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Molecular crystals and liquid crystals science technology. Section A, Molecular crystals and liquid crystals/Molecular crystals and liquid crystals science and technology. Section A, Molecular crystals and liquid crystals

Fast Dephasing in Glasses Induced by Tunneling States and Local Modes

Stephan J. Zilker, D. Haarer, Yuri G. Vainer, Andrei V. Deev, Michail A. Kol'chenkq, Roman I. Personov

Published May 1, 1998• 0 citations
Publisher

Abstract

We present a comprehensive investigation of fast line broadening processes in two amorphous polymers, polymethylmethacrylate and polyisobutylene. Using a combination of two-pulse and incoherent photon echo spectroscopy, we have studied the homogeneous linewidth of embedded impurity molecules in a wide temperature range, namely 0.4–22 K. Thus, we are able to separate clearly between the two active dephasing mechanisms, two-level systems and low-frequency modes.

DOI
10.1080/10587259808042470

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We have performed optical photon echo measurements on amorphous toluene doped with zinc-octaethylporphine over a wide temperature interval (0.35–100 K). This broad temperature range, reaching from subkelvins up to the Debye temperature of toluene (100.7 K), enabled a clear separation between the low and high temperature contributions of line broadening processes to the total optical dephasing. The high temperature part of the measured dephasing times was analyzed using recent data from light scattering experiments which determined the spectrum of low-frequency modes of the toluene matrix. The analysis shows that two line-broadening mechanisms contribute mainly to total dephasing: the tunneling transitions of two-level systems, which lead to a power-law temperature dependence of the linewidth at low temperatures (T<3–4 K), and the interaction of the chromophore with low-frequency phonon modes of the glassy matrix, which gives rise to an exponential behavior at high temperatures. It was shown that the contribution of acoustic phonons to optical dephasing in the studied system is negligible over the whole temperature region of the measurements.

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Low temperature dynamics in amorphous solids: A photon echo study

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The long-lived stimulated photon echo is put forward as a powerful technique to probe structural dynamics in glasses and other amorphous solids. We present results of optical dephasing measurements on several doped organic glasses (deuterated ethanol, toluene, and triethylamine) and polymers (polystyrene, polyvinylbutyral, and polymethylmethacrylate). Using a stochastic analysis within the so-called two-level-system model, we can extract from these measurements the distribution function of fluctuation rates over a huge range of time scales, varying from picoseconds to hundreds of milliseconds. We show that the dynamics, which lies at the origin of spectral diffusion, is very dependent on the glass structure and can be described by two separate hyperbolic distribution functions of fluctuation rates. From longitudinal photon echo measurements in the population dimension, we find evidence for structural dynamics which is not accounted for in the standard two-level-system model and which is related to a spatially inhomogeneous distribution of fluctuation centers.

