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.
We have measured the temperature dependence of the homogeneous width of spectral holes in the incommensurate phase III of chlorin-doped biphenyl from 60 mK to 10 K. Below 2 K a power law (∼ T α , α = 1.36, 1.74 for different spectral peaks) prevails. Above 2 K a strong coupling to low-energy (5 and 9 cm −1 ) vibrational modes dominates. No indication of a low-temperature lock-in transition was found.
The dependence of frequency, width, and area of spectral holes on pressure were measured at 1.6 K in the pressure range up to 2.5 MPa for dimethyl-s-tetrazine (DMST) doped n-hexane (Shpol’skii system), and as reference systems, for DMST-doped durene (“hard” molecular crystal) and ethanol:methanol glass. For the Shpol’skii system, in addition the inhomogenous fluorescence spectra were measured for normal and high (200 MPa) pressures. The main observations were the following: (i) spectral holes in the Shpol’skii system exhibit very large pressure-induced broadening (up to 65 GHz/MPa) depending essentially on the prehistory (freezing pressure) and exceeding the corresponding values for durene (by far) and glass; (ii) spectral holes in the Shpol’skii system exhibit strong, and to a large extent, reversible, area reduction with applied pressure; and (iii) the inhomogeneous fluorescence lines show quite a moderate (as compared to holes) pressure broadening of about several GHz/MPa. The results for the Shpol’skii system are shown to be inconsistent with existing theories. They are qualitatively explained by pressure-induced dynamics of vacancy defects in the frozen n-alkanes.
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