J. Phys. Chem. B 2006, 110, 227-233
227
High-Pressure Studies of Optical Dephasing in Polymer Glasses Michael J. McIntire,† Masashi Yamaguchi,‡ Misha A. Kol’chenko,§ Yuri G. Vainer,§ and Eric L. Chronister*,| Department of Chemistry, Natural and Applied Sciences, UniVersity of WisconsinsGreen Bay, Green Bay, Wisconsin 54311, Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, Institute of Spectroscopy, Russian Academy of Sciences, Moscow Reg., Troitsk 142190, Russia, and Department of Chemistry, UniVersity of California, RiVerside, California 92521 ReceiVed: October 12, 2005
The effect of high pressure on the optical dephasing of chromophores in organic polymers at low temperature is evaluated within the stochastic sudden jump two-level-system (TLS) model. The approximations within the “standard” TLS model cannot account for the observed pressure dependence of the pure dephasing rate without ad hoc assumptions about changes in the TLS density of states. However, the photon echo model of Geva and Skinner for disordered systems can be used to model pressure-dependent optical dephasing results for a variety of doped polymer systems without assuming changes in the TLS density of states. The relative importance of pressure-induced changes in TLS density, chromophore-TLS coupling, and TLS-phonon coupling is evaluated by fitting experimental high-pressure photon echo results to the TLS model.
1. Introduction theoretical1-4
High pressure has recently been utilized as a and experimental5-11 probe of dynamics in glasses. These studies have revealed a variety of pressure-induced effects, including irreversible structural changes in covalent glasses,1-3 narrowing of spectral holes in molecular glasses,5,6 narrowing of spectral holes in polymer glasses above 4 K,7,8,12 and dephasing in photon echo studies of polymeric glasses at low temperature.9-11 These results have been interpreted in terms of irreversible collapse of the tunneling two-level system (TLS) in covalent glasses,1-3 crystallization in a glass-forming liquid,5 collapse of the TLS in molecular glasses,6 the conversion of the TLS in strongly localized modes,7,8 and a change in the form of the TLS density of states.11 The present study will investigate the effect of compression on TLS dynamics by utilizing the stochastic optical dephasing model of Geva and Skinner13,14 to model pressure-dependent photon echo measurements of chromophores in polymeric glasses at variable low temperature. The relative importance of pressure-induced changes in TLS density, chromophore-TLS coupling, and TLS-phonon coupling will be evaluated by comparison with experimental high-pressure photon echo results. The tunneling TLS model15,16 postulates that a distribution of phonon-assisted tunneling rates accounts for the characteristic low-temperature dynamics of glasses. The TLS model has accurately accounted for much of the thermal, acoustical, and optical properties of amorphous solids at low temperature.17,18 The TLS energy E ) x∆2+(pΩ)2 is determined by the asymmetry in the energies of two adjacent potential wells, ∆, and the TLS tunneling frequency, Ω. The tunneling frequency Ω ) ω exp(-λ) is proportional to the single-well frequency, ω, and depends exponentially on the tunneling parameter λ ) * To whom correspondence should be addressed. † University of WisconsinsGreen Bay. ‡ Rensselaer Polytechnic Institute. § Russian Academy of Sciences. | University of California.
dx2mU/p, where d is the tunneling distance, m is the reduced mass, and U is the potential energy barrier between the two minima. A relatively flat distribution of TLS energies can be used to model the low-temperature thermal properties of glasses. The seemingly ubiquitous power law temperature-dependent properties of glasses have been related to the form of the TLS density of states; however, more exact TLS model analyses have shown that even a completely flat TLS distribution can yield different “effective” power law exponents.13,14,20 Despite its success at modeling low-temperature phenomena in amorphous solids, the microscopic nature of the TLS in amorphous solids remains unclear. Although it has been postulated that TLSs may be associated with voids that form near the glass transition temperature,19 experimental determination of the microscopic nature of the TLS has been elusive. In the present study, high pressure is utilized as an experimental parameter that can control the free volume of amorphous solids. Compression of the glass can induce changes in several important TLS parameters, such as the TLS density, the chromophore-TLS coupling, and the TLS-phonon coupling. Pressure-induced changes in these parameters can affect the temperature dependence of the homogeneous line width, even in the absence of a change in either the magnitude or the form of the TLS density of states. Previous high-pressure photon echo results at low temperature (
