Spectral dynamics

Skinner J L 1997 Theoretical models for the spectral dynamics of individual molecules in solids Single Molecule Optical Detection, Imaging and Spectroscopy ed T Basche, W E Moerner, M Orrit and U P Wild (Weinheim VCFI)... 

Geva E, Reilly P D and Skinner J L 1996 Spectral dynamics of individual molecules in glasses and crystals Acc. Chem. Res. 29 579-84... 

Nanosecond Absorption Spectroscopy Absorption apparatus, 226, 131 apparatus, 226, 152 detectors, 226, 126 detector systems, 226, 125 excitation source, 226, 121 global analysis, 226, 146, 155 heme proteins, 226, 142 kinetic applications, 226, 134 monochromators/spectrographs, 226, 125 multiphoton effects, 226, 141 nanosecond time-resolved recombination, 226, 141 overview, 226, 119, 147 probe source, 226, 124 quantum yields, 226, 139 rhodopsin, 226, 158 sample holders, 226, 133 singular value decomposition, 226, 146, 155 spectral dynamics, 226, 136 time delay generators, 226, 130. 

Figure 9.29 Spectral dynamics of FM synthesis with linearly changing modulation index/( ). (Reprinted with permission from [Moorer, 1977], 1977, IEEE)...

Spectral dynamics of trumpet-like sound using FM synthesis 408... 

As discussed in detail in ref. 21, a major problem in FT/ICR is the limited mass spectral dynamic range (factor of - 1000 1). Briefly, it is not easy to detect fewer than - 100 ions at easily managed chamber pressures (>/IO"9 torr) alternatively, the spectrum can be distorted by Coulomb forces when more than - 100,000 ions are present. 

Chang, S. L., Lyezkowski, R. W., and Berry, G. R, Spectral dynamics of computer simulated two-dimensional fewtube fluidized beds. AIChE. Symp. Ser. 2b9, 2b7 (1989). 

Schleifenbaum F, Blum C, Subramaniam V, Meixner AJ (2009) Single-molecule spectral dynamics at room temperature. Mol Phys 107(18) 1923-1942... 

The current waveform on the anode of the photodetector is fed to the input connector of a transient digitizer/digital oscilloscope where it is converted into digital format and read into a PC. The resulting stored data can be processed for spectral/ dynamic content by one of several available commercially available data manipulation software packages. 

C. Hofmaim, T.J. Aartsma, H. Michel, J. K5hler, Spectral dynamics in the B800 band of LH2 from Rhodospirillum molischianum A single-molecule study. New J. Phys. 6, 1-15 (2004b)... 

The H93G mutant and modified Mb proteins serve as excellent model systems and have been used in theory and experiment to probe the fundamental principles of energy transfer and heme cooling. Similar Mb models were studied in subsequent experiments. Champion and coworkers studied the spectral dynamics of the photo-excited horse heart Mb, both the native and H93G mutant [63]. It was found that the transient relaxation of each compound is completed within 10 ps the optical transient of H93G was observed to be quite similar to that of native deoxyMb. When the protoheme in Mb was replaced by an iron porphine, in which all heme side chains were replaced by hydrogens, the relaxation is also completed within 10 ps but is obviously slower than the native Mb, consistent with simulation results [62],... 

In tins chapter we continue the discussion of experiments performed on stable single molecules and introduce the technique of single molecule fluorescence microscopy which will be applied to the study of spectral dynamics. 

Theoretical Models for the Spectral Dynamics of Individual Molecules in Solids... 

For inhomogeneously broadened line shapes it necessarily follows that no information about time-dependent fluctuations of the chromophore s transition frequency (which I will call spectral dynamics) can be obtained from the line shape itself. This does not mean that such dynamic fluctuations do not occur it simply means that either their amplitude is much smaller than the inhomogeneous line width or that their time scale is much longer than the inverse of the inhomogeneous line width. In either case these dynamic fluctuations are of great interest because they result from time-dependent changes in the local environments of chromophores, and hence can provide information about solid-state dynamics. 

