The electric-charge distribution in dark molecular clouds

Abstract We have extended previous quantum mechanical calculations of cross sections and rate coefficients for electron attachment to small, spherical, uncharged grains. It is found that potential-dependent resonances can have large effects on the low-energy cross sections and hence on the low-temperature rate coefficients. Comparison is made with previous calculations of electron attachment to PAH molecules, which adopted a similar approach. At low temperatures ($T \mathrel {\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim $}}}\hbox{$<$}}}100$ K), the value of the rate coefficient for electron attachment to PAH molecules is subject to the same uncertainties as for small grains. At higher temperatures, on the other hand, we would expect the rate coefficients for both PAH molecules and small grains to converge to classical (Langevin) values. Whilst this is observed to be the case for small grains, the values for PAH molecules remain much lower than the classical limit. The reasons for this discrepancy are considered, and we conclude that the rate coefficients previously reported for electron attachment to PAH molecules are underestimated by between 2 and 3 orders of magnitude. The consequences for the electric-charge distribution in dark clouds are investigated.