Absorption spectra of electrified hydrogen molecules
  Molecular hydrogen normally has only weak, quadrupole transitions between its
rovibrational states, but in a static electric field it acquires a dipole
moment and a set of allowed transitions. Here we use published ab initio
calculations of the static electrical response tensors of the H2 molecule to
construct the perturbed rovibrational eigensystem and its ground state
absorptions. We restrict attention to two simple field configurations that are
relevant to condensed hydrogen molecules in the interstellar medium: a uniform
electric field, and the field of a point-like charge. The energy eigenstates
are mixtures of vibrational and angular momentum eigenstates so there are many
transitions that satisfy the dipole selection rules. We find that mixing is
strongest amongst the states with high vibrational excitation, leading to
hundreds of absorption lines across the optical and near infrared. These
spectra are very different to that of the field-free molecule, so if they
appeared in astronomical data they would be difficult to assign. Furthermore in
a condensed environment the excited states likely have short lifetimes to
internal conversion, giving the absorption lines a diffuse appearance. We
therefore suggest electrified H2 as a possible carrier of the Diffuse
Interstellar Bands (DIBs). We further argue that in principle it may be
possible to account for all of the DIBs with this one carrier. However, despite
electrification the transitions are not very strong and a large column of
condensed H2 would be required, making it difficult to reconcile this
possibility with our current understanding of the ISM.