The experimental techniques of fluorescence line narrowing and hole burning were invented, in part, to access this dynamic information. They each involve selective excitation by a narrow-band laser of a nearly resonant subset of chromophores. The resulting fluorescence line shape or hole shape reflects the spectral dynamics of the members of this subset, unobscured by the other chromophores. In a similar vein, in the time-domain photon echo experiment, after the application of a short pulse the inhomogeneous dephasing of all of the chromophores is then rephased by a second pulse, and so the echo decay again reflects only transition frequency fluctuations. 

Consider first the case of (substitutional) chromophores in crystals. The above techniques have all been used to measure the chromophores spectral dynamics. 

In fact, single molecule spectroscopy (SMS) experiments have recently become a reality. The first experiments were performed on pentacene (the chromophore) in a p-terphenyl crystal [8-10]. I will focus here on the experiments of Ambrose, Basche, and Moemer [9, 10], which involved repeated fluorescence excitation spectrum scans of the same chromophore. For each chromophore molecule they found an identical (except for its center frequency) Lorentzian line shape whose line width is determined by fast phonon-induced fluctuations (and by the excited state lifetime), as discussed above. However, for each of a number of different chromophore molecules Moemer and coworkers found that the chromophore s center frequency changed from scan to scan, reflecting spectral dynamics on the time scale of many seconds The transition frequencies of each of the chromophores seemed to sample a nearly infinite number of possible values. Plotting the transition frequency as a function of time produces what has been called a spectral diffusion trajectory (although the frequency fluctuations are not necessarily diffusive ). These fascinating and totally... 

Similar spectral dynamics of individual chromophores from repeated fluorescence excitation scans have subsequently been seen in amorphous hosts, for the systems Tr in polyethylene (PE) [14] and tetra-r-butyl-terrylene (TBT) in polyisobutylene (PIB) [15, 16]. In at least one instance [16] the chromophore samples far fewer frequencies than in the case of pentacene in p-terphenyl. The spectral difl usion trajectories are assumed to result from the flipping of those TLSs whose dynamics is slower than the scan time. 

As mentioned earher, the study of fluorescence excitation line shapes themselves can also provide a useful probe of the spectral dynamics of individual molecules in glasses. Unlike the case of pentacene in p-terphenyl crystal, where all molecules have the same line shape, individual chromophores in glasses do indeed have a variety of line shapes. This has been seen for perylene (Py) in PE [17], Tr in PE [18], Tr in polyvinylbutyral (PVB), polymethylmethacrylate (PMMA), and polystyrene (PS) [19], and for TBT in PE and PIB [15,16]. The spectral broadening presumably is due to the flipping of those TLSs with dynamics faster than the scan time. There is a distribution of line shapes because, as noted earher, different chromophore molecules are coupled to different sets of TLSs. 

A third method for measuring spectral dynamics of individual molecules in glasses involves fluorescence intensity fluctuations during steady-state excitation at a fixed frequency. [20] This method has been applied to the systems of Tr in PE [18,20] and TBT in PIB [15, 16]. In these experiments the fluorescence intensity fluctuates as the chromophore moves in and out of resonance because of coupling to flipping TLSs. Thus this very clever technique can provide a direct probe of TLS dynamics on all time scales. 

It should be clear from the above that SMS presents a terrific opportunity to probe dynamics in both crystaUine and amorphous solids at low temperatures. In order to provide a microscopic understanding of spectral dynamics and to analyze experimental results one needs theoretical models. The spectral dynamics in all of the experiments discussed above is assumed to arise from the coupling of the chromophore to one or more TLSs. In this chapter I will discuss the TLS model, and will attempt to provide a unified theoretical framework within which both the crystal and glass results, involving aU three different experimental techniques, can be understood. 

Spectral dynamics of a chromophore coupled to one or many two-level systems... 

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