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The Journal of Chemical Physics

A photon echo study of two-level systems in polyisobutylene under high pressure

Masashi Yamaguchi, Michael McIntire, Eric L. Chronister

Published Jan 22, 2002• 7 citations
Publisher

Abstract

We report pressure (0–28 kbar) and temperature (1.1–4.3 K) dependent photon echo results for tetra-tert-butyl-terrylene (TBT) in a polyisobutylene (PIB) host. Increased pressure is found to increase the homogeneous dephasing rate over the temperature range of this study, in contrast to pressure dependent hole burning results reported for other polymer systems at somewhat higher temperatures (⩾4.2 K). A relatively small pressure increase (0 to 9 kbar) resulted in a change in the temperature dependence of the dephasing characterized by a lowering of the temperature power law exponent from ∼1.3 to ∼0.9. Further pressure increase above 9 kbar was characterized by an additional increase in the homogeneous dephasing rate without further change in the temperature exponent. The results are compared to high pressure photon echo studies of other polymer systems, and discussed in terms of the TLS (two-level-systems) model.

DOI
10.1063/1.1429656

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We report pressure (0–28 kbar) and temperature (1.1–4.3 K) dependent photon echo results for tetra-tert-butyl-terrylene (TBT) in a polyisobutylene (PIB) host. Increased pressure is found to increase the homogeneous dephasing rate over the temperature range of this study, in contrast to pressure dependent hole burning results reported for other polymer systems at somewhat higher temperatures (⩾4.2 K). A relatively small pressure increase (0 to 9 kbar) resulted in a change in the temperature dependence of the dephasing characterized by a lowering of the temperature power law exponent from ∼1.3 to ∼0.9. Further pressure increase above 9 kbar was characterized by an additional increase in the homogeneous dephasing rate without further change in the temperature exponent. The results are compared to high pressure photon echo studies of other polymer systems, and discussed in terms of the TLS (two-level-systems) model.
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.
Pressure- and temperature-dependent photon echo results are obtained for pentacene doped polymethyl methacrylate (PMMA). A unique pressure effect is observed in which the optical dephasing rate increases as the pressure is increased from ambient pressure to 4 kbar, above which the optical dephasing rate is pressure independent up to 43 kbar. The present results are also compared with pressure- and temperature-dependent photon echo results for rhodamine 101 in PMMA, in which the optical dephasing rate was completely insensitive to pressure over the range 0 to 30 kbar. A negative correlation is also observed between the optical dephasing rate and the spectral hole burning efficiency. Line broadening due to pressure induced spectral diffusion may be responsible for both the increased dephasing rate and the reduced spectral hole-burning at high pressure.
The effect of high pressure on the optical dephasing of chromophores in organic polymers at low temperature is evaluated within the stochastic sudden jump two-level-system (TLS) model. The approximations within the "standard" TLS model cannot account for the observed pressure dependence of the pure dephasing rate without ad hoc assumptions about changes in the TLS density of states. However, the photon echo model of Geva and Skinner for disordered systems can be used to model pressure-dependent optical dephasing results for a variety of doped polymer systems without assuming changes in the TLS density of states. The relative importance of pressure-induced changes in TLS density, chromophore-TLS coupling, and TLS-phonon coupling is evaluated by fitting experimental high-pressure photon echo results to the TLS model.
A high-pressure photon echo study of the low-temperature dynamics of rhodamine 640 in a 2-methyltetrahydrofuran (2-MTHF) molecular glass is reported. High pressure is found to decrease the homogeneous dephasing rate comparably to the volume compression ratio. The pressured-induced line narrowing is also qualitatively similar to high-pressure hole burning results reported for a 2-MTHF glass doped with bacteriochlorophyll a . The results are consistent with a pressure-induced decrease in the number of tunneling two-level systems (TLSs) that may be associated with collapsible void space in the molecular glass. Comparable high-pressure photon echo studies of organic polymer glasses over the same low-temperature range do not show line narrowing at high pressure, suggesting an intrinsic difference in the nature of TLSs for molecular versus polymer glasses.
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.
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.