Pure Vibrational Excitations
The rotationless vibrational levels of hydrogen molecules are ideally suited to illustrate the typical solid-state properties resulting from the effect of the intermolecular interaction on the dynamical and optical properties of molecules when condensed into the solid state. The vibrational coupling between neighboring molecules in solid H2 and D2 broadens the excited vibrational levels into energy bands which are the simplest and most perfect examples of single-band Bloch states in all of solid state physics. Moreover, the fortunate circumstance that the fundamental vibrational frequencies of the ortho and para species differ by an amount of the same order of magnitude as the width of the v=1 vibrational bands makes mixed ortho-para crystals, at low concentrations of either species, equally ideal examples of energy-band impurity problems which have been studied theoretically by perturbation, walk-counting, and Green function techniques, and experimentally by means of infrared and Raman spectroscopy. At low concentrations the very small deloalization of the bound impurity states gives rise to dramatic changes in the Raman scattering cross sections, while at higher concentrations the vibrational Raman spectra provide examples of the breakdown of the energy-band picture and the onset of localized states, in excellent agreement with the results of percolation theory. These various properties of the purely vibrational excitations in the solid hydrogens are discussed in this chapter.
KeywordsImpurity State Solid Hydrogen Vibrational Coupling Raman Cross Section Incoherent Excitation
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