The diffuse interstellar bands: a dipole-bound state hypothesis

Abstract

It is proposed that some, possibly many, of the unidentified diffuse interstellar absorption bands arise from rovibronic transitions between the ground states of negatively charged molecules and/or small grains, and shallow dipole–bound electronic states which lie close to the electron detachment threshold. Under this hypothesis the attributes for the neutral ‘molecular’ frameworks are electron affinities between 1 and 3 eV and permanent electric dipole moments of ∼2 debye or greater. Bound–bound spectra involving the lowest rotational levels have not been detected in the laboratory, but these proposed carriers appear to be capable of satisfying the main observational astronomical constraints: transitions that lie in the range from the near-ultraviolet to the near-infrared; a wide range of widths; band wavelengths that are invariant; and a large number of related but distinct carriers. The wavelengths of the lowest rotational lines of the band of the transition between the ground and a dipole–bound electronic state of the CH2CN− molecule appear to be consistent with a diffuse band near 8037 A.