R. I. Personov

COMPARISON OF PHOTON ECHO, HOLE BURNING, AND SINGLE MOLECULE SPECTROSCOPY DATA ON LOW-TEMPERATURE DYNAMICS OF ORGANIC AMORPHOUS SOLIDS

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. ...

Dephasing and diffusional linewidths in spectra of doped amorphous solids: Comparison of photon echo and single molecule spectroscopy data for terrylene in polyethylene

Abstract The concept of “homogeneous” spectral linewidths in doped amorphous solids and some possibilities to separate the linewidth parts related to the optical dephasing and spectral diffusion (SD) are discussed. The results of the model calculations of the photon echo (PE) decay under conditions of a large dispersion of homogeneous linewidths are presented. The deviations of the PE decay from an exponential due to this dispersion are discussed. The experimental data on incoherent PE in the terrylene/polyethylene system are presented and compared with literature single molecule spectroscopy (SMS) data. It is shown that in this case the dephasing time dispersion plays an important role for the linewidth distribution in SMS. A simple method for separation of the dephasing and SD linewidths in the SMS, based on intensity saturation effects, is considered. The facilities of this method using some SMS data are demonstrated.

Optical dephasing in doped polymers and the soft potential model : Analysis of photon echo in doped PMMA

The approach for description of the optical dephasing in doped amorphous solids in the framework of the soft-potential model (SPM) was developed. A procedure for calculation of the homogeneous line width at low temperatures within this model was suggested. The temperature dependencies of the optical dephasing for two amorphous systems, polymethimetacrylate doped with tetra-tert-butylterrylene and zinc-tetraphenylporphine were calculated. Calculations were performed using the parameters known from the literature. The results were compared with the photon echo data for these systems, measured in the broad temperature range. It was found that the SPM describes the temperature behavior of the homogenous line width qualitatively correctly. At temperatures below 2 K, where the main contribution to optical dephasing is associated with tunneling two-level systems, the predicted line width values agree well with the experimental data. At higher temperatures, some difference between the prediction and experimental data was observed and attributed to the effect of impurity on the formation of quasilocal oscillations of the matrix.

Investigation of low-temperature line broadening mechanisms in organic amorphous solids with photon echo, hole-burning and single-molecule spectroscopy

Abstract We present the first comparative analysis of spectral data obtained at low temperatures with three different highly selective techniques: two-pulse photon echo (2PE), hole-burning (HB) and single-molecule spectroscopy (SMS) of a polyisobutylene glass (PIB) doped with tetra-tert-butylterrylene (TBT). Our 2PE- and HB-experiments are compared with literature data [Tittel et al., J. Lumin. 64 (1995) 1] obtained by SMS. It is found that the linewidth measured by HB is significantly larger than the mean of the linewidth distribution obtained by SMS. We conclude that a T 2 -dispersion plays an important role in the formation of the linewidth dispersion in TBT/PIB.

Contributions of The Different Line Broadening Mechanisms in Photon Echoes and Single Molecule Spectra in Amorphous Solids

Abstract Experimental data on Photon Echoes (PE) in the system terrylene/poly(isobutylene) in the temperature range 0.4–8K are presented and compared with Single Molecule Spectroscopy (SMS) results. From this comparison we conclude that the linewidth distribution in SMS is caused mainly by a large T 2-dispersion (but not slow spectral diffusion). The temperature line broadening is discussed.

COMPARISON OF INTENSITY DISTRIBUTION IN THE SPECTRA OF COMPLEX MOLECULES COOLED IN A SUPERSONIC JET AND IN A CRYSTALLINE MATRIX

Comparative investigations of intensity distribution in excitation and fluorescence spectra of 1,4-dihydroxyanthraquinone (quinizarin) and 1,4,5,8-tetrahydroxy-9,10-anthraquinone cooled in a supersonic jet and in a crystalline n-octane matrix were made. It is shown that in a jet the intensity distribution is appreciably affected by saturation effects. A special analysis of the effects has been made. It is noted that the investigation of saturation effects in jet spectra may serve as a tool for the lifetime measurements of the molecular excited states. In the absence of saturation, one has defined the constants of Franck-Condon and Herzberg-Teller interactions for the molecules under investigation. It has been established that the differences of intensity distribution in the spectra of molecules in a jet and in n-octane are mainly attributed to the Herzberg-Teller parameter variation.

Optical dephasing in doped organic glasses over a wide (0.35–100 K) temperature range: Solid toluene doped with Zn–octaethylporphine

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 contr...

Optical dephasing in solid toluene activated by octaethylporphine zinc

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 (TD=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.

Temperature-dependent line broadening of chromophores in amorphous solids: Differences between single-molecule spectroscopy and photon echo results

Abstract Spectroscopic techniques exhibit different sensitivities for line broadening processes in amorphous solids. Photon echo and hole-burning spectroscopy yield averages over the chromophore ensemble. At low temperatures, the results can usually be fitted with a combination of a power-law term — corresponding to the relaxations of two-level systems- and of an exponentially activated contribution of pseudo-local phonon modes. Single-molecule spectroscopy (SMS). in contrast, can resolve the behavior of single dye molecules and yields a distribution of power laws as well as of activation energies. We compare photon echo results for tetra-tert-butylterrylene (TBT) in polyisobutylene (PIB) with SMS data for the same system. The latter were used to simulate numerically the data which would be obtained in an ensemble-averaging experiment. The results of the numerical calculation can be well fitted without assuming a distribution of parameters.

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

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.