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Journal of Experimental and Theoretical Physics

Soft-potential model and homogeneous width of spectral lines of impurity centers in molecular amorphous media

Yu. G. Vainer, M. A. Kol’chenko, Roman I. Personov

Published Apr 1, 2001• 1 citation
Publisher

Abstract

A study is made to analyze the possibility of using the soft-potential model in optical investigations of disordered molecular systems with impurities. A procedure is suggested for calculating the temperature dependence of the homogeneous width of a phononless line in amorphous media with impurities within the soft-potential model. A calculation is performed of the temperature dependence of the width of a phononless line (optical dephasing) in an amorphous system of polymethyl methacrylate (PMMA) with an addition of tetratert-butylterrylene (TBT) using the parameters of this system known from the literature. Calculations are performed of the contributions to the width of a phononless line due to the interaction of an impurity with tunneling two-level systems, with thermally activated barrier crossings in double-well potentials, and with quasilocal modes of the matrix. The model calculation results are compared with the experimental data on the photon echo for TBT/PMMA, measured by us in the temperature range from 0.3 to 20 K. It is found that the soft-potential model describes qualitatively correctly the temperature behavior of the homogeneous width of a phononless line. In the temperature range of T < 2 K, where the main contribution to optical dephasing is associated with tunneling two-level systems, the predicted values of phononless line width agree well with the experimental data. At higher temperatures, some difference is observed between the prediction and experimental data, which may be due to the effect of impurity on the formation of quasilocal oscillation of the matrix.

DOI
10.1134/1.1371346

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A study is made to analyze the possibility of using the soft-potential model in optical investigations of disordered molecular systems with impurities. A procedure is suggested for calculating the temperature dependence of the homogeneous width of a phononless line in amorphous media with impurities within the soft-potential model. A calculation is performed of the temperature dependence of the width of a phononless line (optical dephasing) in an amorphous system of polymethyl methacrylate (PMMA) with an addition of tetratert-butylterrylene (TBT) using the parameters of this system known from the literature. Calculations are performed of the contributions to the width of a phononless line due to the interaction of an impurity with tunneling two-level systems, with thermally activated barrier crossings in double-well potentials, and with quasilocal modes of the matrix. The model calculation results are compared with the experimental data on the photon echo for TBT/PMMA, measured by us in the temperature range from 0.3 to 20 K. It is found that the soft-potential model describes qualitatively correctly the temperature behavior of the homogeneous width of a phononless line. In the temperature range of T < 2 K, where the main contribution to optical dephasing is associated with tunneling two-level systems, the predicted values of phononless line width agree well with the experimental data. At higher temperatures, some difference is observed between the prediction and experimental data, which may be due to the effect of impurity on the formation of quasilocal oscillation of the matrix.
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.
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.
Picosecond photon echo experiments on pentacene in polymethylmethacrylate (PMMA) show that the homogeneous width exhibits a T1.3 temperature dependence. This behavior is ascribed to coupling of the pentacene transition to randomly distributed low-frequency excitations in the amorphous host. A calculation of the resulting homogeneous width is presented which uses optical Redfield theory as a starting point. From the results of this calculation we conclude that the coupling between the pentacene molecule and the host low frequency excitations is of static dipolar nature and that the density of states of these excitations varies as ω0.3. Results of nonphotochemical hole-burning experiments on the same system are also reported. Comparison of these results with the ones obtained by the photon echo method indicates, that in the hole-burning experiments, the hole width and its temperature-induced broadening are dominated by spectral diffusion.
A modified theoretical model of photon echoes (PE) in low-temperature doped glasses based on the tunneling two-level system (TLS) and stochastic sudden jump models has been developed. The distinguishing feature of the model is its adaptation for Monte Carlo technique. The developed approach is more general than usual PE theories and allows us to take into account the different microscopic parameters of spectral dynamics in doped amorphous solids: in particular, the minimal radius of interaction between TLS and chromophore molecules, the dispersion of this coupling constant, and the changes of matrix properties nearby chromophore (shell-effect). The developed theory was applied for calculations of two-pulse PE decays for an organic amorphous system with typical parameters. It was found that the slope of temperature dependence of the homogeneous line width is decreased with an increase in the above-mentioned minimal radius. This effect could be one of the main reasons for the discrepancy between some PE experimental results versus calculation using standard PE theory.