COMMENTS ON 2-PHOTON ABSORPTION BY H2 MOLECULES AS A SOURCE OF DIFFUSE INTERSTELLAR BANDS

It has been proposed that some of the diffuse interstellar absorption bands which have puzzled astronomers for several decades may be assigned to transitions in vibrationally excited H-2 molecules. These molecules would arise in circumstellar environments where stellar Lyman-alpha photons create a population of H-2 in the C (1) Pi(u), nu = 9 state through a two-photon process. From this state absorptions in the range between 7660 and 7880 Angstrom can occur. While intriguing, this suggestion overlooks several critical spectroscopic and astrophysical problems, and for now must be regarded as unproven and probably not viable. In this Letter, we summarize some problems with the hypothesis.