The Journal of Chemical Physics•Oct 15, 1994•56 citations
We present a comprehensive investigation of fast line broadening processes in two amorphous polymers, polymethylmethacrylate and polyisobutylene. Using a combination of two-pulse and incoherent photon echo spectroscopy, we have studied the homogeneous linewidth of embedded impurity molecules in a wide temperature range, namely 0.4–22 K. Thus, we are able to separate clearly between the two active dephasing mechanisms, two-level systems and low-frequency modes.
We have performed optical photon echo measurements on amorphous toluene doped with zinc-octaethylporphine over a wide temperature interval (0.35–100 K). This broad temperature range, reaching from subkelvins up to the Debye temperature of toluene (100.7 K), enabled a clear separation between the low and high temperature contributions of line broadening processes to the total optical dephasing. The high temperature part of the measured dephasing times was analyzed using recent data from light scattering experiments which determined the spectrum of low-frequency modes of the toluene matrix. The analysis shows that two line-broadening mechanisms contribute mainly to total dephasing: the tunneling transitions of two-level systems, which lead to a power-law temperature dependence of the linewidth at low temperatures (T<3–4 K), and the interaction of the chromophore with low-frequency phonon modes of the glassy matrix, which gives rise to an exponential behavior at high temperatures. It was shown that the contribution of acoustic phonons to optical dephasing in the studied system is negligible over the whole temperature region of the measurements.
The optical dephasing in frozen amorphous toluene doped with octaethylporphine zinc is investigated using the photon echo technique over a wide range of temperatures (0.4–100 K) up to the Debye temperature of solid toluene (T D=100.7 K). The contributions of different mechanisms to the broadening of the zero-phonon line (ZPL) are reliably separated owing to the measurements performed over such a wide range of temperatures. Analysis of the experimental data demonstrates that, at low temperatures, the main contribution to the optical dephasing is determined by the interaction of impurities with fast tunneling transitions in two-level systems. The temperature dependence of the linewidth exhibits a quasi-linear behavior at temperatures below 3–4 K. At higher temperatures, the dominant contribution to the dephasing is made by the interaction of impurities with quasi-local phonons, which leads to a quasi-exponential temperature dependence of the linewidth. It is shown that the latter contribution can be described in the framework of the soft-potential model allowing for a broad spectrum of low-frequency phonon vibrations in the matrix. The temperature of the crossover between the aforementioned two mechanisms of line broadening is determined.
The photon echo (PE) spectroscopy and single-molecule spectroscopy (SMS) may be combined to give a very powerful tool for comprehensive study of low-temperature dynamics in dye-doped disordered solids (polymers, glasses). At the same time, this type of studies are likely to reveal discrepancies when comparing characteristic times of optical dephasing T2 and single-molecule zero-phonon spectral lines (ZPL) broadening obtained from PE and SMS, correspondingly, for tetra-tert-butylterrylene in polyisobutylene in the temperature range of a few–dozen of Kelvins [see Phys. Status Solidi B 241, 3480 and 3487 (2004)]. Inexplicably, PE-experiments demonstrated T2-times to be much shorter than it is sufficient to cause the corresponding ZPL broadening. Here we experimentally solve this problem and show that at T = 4.5–15 K the incoherent PE gives T2-times which correspond to the narrowest SM ZPL. On the SM-level there is a pronounced additional ZPL-broadening due to spectral diffusion processes which are strongly dependent on the characteristics time of the measurement (tens of nanoseconds for PE and seconds for SMS). There is also a broad distribution of ZPL spectral widths for different SMs due to different local environments, that contribute differently to both the optical dephasing and the spectral diffusion processes, but always in addition to the value of inverse optical dephasing times measured using a PE technique.
A joint analysis of spectroscopic data obtained at liquid–helium temperatures by three line-narrowing techniques, photon echo (PE), persistent hole burning (HB), and single molecule spectroscopy (SMS), is presented. Two polymer systems, polyisobutylene (PIB) and polymethylmethacrylate (PMMA), doped with tetra-tert-butylterrylene (TBT) were studied via PE and HB techniques and the results are compared with literature data [R. Kettner et al., J. Phys. Chem. 98, 6671 (1994); B. Kozankiewicz et al., J. Chem. Phys. 101, 9377 (1994)] obtained by SMS. Both systems behave quite differently. In TBT/PIB a rather strong influence of a dispersion of the dephasing time T2 was found which plays only a minor role in TBT/PMMA. We have also measured the temperature dependence of T2 for both systems in a broad temperature range (0.4–22 K). Using these data we separated the two different contributions to the optical dephasing — due to an interaction with two-level systems and due to coupling with local low-frequency modes. The data are compared with calculations using a numerical and a semianalytical model in the presence of a large dispersion of the single molecule parameters. Furthermore, we discuss the differences of the linewidths as measured by different experimental methods.
The long-lived stimulated photon echo is put forward as a powerful technique to probe structural dynamics in glasses and other amorphous solids. We present results of optical dephasing measurements on several doped organic glasses (deuterated ethanol, toluene, and triethylamine) and polymers (polystyrene, polyvinylbutyral, and polymethylmethacrylate). Using a stochastic analysis within the so-called two-level-system model, we can extract from these measurements the distribution function of fluctuation rates over a huge range of time scales, varying from picoseconds to hundreds of milliseconds. We show that the dynamics, which lies at the origin of spectral diffusion, is very dependent on the glass structure and can be described by two separate hyperbolic distribution functions of fluctuation rates. From longitudinal photon echo measurements in the population dimension, we find evidence for structural dynamics which is not accounted for in the standard two-level-system model and which is related to a spatially inhomogeneous distribution of fluctuation centers.