Calculations on the Formation Rates and Mechanisms for C n H Anions in Interstellar and Circumstellar Media

The rate coefficients for the radiative attachment reactions between the radicals C n H ( n = 2–8) 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 = 4,5, the new results are larger than the old estimates. The calculated rate coefficient for C 4 H + e − is now much too large to explain the current observed abundance ratio of C 4 H − /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 "dipole-bound" 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 2 m ( m = 2–4) and leading to the syntheses of the anions C 2 m H − .