Raman relaxation mechanism

When Wqi / Wq2 the magnetization recovery may appear close to singleexponential, but the time constant thereby obtained is misleading [50]. The measurement of 7) of quadrupolar nuclei under MAS conditions presents additional complications that have been discussed by comparison to static results in GaN [50]. The quadrupolar (two phonon Raman) relaxation mechanism is strongly temperature dependent, varying as T1 well below and T2 well above the Debye temperature [ 119]. It is also effective even in cases where the static NQCC is zero, as in an ideal ZB lattice, since displacements from equilibrium positions produce finite EFGs. 

The Cua site, common in biology (inset in Fig. 5.42), is dinuclear with two copper atoms bridged by the thiolate sulfurs of two cysteine ligands. One unpaired electron is delocalized over two metals, which are thus Cul 5+. The NMR spectra show narrow lines from the copper ligands (Fig. 5.42) [120,121], corresponding to an electron relaxation time of 10 11 s, as in Cu2+-Cu2+ dimers (see Section 6.3.2). However, in Cua there is no magnetic coupling between the two centers, as they contain only one unpaired electron just as an isolated Cu2+ ion. Electron relaxation of Cua may be fast because the orbital overlap between the two copper centers provides new relaxation mechanisms not available to a monomer (as Orbach or Raman relaxation). 

Figure 3.21. Scheme of the expected excitation profile due to the relaxation mechanisms illustrated in Fig. 3.22. We expect A) a Raman peak (quasi-resonant), (B) a dip where the intrasurface relaxation ris is quenched by the relaxation to the bulk rt, and (C) a bump / s competing with rB and overhelmed at higher energies f s. depending on the exact exciton vibration coupling, and different for the modes at 390 and 45 cm. ...

Av is the frequency difference between two anisotropic ESR resonance lines, the resulting spectrum is the superposition of the individual configurations. On the contrary, if r < (2 JtAv), we have an isotropic spectrum the resonance frequency is the average of the anisotropic components of the individual configurations As discussed in detail by Ham , motional narrowing can be produced by three relaxation mechanisms, which are characterized by a different temperature dependence an Arrhenius-type dependence (r" = Voe ) for an Orbach process, and a linear dependence or proportional to T for direct and Raman processes, respectively. Therefore, the temperature dependence of the isotropic spectrum gives information about the relaxation mechanism and consequently on the vibronic level scheme. 

Lead. It has been proposed that the relaxation of Pb in lead nitrate is by a spin-phonon Raman scattering mechanism. The recrystallisation behaviour... 

From our data we deduced the Debye temperature of 135 K for the crystal, in good agreement with the value of 148 K found by Nusawa et al. (1975). At yet lower temperatures (less than 70 K), the curve indicates coexistence of two opposing effects the Raman process and occurrence of antifeiromagnetic ordering with a Neel temperature close to absolute zero. As a consequence, we could not confirm the influence of phase transition on relaxation mechanisms. 

Time-resolved IR spectra of similar peptides following a laser-excited temperature jump showed two relaxation times, unfolding 160 ns and faster components <10 ns (Williams et al., 1996). These times are very sensitive to the length, sequence, and environment of these peptides, but do show that the fundamental helix unfolding process is quite fast. These fast IR data have been contrasted with Raman and fluorescence-based T-jump experiments (Thompson et al., 1997). Raman experiments at various temperatures have suggested a folding in 1 /xs, based on an equilibrium analysis (Lednev et al., 2001). But all agree that the mechanism of helix formation is very fast. 

During the last two decades, studies on ion solvation and electrolyte solutions have made remarkable progress by the interplay of experiments and theories. Experimentally, X-ray and neutron diffraction methods and sophisticated EXAFS, IR, Raman, NMR and dielectric relaxation spectroscopies have been used successfully to obtain structural and/or dynamic information about ion-solvent and ion-ion interactions. Theoretically, microscopic or molecular approaches to the study of ion solvation and electrolyte solutions were made by Monte Carlo and molecular dynamics calculations/simulations, as well as by improved statistical mechanics treatments. Some topics that are essential to this book, are included in this chapter. For more details of recent progress, see Ref. [1]. 

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