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Optics and Spectroscopy

Low-temperature photon echo study of 2-MTHF glass under high pressure

Michael McIntire, E. L. Chronister

Published May 1, 2005• 1 citation
Publisher

Abstract

A high-pressure photon echo study of the low-temperature dynamics of rhodamine 640 in a 2-methyltetrahydrofuran (2-MTHF) molecular glass is reported. High pressure is found to decrease the homogeneous dephasing rate comparably to the volume compression ratio. The pressured-induced line narrowing is also qualitatively similar to high-pressure hole burning results reported for a 2-MTHF glass doped with bacteriochlorophyll a . The results are consistent with a pressure-induced decrease in the number of tunneling two-level systems (TLSs) that may be associated with collapsible void space in the molecular glass. Comparable high-pressure photon echo studies of organic polymer glasses over the same low-temperature range do not show line narrowing at high pressure, suggesting an intrinsic difference in the nature of TLSs for molecular versus polymer glasses.

DOI
10.1134/1.1929054

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A high-pressure photon echo study of the low-temperature dynamics of rhodamine 640 in a 2-methyltetrahydrofuran (2-MTHF) molecular glass is reported. High pressure is found to decrease the homogeneous dephasing rate comparably to the volume compression ratio. The pressured-induced line narrowing is also qualitatively similar to high-pressure hole burning results reported for a 2-MTHF glass doped with bacteriochlorophyll a . The results are consistent with a pressure-induced decrease in the number of tunneling two-level systems (TLSs) that may be associated with collapsible void space in the molecular glass. Comparable high-pressure photon echo studies of organic polymer glasses over the same low-temperature range do not show line narrowing at high pressure, suggesting an intrinsic difference in the nature of TLSs for molecular versus polymer glasses.
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.
We report pressure (0–28 kbar) and temperature (1.1–4.3 K) dependent photon echo results for tetra-tert-butyl-terrylene (TBT) in a polyisobutylene (PIB) host. Increased pressure is found to increase the homogeneous dephasing rate over the temperature range of this study, in contrast to pressure dependent hole burning results reported for other polymer systems at somewhat higher temperatures (⩾4.2 K). A relatively small pressure increase (0 to 9 kbar) resulted in a change in the temperature dependence of the dephasing characterized by a lowering of the temperature power law exponent from ∼1.3 to ∼0.9. Further pressure increase above 9 kbar was characterized by an additional increase in the homogeneous dephasing rate without further change in the temperature exponent. The results are compared to high pressure photon echo studies of other polymer systems, and discussed in terms of the TLS (two-level-systems) model.
Pressure- and temperature-dependent photon echo results are obtained for pentacene doped polymethyl methacrylate (PMMA). A unique pressure effect is observed in which the optical dephasing rate increases as the pressure is increased from ambient pressure to 4 kbar, above which the optical dephasing rate is pressure independent up to 43 kbar. The present results are also compared with pressure- and temperature-dependent photon echo results for rhodamine 101 in PMMA, in which the optical dephasing rate was completely insensitive to pressure over the range 0 to 30 kbar. A negative correlation is also observed between the optical dephasing rate and the spectral hole burning efficiency. Line broadening due to pressure induced spectral diffusion may be responsible for both the increased dephasing rate and the reduced spectral hole-burning at high pressure.
Temperature-dependent (1.2–2.1 K) photon echo measurements have been performed on quinizarin and rhodamine 101 doped into aluminosilica sol-gel glasses as well as rhodamine 101 doped poly-methyl-methacrylate (PMMA) polymer. We observe a temperature-dependent homogeneous linewidth for both quinizarin and rhodamine in the aluminosilica xerogel (ASX) matrix proportional to T2, in contrast to the T1.3±0.1 linewidth dependence typically observed for a variety of organic and inorganic glass hosts in this temperature range (including rhodamine 101 doped PMMA). The contrasting optical dephasing results for the ASX glass and PMMA suggest that the ASX glass may have unique two-level-system dynamics.
The homogeneous optical dephasing times (T2) and the non-photochemical hole burning (NPHB) linewidths of resorufin in glycerol glass are reported from 1.1 to 25.5 K. At low temperature, the linewidths obtained from NPHB are substantially wider than the linewidths (1/πT2) obtained from photon echo measurements, but the two become equal at high temperatures. The extra broadening of the NPHB is found to increase as T0.8. A model of homogeneous dephasing including both two-level system and pseudolocal mode contributions is successful over the entire temperature range.
High pressure photon echo results are presented for pentacene doped naphthalene, pentacene doped (triclinic and monoclinic) p-terphenyl, and rhodamine doped PMMA. Figure 1 illustrates typical photon echo intensity decays in a high pressure diamond anvil cell at variable low temperatures. The effect of pressure and temperature on the homogeneous dephasing is used to characterize the interactions between the impurity molecule and the host crystal, as well as provide a unique probe of local polymorphic phase changes.