Dipole strength dimer

Figure 5. Slick" molecules in the possible modes of interaction during an optical transition, with transition density represented by vertical displacement from the horizontal sticks. Right, the transition densities cancel each other and are silent, and left, the transition densities add so that the dimer becomes a supermolecule with a single allowed optical transition having the combined dipole strength of the two participating molecules. (Reproduced, with permission, from Ref. 30. Copyright J973, Society of Photographic Scientists and Engineers.)...

Fig. 8.4 Calculated absorption spectra of homodimers with four different geometries. The orientations of the transition dipoles and the geometrical factor k (Fig. (7.17)) are indicated in the box below each spectrum. The absorption spectrum of the monomer is shown with a dotted line in each panel and the spectrum of the dimer with a solid line. From left to right, the relative dipole strengths of the and Fs+ transitions are 0 2, 1 1, 0 2 and 0.5 1.5. ( b is the high-energy transition in A and the low-energy transition in C and D.) The sum of the dipole strengths is always twice the dipole strength of the monomer. The exciton bands have been given Gaussian shapes with an arbitrary width. See Fig. 9.7A for another illustration...

Calculate the dipole strengths of the two exciton absorption bands of the dimer. (Assume that each monomeric molecule has only one excited state.)... 

Fig. 9.7 SHex contributes opposite rotational strengths to the exciton bands of a dimer. The dashed lines in (A) show the exciton absorption bands of a dimer the solid curve is the total absorption spectrum. In (B), the dashed lines are the circular dichroism (CD) of the two bands and the solid curve Is the total CD spectrum. The spectra are for a homodimer with = Di,a 2) =10 D, I/ 2iI = 7 A, 9 = 71°, a = P = 90° (Fig. 7.2) and ba = 4,444 A. This geometry makes H21 positive (7 21 = 50 cm in the point-dipole approximation) and gives the exciton band the higher transition energy, the larger dipole strength, and a positive rotational strength. For purposes of Illustration, the exciton bands were assigned Gaussian shapes with arbitrary widths...

By way of illustration of the above treatment we will now calculate the various contributions of the monomer transitions to a particular exciton band of Rps, viridis. We use the results of the exciton calculation of [7] to find the coefficients a. In [7] the interaction V is approximated by a point dipole-dipole interaction with a correction for the primary donor dimer. The direction of the unperturbed transition moment is taken along the NJ-N3 axis of each pigment. The zero-order energies e of the six monomer transitions were used as fit parameters. The dipole strengths were estimated from the in-vitro monomeric... 

The excimer fluorescence (with respect to the excited vdW dimer emission) is red shifted and structureless because the emission is terminated in a repulsive ground-state potential energy surface (Figure 15). For parallel transition moments, emission from the out-of-phase exciton state to the ground state is forbidden and for the in-phase exciton state emission is allowed [28a]. It should be noted, however, that the forbidden emission from the out-of-phase exciton state is expected to have a similar transition dipole moment as the Lb So emission. The actual dynamics of the initially excited vdW dimer depend on the energy gap and the coupling strength between the primary excited (LE) state and the excimer state. 

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