Exciton pressure dependence

Huffman, D. R., L. A. Schwalbe, and D. Schiferl, 1982. Use of smoke samples in diamond-anvil cells to measure pressure dependence of optical spectra application to the ZnO exciton, Solid State Commun. (in press). 

HMX or 1,3,5,7-tetranitro 1,3,5,7-tetrazacyclooctane exists in four polymorphic forms. These are molecular crystals. Both intramolecular excitons and intermolecular (charge transfer) excitons are predicted. Electronic charge transport depends on overlap of the molecular wave-functions and is therefore enhanced by the pressures (10 atm) in the shock wave during detonation. The mobilities of both types of excitons are also enhanced by pressure. 

In molecular solids the molecules cannot move around freely, but they are trapped in relatively deep potential wells, caused by the intermolecular potential. In these wells they can vibrate and since the vibrations of individual molecules are coupled, again by the intermolecular potential, one obtains collective vibrations of all the molecules in the solid, called lattice vibrations or phonons. Phonons associated with the center of mass motions of the molecules are called translational phonons, phonons associated with their hindered rotations or librations are called librons. The degree of hindrance of the rotations may vary. If the molecules have well-defined equilibrium orientations and perform small amplitude librations about these, one speaks about ordered phases. If the molecular rotations are nearly free or if the molecules can oscillate in several orientational pockets and easily jump between these pockets, then the solid is called orientationally disordered or plastic. Several molecular solids may occur in each of these phases, depending on the temperature and pressure they undergo order/disorder phase transitions. Also the intramolecular vibrations are coupled by the intermolecular potential, via its dependence on the internal coordinates. The excitations of the solid associated with such vibrations are called vibrational excitons or vibrons. 

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