claim. Therefore, in this study, we improve previous estimates of the radiative association rate coefficients for forming CO+ from C+(2P) and O(3P). To do this, we perform quantum mechanically based perturbation theory calculations as well as semiclassical calculations, which are combined with Breit–Wigner theory in order to add the effect of shape resonances. We explicitly include four electronic transitions. The required potential energy and transition dipole-moment curves are obtained through large basis set multireference configuration interaction electronic structure calculations. We report cross-sections and from these we obtain rate coefficients in the range of 10 –10 000 K, finding that the CO+ formation rate coefficient is larger than the previous estimate by Dalgarno et al. Still our results support their claim that in SN 1987A, CO is mainly formed through radiative association and not through the charge transfer reaction CO+ + O → CO + O+ as earlier suggested by Petuchowski et al.

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 silicon monoxide by radiative association: the impact of resonances

Detailed quantum chemistry calculations within the multireference configuration interaction approximation with the Davidson correction are presented using an aug-cc-pV6Z basis set, for the potential energy curves and transition dipole moments between low lying molecular states of singlet spin symmetry for the SiO molecule. The high quality molecular data are used to obtain radiative association cross sections and rate coefficients for collisions between ground state Si and O atoms. Quantal calculations are compared with semiclassical results. Using a quantum kinetic theory of radiative association in which quasibound levels are assumed to be in local thermodynamic equilibrium, we find that resonances play an important role in enhancing the rate coefficients at low temperatures by several orders of magnitude from that predicted by standard quantum scattering formulations. These new molecular formation rates may have important implications for applications in astrophysics.

STATE RESOLVED DATA FOR RADIATIVE ASSOCIATION OF H AND H + AND FOR PHOTODISSOCIATION OF ${\rm H}_2^+$

The matrix elements and energies needed to calculate vibrational-rotational state resolved cross sections and rate coefficients for radiative association of H and p and for photodissociation of H${}_2^+$ are presented for applications to simulations of chemistry in the early Universe and to stellar atmospheres.

Depopulation of metastable helium He(21S) by radiative association with hydrogen and lithium cations

Depopulation of metastable He(21S) by radiative association with hydrogen and lithium ions resulting in the formation of the HeH+ and HeLi+ molecular ions is investigated. Energy dependent cross-sections for spontaneous and stimulated processes on the spin-singlet manifold are calculated using a fully quantal approach and considering the association to rotational–vibrational states of the lowest singlet electronic states XΣ+1, AΣ+1, and BΣ+1 from the continuum states of the BΣ+1 electronic state. Evaluation of the cross-sections is based on highly accurate quantum calculations taking into account all possible state-to-state transitions at thermal energies (for spontaneous association) or at higher background temperatures (stimulated association). The corresponding rate coefficients are then presented as functions of temperature. As expected calculations show that the rate coefficients for radiative association to the X and A states are several orders of magnitude larger than the one for the B state formation. On the other hand, stimulation of the radiative association by black-body background radiation has a noticeable effect only on the B→B process.

DEPOPULATION OF METASTABLE HELIUM BY RADIATIVE ASSOCIATION WITH HYDROGEN AND LITHIUM IONS

Depopulation of metastable He(23S) by radiative association with hydrogen and lithium ions is investigated using a fully quantal approach. Rate coefficients for spontaneous and stimulated radiative association of the HeH+, HeD+, and LiHe+ molecular ions on the spin-triplet manifold are presented as functions of temperature considering the association to rotational-vibrational states of the lowest triplet electronic states a 3Σ+ and b 3Σ+ from the continuum states of the b 3Σ+ electronic state. Evaluation of the rate coefficients is based on highly accurate quantum calculations, taking into account all possible state-to-state transitions at thermal energies (for spontaneous association) or at higher background energies (stimulated association). As expected, calculations show that the rate coefficients for radiative association to the a state are several orders of magnitude larger than the one for the b state formation. A noticeable effect by blackbody background radiation on the radiative association is only obtained for the b → b process. Aspects of the formation and abundance of the metastable HeH+(a 3Σ+) in astrophysical environments are briefly discussed.

