Excited dipole moments

Equations (7.6) and (7.7) provide a means of determining excited dipole moments together with dipole vector angles, but they are valid only if (i) the dipole moments in the FC and relaxed states are identical, (ii) the cavity radius remains unchanged upon excitation, (iii) the solvent shifts are measured in solvents of the same refractive index but of different dielectric constants. 

Three-Dimensional Model. On the other hand, if the interfacial layer is thick enough compared to the molecular size of SRIOI and if SRIOI molecules adsorbed on the interface are weakly oriented, the rotational motions of SRIOI take place in three dimensions, similar to those in a bulk phase. If this is the case, the contribution of the fluorescence with the excited dipole moment of SR 101 directed along the z-axis cannot be neglected, so that the time profile of the total fluorescence intensity must be proportional to / (0 + 2/i(t). Thus, fluorescence dynamic anisotropy is given by Equation (15), as is well known for that in a macroscopically isotropic system [10,13] ... 

Dispersion forces are the result of the dipolar interactions between the virtually excited dipole moments of the solute and the solvent, resulting in a nonzero molecular polarizability. Although the average of every induced dipole is zero, the average of the product of two induced dipoles is nonzero (Figure 13.1.6). 

Figure 12.1.9. Dispersion forces as aresult of dipolar interactions between the virtually excited dipole moments of solute and solvent.

Infrared and Raman spectroscopy each probe vibrational motion, but respond to a different manifestation of it. Infrared spectroscopy is sensitive to a change in the dipole moment as a function of the vibrational motion, whereas Raman spectroscopy probes the change in polarizability as the molecule undergoes vibrations. Resonance Raman spectroscopy also couples to excited electronic states, and can yield fiirtlier infomiation regarding the identity of the vibration. Raman and IR spectroscopy are often complementary, both in the type of systems tliat can be studied, as well as the infomiation obtained. 

Figure Bl.25.12 illustrates the two scattering modes for a hypothetical adsorption system consisting of an atom on a metal [3]. The stretch vibration of the atom perpendicular to the surface is accompanied by a change m dipole moment the bending mode parallel to the surface is not. As explained above, the EELS spectrum of electrons scattered in the specular direction detects only the dipole-active vibration. The more isotropically scattered electrons, however, undergo impact scattering and excite both vibrational modes. Note that the comparison of EELS spectra recorded in specular and off-specular direction yields infomiation about the orientation of an adsorbed molecule.

Figure Bl.25.12. Excitation mechanisms in electron energy loss spectroscopy for a simple adsorbate system Dipole scattering excites only the vibration perpendicular to the surface (v ) in which a dipole moment nonnal to the surface changes the electron wave is reflected by the surface into the specular direction. Impact scattering excites also the bending mode v- in which the atom moves parallel to the surface electrons are scattered over a wide range of angles. The EELS spectra show the higlily intense elastic peak and the relatively weak loss peaks. Off-specular loss peaks are in general one to two orders of magnitude weaker than specular loss peaks.

The polarization properties of single-molecule fluorescence excitation spectra have been explored and utilized to detennine botli tlie molecular transition dipole moment orientation and tlie deptli of single pentacene molecules in a /7-teriDhenyl crystal, taking into account tlie rotation of tlie polarization of tlie excitation light by tlie birefringent... 

Colvin V L and Alivisatos A P 1992 CdSe nanocrystals with a dipole moment in the excited state J. Chem. Phys. 97 730... 

C3.4.13)). The dimer has a common ground state and excitation may temrinate in eitlier tire or excited state (see tire solid arrows in figure C3.4.3). The transition dipole moments of tliese transitions are defined as ... 

ZINDO is an adaptation of INDO speciflcally for predicting electronic excitations. The proper acronym for ZINDO is INDO/S (spectroscopic INDO), but the ZINDO moniker is more commonly used. ZINDO has been fairly successful in modeling electronic excited states. Some of the codes incorporated in ZINDO include transition-dipole moment computation so that peak intensities as well as wave lengths can be computed. ZINDO generally does poorly for geometry optimization. 

As a rule, the fluorosolvatochromic effects are less as the dipole moment decreases on excitation, but the media environment can considerably influence quantum yield (61). 

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