Excitation elliptically polarized

Fig. 4.4. The geometry adopted in the calculation of the degree of circularity C = (Ir — Ii)/(Ir + h) under elliptically polarized excitation.

Auzinsh, M.P. and Ferber, R.S. (1990). Orientation of atoms and molecules under excitation by elliptically polarized light, Opt. Spectrosc. (USSR), 68, 149-152. 

Fig. 3. Energy level diagram illustrating S-type MCD. Two nondegenerate excited states, 1) and 12), are mixed by the external magnetic field, H. Transitions from the ground state of the system, 0), to the perturbed excited states, 1 ) and 2 ), are elliptically polarized in opposite senses.

The n-jt transition (222 nm CD band) is responsive to the a-helical content. The n n excitation band at 208 nm polarizes parallel to the helix axis and is sensitive to whether the a-helix is single-stranded or is an interacting helix, as in the case of two-stranded coiled coils. 31,131 The decrease in parallel band intensity, coupled with the red shift in the parallel band maximum, corresponds to the conversion of a rigid single-stranded a-helix to an a-helical coiled coil.1 31 The maximum ellipticity at 208 nm in benign medium shifts to 206 nm in 50% TFE[861 and the ratio changes from >1.0 to <1.0 (range ca. 0.85-0.90 in 50%... 

Figure 10.11 Near-field imaging of SHG signals from a single Au nanoparticle.250 (a) Schematic of experimental setup (b) AFM image of an elliptical nanoparticle (c, d) near-field SHG signal mapping with emission detection polarized parallel and perpendicular to incident excitation. (Reprinted with permission from M. Zavelani-Rossi et al., Appl. Phys. Lett. 2008,92, 093119. Copyright 2008 American Institute of Physics.) (See color insert.)...

Since the relaxation of the higher exciton states occurs on an ultrafast timescale of about 100 fs [23,26], the absorption spectrum for a closed structure, Fig. 26.4, top and centre, consists of either one or a few relatively broad spectral bands, respectively. For both cases, i.e., circular and elliptical arrangement, the transitions from the k = 1 exciton states are polarized perpendicular with respect to each other. Moreover, the lowest exciton state is optically forbidden, because a C2-type symmetry reduction alone, i.e., an ellipse, does not give rise to oscillator strength in the k = 0 state. This situation is reminescent to the electronically excited states of the B850 BChl a... 

Figure 4 shows the SCF results in spheroidal coordinates for the excited bending state (0,4) as a function of the coordinate parameter a. For each of the states, the result is compared with the energy given by SCF in polar (hyperspherical) coordinates. Also shown are the results of a bare-mode approximation, a crude model which assumes for the mode a potential = 0), and similarly postulates a separate potential F( cq, > ) for the q mode, without any self-consistency in the treatment of the two modes. It is evident from Fig. 4 that the physically motivated elliptical (spheroidal) SCF modes do better in this case than the hyperspherical coordinates. Also, the SCF correction gives an important improvement on the bare-mode results. Most important, coordinate optimization, that is, imposing condition, (25) yields a noticeably better result than the SCF energy in a spheroidal system that is not refined for the best a value. 

Exciton-coupled circular dichroism (ECCD) spectroscopy was performed on the Aib-Stb-D peptide to fiirther substantiate the interaction between the methyl stilbene side-chains, which was observed via PL experiments. The interaction between the excited state of chromophores in a chiral environment causes split Cotton effects upon absorption of circularly polarized light by the chromophores (29, 30). It can be observed from Figure 7 that Aib-Stb-D exhibits a split CD Cotton effect presumably because of the chiral presentation of the methyl stilbene molecules on the same side of the a-helical peptide the asymmetric nature of the split observed may be due to some other electronic transitions or of additional background ellipticity, as has been observed in other systems (29, 31). The ECCD results confirm the close proximity of the methyl stilbene side chains mediated by the peptide backbone, which permits interaction between the side-chains in the excited state. 

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