Absorption spectroscopy in solid hydrogen: challenges to experimentalists and theorists

Hydrogen is the most abundant molecule in the universe, and over the years has provided a fundamental testing ground for both theory and experiment. By symmetry, isolated H2 molecules do not have allowed dipole rotational or vibration-rotational spectra. However, when they interact in the gas, liquid, or solid, there are induced dipoles that can interact with radiation. In the solid because of the large lattice constant, these dipoles arise primarily from the long-range induction by multipole moments of one molecule with the polarizability in its neighbors. By analyzing the intensities, one is able to obtain experimental values for not only the quadrupole moment, but also for higher-order moments as well. In the present paper, we review only a very limited part of the extensive research that has been carried out; namely, that of solid H2, although extensive experimental and theoretical results for other phases and for other isotopes exist. Solid H2 is a quantum crystal, in which the individual molecules undergo almost free rotation and vibration in the hcp lattice. This simplifies the identification of the observed transitions based on their frequencies calculated from well-known gaseous spectroscopic constants. Although high-resolution studies have revealed subtle effects such as crystal-field splittings, interferences between the allowed and induced dipoles in HD, structure of the phonon density of states, triple transitions, etc. we limit ourselves to only the zero-phonon single and double transitions in para-hydrogen, or for an isolated ortho-hydrogen molecule in a para-hydrogen environment. We review the extensive literature over the last four decades, and present comparisons between theory and experiment. From this analysis, we can draw a number of conclusions about the accuracy and consistency of the experimental data and the need for improvements in theory. q 2003 Elsevier B.V. All rights reserved.