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Chemical Physics Letters

Optical dephasing of chromophores in an organic glass: picosecond photon echo and hole burning experiments

Cecilia A. Walsh, Mark A. Berg, L. R. Narasimhan, M. D. Fayer

Published Jan 1, 1986• 61 citations
Publisher

Abstract

The homogeneous optical dephasing time T2 for the organic glass system resorufin in ethanol is measured by picosecond two-pulse photon echoes from 1.5 to 11.4 K. A non-power law temperature dependence of T2 is measured. Non-photochemical hole burning (NPHB) linewidths are four times broader than the homogeneous linewidth, demonstrating that additional factors contribute to NPHB.

DOI
10.1016/0009-2614(86)80414-6

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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.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Apr 1, 1990•0 citations
The homogeneous optical dephasing time T2 for the organic glass system resorufin in ethanol is measured by picosecond two-pulse photon echoes from 1.5 to 11.4 K. A non-power law temperature dependence of T2 is measured. Non-photochemical hole burning (NPHB) linewidths are four times broader than the homogeneous linewidth, demonstrating that additional factors contribute to NPHB.
The first two-pulse photon echo experiments on a chromophore in an organic glass are reported. The homogeneous electronic dephasing of resorufin in ethanol glass is measured from 1.5–11.4 K. The temperature dependence of the dephasing time does not fit the power law frequently predicted by theory for the dephasing characteristic of glasses. However, the temperature dependence can be accounted for by including dephasing from librations or acoustic phonons, mechanisms known to be important in crystals. The dephasing decay is found to be a single exponential for over six factors of e. The dephasing is also shown to be uncorrelated with the extent of nonphotochemical hole burning (NPHB). However, the homogeneous linewidth deduced from the photon echo is four times narrower than the linewidth obtained from NPHB, demonstrating that the hole is broadened by additional processes.
The homogeneous optical dephasing times (T2) and the non-photochemical hole burning (NPHB) linewidths of resorufin in glycerol glass are reported from 1.1 to 25.5 K. At low temperature, the linewidths obtained from NPHB are substantially wider than the linewidths (1/πT2) obtained from photon echo measurements, but the two become equal at high temperatures. The extra broadening of the NPHB is found to increase as T0.8. A model of homogeneous dephasing including both two-level system and pseudolocal mode contributions is successful over the entire temperature range.
The range of relaxation rates present in amorphous systems is shown to affect the results of optical dephasing measurements. The interpretation of these experiments in terms of four-time correlation functions is introduced. Photon echo and hole burning results are shown for resorufin in three glass hosts: ethanol, d-ethanol, and glycerol. The hole burning data are shown to be affected by slow relaxation processes whereas the echo results are not. The implications of a slowest relaxation rate in amorphous systems are also considered.
Photon echo and non-photochemical hole burning experiments on cresyl violet in ethanol glass are reported. At 1.5 K the optical dephasing time measured by the picosecond echo is eight times longer than the time measured by hole burning because of the difference in the time scales associated with the measurements. The observed temperature dependence arises from the glass's two-level systems and from a pseudolocal phonon mode at higher temperatures. Comparing the ratios of the echo to hole burning dephasing times for cresyl violet and resornfin, suggests that their dephasing is influenced by the nature of their ethanol solvation shells in addition to the dynamics of the bulk ethanol.
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.