The diffuse interstellar bands: a dipole-bound state hypothesis

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 similar to 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 0(0)(0) 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 Angstrom.