The infrared dielectric function of solid para-hydrogen
  We report laboratory measurements of the absorption coefficient of solid
para-H2, within the wavelength range from 1 to 16.7 micron, at high spectral
resolution. In addition to the narrow rovibrational lines of H2 which are
familiar from gas phase spectroscopy, the data manifest double transitions and
broad phonon branches that are characteristic specifically of hydrogen in the
solid phase. These transitions are of interest because they provide a spectral
signature which is independent of the impurity content of the matrix. We have
used our data, in combination with a model of the ultraviolet absorptions of
the H2 molecule, to construct the dielectric function of solid para-H2 over a
broad range of frequencies. Our results will be useful in determining the
electromagnetic response of small particles of solid hydrogen. The dielectric
function makes it clear that pure H2 dust would contribute to IR extinction
predominantly by scattering starlight, rather than absorbing it, and the
characteristic IR absorption spectrum of the hydrogen matrix itself will be
difficult to observe.
