Two-Photon Absorption by Hydrogen Molecules: Origin of the 2175 Angstrom   Astronomical Band?
  The near UV spectra of OB stars in our galaxy are often dominated by a very broad extinction band peaking at ~2175 Angstroms. Forty years after its discovery, the origin of this band remains unknown, although interstellar dust particles are generally assumed to be the carriers. Here we report that two-photon absorption by hydrogen molecules in gaseous clouds enveloping OB stars can lead to a band peaking at ~2175 Angstroms. We present astronomical spectral evidence supporting our proposal that this nonlinear absorption mechanism accounts for the lambda2175 feature. 
Peter P. Sorokin and James H. Glownia