Crystals spectral jumps

When the number of perturbing defects in the molecular surroundings is large, either because the molecule is very close to an extended crystal fault, or because the crystal is extremely disordered, the number of preferred frequencies is so large that it becomes impossible to recognize a preferred set. Then, the number of jumps between two scans of the laser is large, and the trajectory looks like a onedimensional random walk, or a spectral diffusion. Such a pattern appears for jumping molecules in strained and disordered crystals at the end of an optical fiber. 

Spectral diffusion behaviors differ deeply from molecule to molecule. For example, one molecule, the creeper (Fig. 4) was found in [41] to drift steadily over 1.6 hours towards the line center. The wandering of this molecule presented small discontinuous jumps in addition to the drift, suggesting a driven random walker. This drift might result from a structural relaxation of the strained crystal over a long timescale. Within the more specific model of interaction of the molecule with a... 

Chachaty and co-workers [8.20, 8.37, 8.38] were first to describe correlated internal motions in alkyl chains of surfactant molecules that form lyotropic liquid crystals. The last section described an extension of the master equation method of Wittebort and Szabo [8.4] to treat spin relaxation of deuterons on a chain undergoing trans-gauche jump rotations in liquid crystals. This method was also followed by Chachaty et al. to deal with spin relaxation of nuclei in surfactants. However, they assumed that the conformational changes occur by trans-gauche isomerization about one bond at a time. In their spectral density calculations (see Section 8.3.1), they used a transition rate matrix that was constructed from the jump rate Wi, W2, and Ws about each bond. Since W3 is much smaller than Wi and W2, the time scale of internal motions was practically governed by Wi and W2 of each C-C bond. Since... 

Fig. 18.15 Spectroscopy of single pentacene molecules in p-terphenyl crystal (W. P. Ambrose, Th. Basche and W. E. Moemer, 1. Chem. Phys. 95, 7150 (1991). (a) Fluorescence excitation spectrum of a single molecule at 1.5 K (0 MHz detuning = 592.407 nm, at the wing of the inhomogeneous hneshape) (b) Fluorescence excitation spectrum of the full inhomogeneous line at 1.5 K. (c) The dependence of the single molecule homogeneous hnewidth on temperature (the solid line is a fit to the data), (d) Two views of spectral diffusion The upper panel shows a time sequence of excitation spectra (each taken over a period of 1 s). The lower panel shows the jumps in the peak frequency as a function of time.

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