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The Journal of Chemical Physics

Low temperature dynamics in amorphous solids: A photon echo study

Hans C. Meijers, Douwe A. Wiersma

Published Oct 15, 1994• 56 citations
Publisher

Abstract

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.

DOI
10.1063/1.468321

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The Journal of Chemical Physics•May 22, 2002•11 citations
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 principles of incoherent photon echoes (IPE) are shortly recalled. The results on dynamics in doped organic glasses and polymers (resorufin in d‐ and d6‐ethanol and terrylene in polyethylene) studied by IPE are presented. In all cases the fast dephasing processes are related to coupling with both two‐level systems (TLS) and pseudolocal mode. The nanosecond spectral diffusion (SD) processes has been observed. The IPE‐results are compared with hole‐burning and single molecule spectroscopy (SMS) data.
Picosecond photon echo (PE) experiments are used to examine the optical dephasing and intermolecular interactions of organic chromophores in a variety of glassy organic and polymer hosts. The inherent disorder in these systems leads to dynamics which span times from picoseconds to hours and longer. The temperature dependent optical dephasing as measured by the PE follows a power law characteristic of tunnelling twolevel systems (TLS) at low temperatures (1 °K to 4 °K) and is exponentially activated at higher temperatures due to dephasing by additional modes. The nature of these modes will be addressed. These data and those from other line narrowing experiments such as hole burning allow the study both of fast dephasing processes and spectral diffusion due to slow fluctuations in the host. The observed lineshapes and holewidths require dipolar coupling between the chromophore and TLS and offer information on the distribution of relaxation rates. Furthermore, techniques sensitive to different timescales are used to examine host dynamics by removing the guest-host coupling from consideration. The data are discussed in conjunction with recently developed theoretical treatments of dephasing in disordered systems.
Accumulated photon echoes (APE) were used to measure the dephasing rates of Nd3+ in various inorganic glasses as a function of excitation wavelength, sample temperature, and laser power. Although dephasing processes of optical transitions in solids are of considerable interest, they are poorly understood, largely due to the lack of experimental data on femtosecond and picosecond time scales. The APE technique using sub-picosecond laser sources has made it possible to measure in the time domain dephasing rates of inhomogeneously broadened transitions in amorphous systems.1,2
Until now, in spite of a great number of experimental and theoretical works in study of organic amorphous solids, the basic features of the low temperature dynamics of these mediums are poorly understood. One of the main reason of this is the lack of experimental information about these dynamical processes. In particular, it’s necessary to learn more about optical dephasing in doped organic glasses and polymers for wider temperature interval. It will allow to test the existent theories more correctly and to create new conceptions.
In the last two decades the dynamics of glasses has been a field of great interest. In order to explain the dynamical properties of glasses the two-level-system model has been proposed. In this model an atom, or group of atoms, can access two potential minima separated by a barrier. Phonon-assisted-tunneling between the two minima of this asymmetric double well potential represents a jump between two different local glass structures. These double well potentials are referred to as two-level-systems (TLS). In the standard TLS-model the potentials do not change in time. The intrinsic disorder of the glass provides a wide range of asymmetries and a broad distribution of transition rates. Consequently the dynamics of the glass takes place over a broad range of time scales.
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.