Rate coefficient for the important interstellar radiative association between [formula omitted] and H2 from classical reaction dynamics

The CH3++H2→CH5++hν radiative associative reaction plays a major role in the synthesis of complex hydrocarbons in interstellar dust clouds and was studied using a part-classical, part-ab initio approach. A first-principles potential energy surface (PES) at CCSD/6-311++G(2df, 2pd) was constructed from a set of 828 data points in configurational space using modified Shepard interpolation, and was used in conjuction with long-range perturbation theory at large intermolecular distances. Data points are chosen so as to optimize PES construction according to the GROW software algorithm. Classical reaction dynamics was employed to infer the reaction rate coefficient.

Radiative charge transfer and radiative association in He++Ne collisions

A fully quantum-mechanical approach is utilized to study the collision process of He${}^{+}$ with neutral neon, and the radiative charge transfer (RCT) and radiative association (RA) cross sections are presented in the energy range from 0.08 meV to 1 eV, while the optical potential and semiclassical methods are adopted to calculate the total radiative decay cross sections for energies from 0.08 meV to 5 keV. The potential energy curves and dipole transition matrix elements are obtained by an ab initio multireference configuration interaction package. For the related three lowest $X$ ${}^{2}{\ensuremath{\Sigma}}^{+}$, $A$ ${}^{2}\ensuremath{\Pi}$, and $B$ ${}^{2}{\ensuremath{\Sigma}}^{+}$ states, the spectroscopic data are in good agreement with other theoretical calculations and experimental measurements. Our results indicate that the RCT cross section is much larger than the nonradiative charge transfer cross section for collision energy $E$ $<$ 20 eV, and when $E$ $>$ 40 eV, the nonradiative process becomes dominant. Especially, we found that in the present collision system the RA process is more important than the RCT process when $E$ $<$ 1 meV. The RCT and RA rate coefficients are also given for temperatures from 1 to 4 $\ifmmode\times\else\texttimes\fi{}{10}^{3}$ K.

Formation of SiC by radiative association

Rate coefficients for radiative association of silicon and carbon atoms to form silicon carbide molecule (SiC) are estimated. The radiative association of Si(3P) and C(3P) atoms mainly occurs through the C3Π state followed by radiative decay to the X3Π state. For the temperature range of 300–14 000 K, the rate coefficients slowly increase with temperature and they can be expressed by K(T) = 2.038 × 10−17(T/300)−0.01263 exp(−136.73/T) cm3 s−1.

Erratum: Radiative association in Li+H revisited: the role of quasi-bound states

The paper 'Radiative association in Li+H revisited: the role of quasi-bound states' was published in Mon. Not. R. Astron. Soc. 341, 361-368 (2003). We have discovered that there was an error in this paper. This error arose in the code for calculating dipole matrix elements involving quasi-bound levels with high rotational angular momentum (and hence high collision energy). The contribution from these quasi-bound levels to the radiative association rate coefficient ...

Silicon monosulphide radiative association

Rate coefficients for radiative association of silicon and sulphur atoms to form silicon monosulphide (SiS) molecule are estimated. The radiative association is due mainly to approach in the E1Σ+ and A1Π states of SiS. For temperatures ranging from ∼1000 to ∼14 000 K, the rate coefficients are found to vary from 8.43 × 10−17 to 2.69 × 10−16 cm3 s−1. Our calculated rate coefficient is higher than the values used in modelling the chemistry of Type Ia supernovae.

Radiative association of C and P, and Si and P atoms

The rate coefficients for the formation of carbon monophosphide (CP) and silicon monophosphide (SiP) by radiative association are estimated for temperatures ranging from 300 to 14 100 K. In this temperature range, the radiative association rate coefficients are found to vary from 1.14 × 10−18 to 1.62 × 10−18 cm3 s−1 and from 3.73 × 10−20 to 7.03 × 10−20 cm3 s−1 for CP and SiP, respectively. In both cases, rate coefficients increase slowly with the increase in temperature.

