Dynamic frequency shifts

Another class of applications involves spectra consisting of a single broad line, such as NMR spectra of quadrupolar nuclei or the ESR spectra of melanin or coal. For example, DISPA experiments are currently in progresses to detect proposed "dynamic frequency shifts" in the NMR spectra of spin-3/2 nuclei such as eSfja. The proton NMR spectrum of water is different for normal than for cancerous tissues exploring the mechanism for this difference offers another possible DISPA application. 

Poupko R, Baram A, Luz Z. 1974. Dynamic frequency shift in the ESR spectra of transition metal ions. Mol Phys 27 1345-1357. 

The calculated spectrum for I 3/2 including the effect of second-order dynamic frequency shift leads to the prediction that the two spectrum components (degenerated only for extreme narrowing conditions) are characterized by different decay curves after a simple resonant pulse. The so-called broad component (3/2-1/2,-3/2-(-l/2)) decays exponentially while the narrow component (1/2-(-1/2)) decays nonexponentially. 

The same group, in Lund, has calculated very recently (88) the corresponding dynamic frequency shifts. The spectral densities J(o)) and Q(o)) are defined by eqs. 13 and 14, i.e., with... 

Dynamic frequency shifts - some general characteristics... 

Figure 1. General characteristics of dynamic frequency shifts ...

Many groups are now trying to fit frequency shift curves in order to understand the imaging mechanism, calculate the minimum tip-sample separation and obtain some chemical sensitivity (quantitative infonuation on the tip-sample interaction). The most conunon methods appear to be perturbation theory for considering the lever dynamics [103], and quantum mechanical simulations to characterize the tip-surface interactions [104]. Results indicate that the... 

GiessibI F J 1997 Forces and frequency shifts in atomic-resolution dynamic-force microscopy Phys. Rev. B 56 16 010... 

When metals have Raman active phonons, optical pump-probe techniques can be applied to study their coherent dynamics. Hase and coworkers observed a periodic oscillation in the reflectivity of Zn and Cd due to the coherent E2g phonons (Fig. 2.17) [56]. The amplitude of the coherent phonons of Zn decreased with raising temperature, in accordance with the photo-induced quasi-particle density n.p, which is proportional to the difference in the electronic temperature before and after the photoexcitation (Fig. 2.17). The result indicated the resonant nature of the ISRS generation of coherent phonons. Under intense (mJ/cm2) photoexcitation, the coherent Eg phonons of Zn exhibited a transient frequency shift similar to that of Bi (Fig. 2.9), which can be understood as the Fano interference [57], A transient frequency shift was aslo observed for the coherent transverse optical (TO) phonon in polycrystalline Zr film, in spite of much weaker photoexcitation [58],... 

For the case of p = 8, the quantum effects of the dynamics become more evident. The CMD method gives the correct short time behavior, but there is a small frequency shift. However, the classical result is much worse at this temperature. 

One of the most striking features of the scattered spectrum for either neutrons or light in the vicinity of a phase transition is the appearance of a divergent elastic or quasielastic peak centred near zero frequency shift that lies entirely outside the quasiharmonic soft-mode description of the dynamics (Fleury Lyons, 1983). The first observation of a divergence in scattered intensity is due to Yakovlev et ai, (1956), who observed the phenomenon in the a-fi transition of quartz. The scattered intensity increases dramatically, sometimes by a factor of 10000 near and the maximum value of line width of the diverging feature is itself rather small ( 1 cm ). In Fig. 4.7, typical central peaks are shown for the purpose of illustration. 

In the case of electron transfer reactions, besides data on the dynamic Stokes shift and ultrafast laser spectroscopy, data on the dielectric dispersion (w) of the solvent can provide invaluable supplementary information. In the case of other reactions, such as isomerizations, it appears that the analogous data, for example, on a solvent viscosity frequency dependence 17 ( ), or on a dynamic Stokes fluorescence shift may presently be absent. Its absence probably provides one main source of the differences in opinion [5, 40-43] on solvent dynamics treatments of isomerization. 

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