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Journal of Experimental and Theoretical Physics

Light-induced temporal broadening of optical lines of individual molecules in polymers and glasses

Игорь Сергеевич Осадько

Published Oct 1, 2001• 3 citations
Publisher

Abstract

The spectral diffusion theory developed for explaining the logarithmic temporal broadening of the optical line of an individual impurity center and based on spontaneous tunnel transitions in a polymer or glass is supplemented by taking into account the tunnel transitions occurring in the center itself upon the absorption of a photon. This light-induced additional tunneling leads to a sharp temporal broadening of the optical line from an individual impurity molecule occurring against the background of a slower logarithmic line broadening. It is shown that the inclusion of light-induced tunneling can explain the difference in the temporal line broadening of three individual Terylene molecules introduced in polyethylene, which was measured in experiments by the group headed by M. Orrit [A. M. Boiron et al. , Chem. Phys. 247 , 119 (1999)].

DOI
10.1134/1.1420457

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Molecular crystals and liquid crystals science technology. Section A, Molecular crystals and liquid crystals/Molecular crystals and liquid crystals science and technology. Section A, Molecular crystals and liquid crystals•May 1, 1998•0 citations
The spectral diffusion theory developed for explaining the logarithmic temporal broadening of the optical line of an individual impurity center and based on spontaneous tunnel transitions in a polymer or glass is supplemented by taking into account the tunnel transitions occurring in the center itself upon the absorption of a photon. This light-induced additional tunneling leads to a sharp temporal broadening of the optical line from an individual impurity molecule occurring against the background of a slower logarithmic line broadening. It is shown that the inclusion of light-induced tunneling can explain the difference in the temporal line broadening of three individual Terylene molecules introduced in polyethylene, which was measured in experiments by the group headed by M. Orrit [A. M. Boiron et al. , Chem. Phys. 247 , 119 (1999)].
A dynamic theory of two-photon correlators for a single impurity center developed recently by the author [I. S. Osad’ko, Zh. Éksp. Teor. Fiz. 113 , 1606 (1998) [JETP 86 , 875 (1998)] has been generalized to the case in which the center interacts with nonequilibrium two-level systems (TLSs) in polymers and glasses. Quantum tunneling transitions in TLS manifest themselves as random jumps of a spectral line of an impurity center. These jumps can be either spontaneous or light-induced. Interaction between the impurity center and many nonequilibrium TLSs, which exist in polymers, results in a time dependence of the optical dephasing rate 1/ T 2 of an impurity molecule, i.e., in spectral diffusion. This paper describes how the jumps of the spectral line manifest themselves in the two-photon correlator, which can be measured in experiments.
The concept of the homogeneous spectral linewidth of an impurity center is more complicated in disordered media such as organic glasses and polymers than it is in crystals. In amorphous media the local environments for impurity centers are continuously changing with time. As a result transition frequencies of individual centers diffuse within the inhomogeneous broadened spectral contour, which is called spectral diffusion. When the timescale for spectral diffusion processes is much longer than the dephasing time, it is possible to distinguish homogeneous dephasing from spectral diffusion. On the other hand, when the timescales of spectral diffusion and dephasing processes are comparable these phenomena become indistinguishable.
Recent advances in the optical detection and spectroscopy of single impurity centers in crystals has provided the possibility of obtaining information about defect-solid interactions on a truly local level. For the system composed of pentacene impurity molecules in the crystal p-terphenyl, single molecules have been studied using both absorption1 and fluorescence excitation2 techniques at liquid helium temperatures. The superior signal-to-noise of the latter technique has led to direct observations of the lifetime-limited Lorentzian homogeneous profile of a single pentacene impurity3 as well as the surprising observation of spontaneous spectral diffusion4. Spectral diffusion, or changes in the resonance frequency of an impurity molecule with time as a result of low-energy excitations (two-level system transitions (TLS)),5 is generally expected in amorphous hosts. Indeed, the presence of TLSs in amorphous solids is intimately connected with nonphotochemical mechanisms for persistent spectral hole-burning (PSHB) in amorphous materials6-8. Using perylene impurity molecules in poly(ethylene), we have observed the optical spectra of single molecules in a polymeric host for the first time. At 1.5K, individual perylene molecules show the expected spectral diffusion; moreover, we observe light-induced changes in resonance frequency, i.e., persistent spectral hole-burning, which allows one to envision optical storage on the single-molecule level.
With high-efficiency fluorescence excitation techniques optical spectra of single impurity molecules of perylene in a polyethylene matrix can be obtained at 1.5 K. Analysis of such spectra shows a variety of spectral diffusion effects, including fast (<2 s) resonance frequency changes on the 1–100-MHz scale, which lead to a range of apparent linewidths, as well as discontinuous jumps in the resonance frequency of 10–1000 MHz on a longer time scale. In addition, light-induced changes in the resonance frequency of a single molecule (persistent spectral hole burning) have been conclusively observed by showing that the burning time decreases with increased laser power. Surprisingly, hole-burned single molecules often spontaneously return to the original frequency in 1–100 s. Measurements of the burning time for a large number of hole-burning events for the same single molecule yield an exponential burn-time distribution, which is the first direct measurement to our knowledge of the stochastic kinetics of a single molecule. Analysis of the signal-to-noise function appropriate to these experiments gives the conditions under which other systems may permit single-molecule detection: strong absorption, high fluorescence yield, weak bottlenecks in the optical pumping process, and low hole-burning quantum efficiency.
Optical spectroscopy provides a useful tool for probing the structure and dynamics of solids. Over the years, optical methods have evolved from absorption spectroscopy to more complex methods such as spectral hole burning, photon echoes, and luminescence line narrowing. The absorption spectrum provides information about inhomogeneous broadening, while the latter approaches provide information about the systematics of the homogeneous line shape information that is often obscured by inhomogeneous broadening. When external perturbations such as strain or electric fields are applied, details of the local site can be obtained. Recently, the ultimate limit of optical spectroscopy has been attained, in which the spectral properties of individual single impurity molecules in a solid can be measured, with all ensemble averaging removed. This novel regime has provided additional, previously unobtainable, information on a highly local scale, such as the direct observation of the spectral shifting of a single molecule produced by configurational transitions of the host. It is to be expected that further surprises await us in this new frontier of single-molecule spectroscopy in solids.