CH+Radiative Association

The results for the calculation of the CH+ formation rate by radiative association are presented. The calculation is performed on the lowest potential energy curves, which are coupled by the spin-orbit and rotational interaction. The association rate coefficient is calculated at temperatures up to 1000 K. The energy-dependent radiative association rate coefficient is marked by the presence of many resonances, which are responsible for significant contributions to the rate. The calculated rate coefficient is lower than that predicted by a previous quantum calculation. An analytical expression for the temperature-dependent association rate coefficient is given for the interval of temperatures 5-1000 K.

Formation of SO, SO+, and S2by Radiative Association

Rate coefficients for radiative association of SO, SO+, and S2 are estimated. For temperatures ranging from 300 to 14,000 K, the direct radiative association rate coefficients are found to vary with temperature from 1.73 × 10-19 to 7.29 × 10-19 cm3 s-1 and from 1.49 × 10-21 to 3.70 × 10-19 cm3 s-1 for S2 and SO, respectively. The rate coefficients for formation through the inverse predissociation for S2 are found to vary from 3.59 × 10-18 to 1.44 × 10-20 cm3 s-1. For SO+, the direct rate coefficient varies rapidly with temperature from 3.62 × 10-27 cm3 s-1 at 2000 K to 2.34 × 10-20 cm3 s-1 at 14,000 K. The direct radiative association rate coefficients increase with the increase in temperature, but the inverse predissociation rate coefficients decrease with the increase in temperature.

Magnesium Monocationic Complexes: A Theoretical Study of Metal Ion Binding Energies and Gas-Phase Association Kinetics

Bond dissociation energies (BDEs) for complexes of ground state Mg+ (2S) with several small oxygen- and nitrogen-containing ligands (H2O, CO, CO2, H2CO, CH3OH, HCOOH, H2CCO, CH3CHO, c-C2H4O, H2CCHOH, CH3CH2OH, CH3OCH3, NH3, HCN, H2CNH, CH3NH2, CH3CN, CH3CH2NH2, (CH3)2NH, H2NCN, and HCONH2) have been calculated at the CP-dG2thaw level of theory. These BDE values, as well as counterpoise-corrected MP2(thaw)/6-311+G(2df,p) calculations on the Mg+ complexes of several larger ligands, augment and complement existing experimental or theoretical determinations of gas-phase Mg+/ligand bond strengths. The reaction kinetics of complex formation are also investigated via variational transition state theory (VTST) calculations using the computed ligand and molecular ion parameters. Radiative association rate coefficients for most of these systems increase by approximately 1 order of magnitude with every 3-fold reduction in temperature from 300 to 10 K. Several of the largest molecules surveyed-notably, CH3COOH, (CH3)2CO, and CH3CH2CN-exhibit comparatively efficient radiative association with Mg+ (k(RA) > or = 1.0 x 10(-10) cm3 molecule(-1) s(-1)) at temperatures as high as 100 K, implying that these processes may have a considerable influence on the metal ion chemistry of warm molecular astrophysical environments known to contain these potential ligands. Our calculations also identify the infrared chromophoric brightness of various functional groups as a significant factor influencing the efficiency of the radiative association process.

The radiative association of Si and N atoms, C and O atoms, and C atoms and S+ ions

Potential energy curves for the SiN.X2Σ+ and A2Πi electronic states, and the SiN(A2Πi--X2Σ+) transition moment function (TMF) are obtained at the MRCI level of theory, with a SA-CASSCF zero-order wavefunction. These potential energy curves and TMF are used to compute radiative association rate coefficients of Si and N atoms for temperatures in the range 300--14 700 K. Rate coefficients for radiative association of carbon and oxygen atoms for the formation of CO, and carbon atoms and sulphur ions for the formation of CS+, are also calculated for temperatures in the range 300--14 700 K. These radiative association rate coefficients (k) are fitted to the standard equation (1) where A=2.02×10−19 cm3 s−1, α=0.029 and β=83.9 for the SiN molecule. The corresponding values for the CO molecule are 3.09×10−17 cm3 s−1, 0.325 and 1629, and those for the CS+ ion are 2.01×10−18 cm3 s−1, 0.066 and 300.9. Radiative association of C and O atoms occurs with emissions of the first overtone band at 2.3 μm and the fundamental band at 4.6 μm found in the ejecta of Supernova 1987A and novae.