Calculations on the Formation Rates and Mechanisms for C_nH Anions in Interstellar and Circumstellar Media

Abstract

The rate coefficients for the radiative attachment reactions between the radicals C n H (n ¼ 2Y8) and electrons have been calculated with the aid of quantum chemistry using a phase-space theory in which strong coupling exists among the ground and any excited anion electronic states with the same spin. The results confirm that the process increases in efficiency with the size of the radical. For n ! 6, the rate coefficients lie at the collisional limit, in agreement with earlier estimates. For n ¼ 4,5, the new results are larger than the old estimates. The calculated rate coefficient for C 4 H þ e À is now much too large to explain the current observed abundance ratio of C 4 H À /C 4 H in IRC +10216 and L1527 and its upper limit in TMC-1. It is quite possible that electronic coupling is weak, and that so-called ''dipolebound'' and other states must act as doorways to radiative attachment. We have also calculated potential surfaces for dissociative attachment starting with the ''carbenes'' H 2 C 2m (m ¼ 2Y4) and leading to the syntheses of the anions C 2m H À .

Astronomical identification of CN-, the smallest observed molecular anion

“…The abundance of these anions relative to the neutral counterparts increases with both size and the electron affinity of the neutral molecule, as expected for formation by radiative electron attachment (Herbst & Osamura 2008). On inspection, however, this process fails to explain the abundance of the shortest observed anions, in particular C 4 H − and C 3 N − .…”

“…Since CN is a small molecule, the rate constant for the reaction of radiative electron attachment is likely to be very small. Here we assumed a value of 2 × 10 −15 cm 3 s −1 at 300 K, similar to that computed for C 2 H by Herbst & Osamura (2008). This process results in a too low formation rate for CN − , more than 5 orders of magnitude lower than that provided by the reactions of C − n and N atoms.…”

The UMIST database for astrochemistry 2012

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Electron attachment rates for PAH anions in the ISM and dark molecular clouds: dependence on their chemical properties

“…see Herbst & Osamura 2008 and the reference quoted therein) also follow phase-space theory reasoning along with the dominance of s-wave electron attachment at thresholds, although no dynamical coupling between nuclear vibration and impinging electrons has ever been explicitly included in that modelling. The approach of the RRKM and conservation of angular momentum (Pechukas & Light 1965), therefore, involves (i) the conjecture that electron in s-wave are captured at vanishing energies; and (ii) that a phase-space analysis of the neutral molecule's vibrational levels, vis-à-vis the vibrational density of states for the final, stable anion is employed to estimate attachment rates.…”

“…The earlier estimates (Herbst 1981), however, did not carry out explicit quantum dynamical analysis of the interaction and attachment mechanism of the impinging electron, but rather argued the possibility of applying phase-space theory to generate vibrational density of states for final species, the stable anion M − , at the scaled internal energy corresponding to the electron affinity value (EA) for the initial species, the neutral partner M. The above treatment involved a careful analysis based on the Rice-Ramsperger-Kassel-Marcus (RRKM) treatment (Marcus 1956) of the electron-attachment kinetics whereby the vibrational structures of the "reagent" (the initial neutral molecule) and of the "products" (the anionic stable molecules) are estimated from structural calculations and are employed to produce the final rates of formation. The conclusion was that radiative attachment of electrons to neutrals could occur efficiently at the low-temperature of the ISM for polyatomic neutrals containing more than three to four atoms and having EA values higer than 2-3 eV (Herbst & Osamura 2008). The above theory was discussed further by Herbst (1985) and Petrie & Herbst (1997), where the conjecture was added that the attachment process is efficiently initiated primarily by s-wave impinging electrons.…”