Dynamic dipole coupling

When the amount of CO decreases to vanishing values, the frequency of CO is practically decoupled and this value can be used to calculate the contribution of the dynamic dipole coupling Av (0) as follows [53] ... 

This is shown in Fig. 8. The coupling of P polarised radiation to the normal and parallel components of the oscillator (the Tiu v(C-O) mode) results in the expected transmission and absorption bands respectively. The former is blue shifted, and the latter is red shifted. This can be understood simply by considering the dynamic dipole coupling lattice sum expected for aligned oscillators perpendicular and pareillel to the surface [59], represented schematically in the inset of Fig. 8. 

In the excited state, the redistribution of electrons can lead to localized states with distinct fluorescence spectra that are known as intramolecular charge transfer (ICT) states. This process is dynamic and coupled with dielectric relaxations in the environment [16]. This and other solvent-controlled adiabatic excited-state reactions are discussed in [17], As shown in Fig. 1, the locally excited (LE) state is populated initially upon excitation, and the ICT state appears with time in a process coupled with the reorientation of surrounding dipoles. 

An ordered monolayer of molecules having a large dynamical dipole moment must not be regarded as an ensemble of individual oscillators but a strongly coupled system, the vibrational excitations being collective modes (phonons) for which the wavevector q is a good quantum number. The dispersion of the mode for CO/Cu(100) in the c(2 x 2) structure has been measured by off-specular EELS, while the infrared radiation of course only excites the q = 0 mode. 

In many cases the temperature dependence of the quadrupolar coupling constant is an indicator of dynamic processes, because the symmetry around the lithium cation is affected by motions which are fast on the NMR time scale. If the rate of these processes exceeds 1/x, the effective symmetry around the lithium cation increases and a decrease in x( Li) results. In Li MAS spectra, a broadening of the satellite transitions can be observed which eventually disappear completely if the rate of the dynamic process comes in the order of the quadrupole frequency. This behaviour was observed for the THF solvated dimer of bis(trimethylsilylamido)lithium, where the Li MAS spectrum at 353 K shows only the central transition and the sidebands caused by CSA and homonuclear Li- Li dipole coupling (Figure 27) . The simulation of the high-temperature spectrum yielded —20 ppm and 1300 Hz for these quantities, respectively. The dipole coupling agrees closely with the theoretical value of 1319 Hz calculated from the Li-Li distance of 2.4 A, which was determined by an X-ray study. 

The usual intensity transfer in the infrared absorption /( ) is given by the relation [43] considering the dynamical dipole-dipole coupling relevant to the frequency shift [44],... 

In multidimensional NMR studies of organic compounds, 2H, 13C and 31P are suitable probe nuclei.3,4,6 For these nuclei, the time evolution of the spin system is simple due to 7 1 and the strengths of the quadrupolar or chemical shift interactions exceed the dipole-dipole couplings so that single-particle correlation functions can be measured. On the other hand, the situation is less favorable for applications on solid-ion conductors. Here, the nuclei associated with the mobile ions often exhibit I> 1 and, hence, a complicated evolution of the spin system requires elaborate pulse sequences.197 199 Further, strong dipolar interactions often hamper straightforward analysis of the data. Nevertheless, it was shown that 6Li, 7Li and 9Be are useful to characterize ion dynamics in crystalline ion conductors by means of 2D NMR in frequency and time domain.200 204 For example, small translational diffusion coefficients D 1 O-20 m2/s became accessible in 7Li NMR stimulated-echo studies.201... 

On metal surfaces, two additional selection mles apply. The first is that only vibrations perpendicular to the surface are HREELS active. This mle follows from two phenomena unique at metal surfaces " (i) Electromagnetic waves polarized perpendicularly to the plane of incidence (parallel to the plane of the surface) undergo a 180° phase shift upon reflection. That is, at the metal surface, the out-of-phase electric-field vectors of the incident and reflected waves cancel each other as a result, no field exists that can couple with dipoles that oscillate parallel to the surface, (ii) The dynamic dipole moment generated by an oscillator that vibrates in the surface-parallel direction is cancelled by that of its image dipole (Figure 1) hence, there the net dynamic dipole moment is zero. On the other hand, if the real dipole is oriented perpendicularly to the surface, its dynamic dipole moment is reinforced by that of its image dipole. This selection mle is the same as that for infrared reflection-absorption spectroscopy (1RAS).°... 

In the second place, we shall study rotational dynamics. Rotational processes are of fundamental importance for dielectric relaxation. To shed light on some controversial issues in dielectric relaxation, Brot and co-workers did a computer simulation of a system of disks interacting via both Lennard-Jones potentials and electric dipole-dipole couplings. This is pre-... 

Using the method suggested by Crossley and King [130], lateral interactions of adsorbed CN ions on gold electrodes were studied using isotopic mixtures of CN and CN [114], as shown in Fig. 38. The principle of the method was described in Sec. 6.5 for adsorbed CO, and consists in separating the dynamic dipole-dipole coupling from the chemical shift. 

The dynamic dipole-dipole coupling for CN is relatively small compared with that of adsorbed CO. For CO on Pt(lll) a shift of c. 30cm is observed for half coverage [55] and for polycrystalline platinum the coupling is so strong that the two bands collapse at intermediate coverages [131]. The chemical shift for CN is of the same order as that for CO adsorbed on a Pt(lll) electrode [55] (see Sec. 6.2). 

The relatively low dynamic dipole-dipole coupling in the case of adsorbed CN could be indicative of a relatively high electrostatic repulsion between adsorbed charged species, generating a large distance between neighboring ions. 

As predicted by the calculation, the absorption features of the P-polarised spectrum (coupling to Pt) correspond to the transmission features of the S polarised spectrum, and also correspond to the expectation values for the assigned modes of the bulk crystalline material [56]. The transmission features of the P-polarised spectrum (coupling to Pn) correspond to the blue shifted bands associated with a net dipole coupling of the normal components of the dynamic dipoles in a thin film. 

The microscopic model, however, cannot take into account net coupling of dynamic dipoles oriented parallel to the surface for the thin (microscopic) film. Such coupling in adsorbed monolayers has been shown [57] by probing an otherwise disallowed transition on a metal through a combination band, to result in a red shift from the singleton frequency. This effect of parallel and normal dipole components can be best exemplified by comparison of the RAIRS spectrum of an isotropic physisorbed molecule with a very strong dipole oscillator, v(C-O) in Mo(CO)6, with the gas phase value (singleton frequency) [58]... 

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