Electric dipole coupling mechanism

A-jg) transition which is symmetry and parity allowed for an electric dipole transition mechanism the spin selection rule is suspended by the large spin orbit coupling of about Ijp 3T 2 400 cm (j 2). Since analysis of the progressions yields, for both compounds, only eg quanta (240 cm 1 for Cl, 154cm 1 for Br) the excitea state must be distorted by Jahn-Teller forces... 

As seen above, synergistic two-photon absorption can in principle take place by either or both of the mechanisms, where (i) each laser photon is absorbed by a different molecule (the cooperative mechanism), or (ii) both laser photons are absorbed by a single molecule (the distributi e mechanism). In each case, the energy mismatch for the molecular transitior s is transferred between the molecules by means of a virtual photon that couples with each molecule by the same electric-dipole coupling as the laser photons. The result, however, is a significant difference in the selection rules applying to the two types of processes. 

The Forster mechanism is also known as the coulombic mechanism or dipole-induced dipole interaction. It was first observed by Forster.14,15 Here the emission band of one molecule (donor) overlaps with the absorption band of another molecule (acceptor). In this case, a rapid energy transfer may occur without a photon emission. This mechanism involves the migration of energy by the resonant coupling of electrical dipoles from an excited molecule (donor) to an acceptor molecule. Based on the nature of interactions present between the donor and the acceptor, this process can occur over a long distances (30—100 A). The mechanism of the energy transfer by this mechanism is illustrated in Figure 11. 

The p-m Mechanism. In this instance, one chromophore contains the electric dipole transition and another contains the magnetic dipole transition. These are coupled in the molecule to produce optical activity in both transitions. 

The dynamic coupling mechanism predicts that hypersensitivity should be observed when the point group of the lanthanide complex contains Y3m spherical harmonics in the expansion of the point potential. The good agreement between the calculated and observed values for Tj or Q.2 parameters shows that the dynamic coupling mechanism makes a significant contribution to the intensities of the quadrupole allowed f-electron transitions in lanthanide complexes. Qualitatively, the mechanism is allowed for all lanthanide group symmetries in which the electric quadrupole component 6,a,fi and the electric dipole moment p, a transform under a common representation. 

Crystal field theory, intensities of 4f-4f transitions, Judd-Ofelt theory of electric-dipole transitions, covalency model of hypersensitivity, dynamic coupling mechanism, solution spectra, spectral data for complexes, solvent effects, fluorescence and photochemistry of lanthanide complexes are dealt with in spectroscopy of lanthanide complexes. 

Serious complications arise, however, if the spins are subject to strong nuclear electric quadrupolar interactions, which tend to modify the echo amplitudes measured. In such cases, it is still possible to extract dipole-dipole coupling information from spin echo decay spectroscopy, if the 71-pulses are applied entirely selectively to the central l/2>o -l/2> transition [6]. If the resonance frequencies between the coupled nuclei are sufficiently similar to allow for spin-exchange via the flip-flop mechanism, Eq. (5) turns into... 

In the semiclassical model the molecule is treated quantum mechanically whereas the held is represented classically. The held is an externally given function of time F that is not affected by any feedback from the interaction with the molecule. We consider the simplest case of a dipole coupling. The formalism is easily extended to other types of couplings. The time dependence of the periodic Hamiltonian is introduced through the time evolution of the initial phase F = F(0 + oat) = electric held and oa is its frequency. The semiclassical Hamiltonian can be, for example, written as... 

The dynamic coupling between the electric hexadecapole moment of a d—d transition in the metal ion and an induced electric dipole in each ligand is forbidden in a four-coordinate complex containing a tetrahedral chromophore, although the mechanism becomes formally allowed on reduction to D2(i or lower chromophoric symmetry. In tetrahedral complexes the dynamic coupling between a d—d quadrupolar... 

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