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Experimentally observed narrowing of spectral holes in a glass under hydrostatic pressure confirms our theoretical finding that the external pressure, in addition to increasing the frequencies of soft localized modes, also reduces their number. This occurs because the majority of soft localized modes in glasses is shown to have a negative cubic anharmonicity. For that reason the applied pressure not only enhances the stiffness of these modes, but also transforms a fraction of them into tunneling two-level systems, whereas the simultaneous reverse transformations of some other two-level systems into soft localized modes are less numerous.

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Quantitative determination of hole burninginduced changes of site energy in glassy matricesWe measured the pressure-induced shift and broadening of spectral holes for 14 different impurity sites in a polycrystalline solid solution of chlorin (17,18-dihydro-21H,23H-porphine) in benzophenone. In addition, we investigated the glassy phase of the same system in a frequency range which covered the entire inhomogeneously broadened band. The temperature range of the experiments was between 1.5 and 4.2 K. Pressure levels up to 10 MPa were applied. In the glassy phase, we observed a linear frequency dependence of the shift per pressure. However, in the crystalline phase, the general trend of the data clearly indicated a nonlinear frequency dependence. Yet, the individual sites, especially those with lower frequencies, showed a kind, of stochastic scattering around the general trend behavior. The results were interpreted within the frame of existing models. The experiments demonstrate that matrix isolation combined with hole burning spectroscopy of large molecular impurities in polymorphic host materials under varying pressure conditions is a useful technique for determining molecular as well as bulk parameters, for examining problems of microelasticity and microplasticity, and for investigating the fundamental differences between glasses and crystals.

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