Polarized excitation

Let (Xgr and jXex be the dipole moments of the ground and excited states. Then if iXgr > the less polar excited state is surrounded by a solvation shell... 

Figure 8-7 illustrates schematically the electronic natures of the polar ground state (an ion pair) and the less polar excited state. This is, therefore, a case of p.g. 

Figure 4.6 shows an apparatus for the fluorescence depolarization measurement. The linearly polarized excitation pulse from a mode-locked Ti-Sapphire laser illuminated a polymer brush sample through a microscope objective. The fluorescence from a specimen was collected by the same objective and input to a polarizing beam splitter to detect 7 and I by photomultipliers (PMTs). The photon signal from the PMT was fed to a time-correlated single photon counting electronics to obtain the time profiles of 7 and I simultaneously. The experimental data of the fluorescence anisotropy was fitted to a double exponential function. 

Many components of food are in the solid state and possess very short T2. The linewidths from solid components are generally too wide to be observed directly by solution state NMR methods. However, these components can be detected by the special techniques of solid state NMR. These techniques involve the use of cross polarization excitation (from 3H to 13C), high power 3H decoupling (to inhibit... 

Recently, a formalism has been developed to determine the second and the fourth order parameters of films using polarized total internal reflection fluorescence (TIRF) [71]. Similarly to IR-ATR spectroscopy (Section 4), the experiment makes use of p- and s-polarized excitation, but the fluorescence emission (analyzed either in p- or s-direction) is detected normal to the substrate. Two approaches are developed based on the measurements of two intensity ratios. In the first one, the S angle has to be known experimentally or theoretically, and the order parameters (P2) and (P4) can be determined. In the second one, the order parameter (R ) is obtained by another technique, for instance IR-ATR spectroscopy, which allows deducing the order parameter (P4) and (cos2<5). 

Evaluation of the Work Term from Charge Transfer Spectral Data. The intermolecular interaction leading to the precursor complex in Scheme IV is reminiscent of the electron donor-acceptor or EDA complexes formed between electron donors and acceptors (21). The latter is characterized by the presence of a new absorption band in the electronic spectrum. According to the Mulliken charge transfer (CT) theory for weak EDA complexes, the absorption maximum hv rp corresponds to the vertical (Franck-Condon) transition from the neutral ground state to the polar excited state (22). 

Fluorescence anisotropy is generally used to provide information about the dipolar orientational dynamics occurring after excitation of a system. This technique has successfully been used to probe ultrafast dynamics of energy transfer in organic conjugated dendrimers. The detected emission intensities Tar and Ter for parallel and perpendicularly polarized excitation respectively, were used to construct an observable emission anisotropy R(t) in accordance with the equation [121] ... 

Throughout this chapter it is assumed that the intensity of the polarized exciting pulse is sufficiently low that only a small fraction of the fluorophores are ever excited.(29) High light intensities are treated elsewhere/73 74) The following subsection presents some very general and basic theory that is not specifically directed toward rotational relaxation of DNA. The reader may wish to skip directly to the final result in Eq. (4.15), or even skip this subsection entirely. 

The polarization study on naphthalene was complemented by Lavalette 39) who determined the polarized excitation spectrum, again using photoselection. The polarization of the strong Tm - Ti band at 4170 A was monitored as a function of the wavelength of polarized excitation into the singlet bands. As expected, a minimum polarization value of —0.18 was obtained at 2900 A near the 0—0 band of the S2 5q Mg) transition. 

Two orientations have been chosen (Fig. 17) for polarized excitation spectroscopy. The first is used to distinguish between in-plane and out-of-plane effects, while the second one allows a comparison of the two in-plane directions. 

The corresponding polarized excitation spectra are given in Figs, 18 and 19. There is a very weak O —0" peak in the Ti- - So transition at 407 nm. Toward lower wavelengths the intensity increases while the fine structure disappears and a rather broad maximum is observed at 388 nm. 

Fig. 18. The polarized excitation spectra in the Sq region comparing the in-plane and...

Fig. 19. The polarized excitation spectra in the - So region comparing the long-axis and short-axis polarizations at 4.2 K and slitwidths for all the curves of 50 fi. The 0—0 band is nearly depolarized while the higher energy bands are favored in the long axis polarization. The gain for curve i is about 5 times that used for curve ii, which is in turn about 5 times that used for curve iii. Comparable curves in the upper and lower spectra are at the same gain. (From Jones, Kearns, and Wing, Ref.49))...

Lavalette, D. Polarized excitation spectrum of the triplet-triplet absorption of aromatic hydrocarbons. Chem. Phys. Letters 3, 67 (A969). 

Figure 8. Absorption onset experiments for photoinduced conversion of compound 10 NH-tautomers in EPIP at 77 K (A) Initial spectrum (B,C) Monitored spectra after irradiation (At 30 min) by the polarized light (Xg = 638 nm). Conditions for polarized excitation and polarized monitoring are shown by arrows. The displacement of inner protons in tautomers conforms to experimental data. The direction of X-axis is chosen for tautomer 1 to be parallel to vector E of exciting light. (Reproduced with permission from Ref. 15. Copyright 1985 North-Holland Physics Publishing Company.)...

With vertically polarized exciting light, p0 — when p = 0. But when P = n/2, p0 becomes negative and is equal to —1/3. The values are +1/3 and—1/7 for unpolarized radiation. Thus negative polarization appears when 6 is small, i.e. absorption probability is high and the transition moment in emission is perpendicular to that in absorption. These observations provide a suitable method for assigning the polarization directions of transition moments in different absorption bands of a given molecule from polarization of the fluorescence excitation spectra. 

Another interesting class of molecules are stilbene derivatives with charge donating groups. These compounds offer the opportunity to explore the role of polar solvation dynamics (dielectric friction) in cis/trans isomerization. Interesting papers on this subject have been published by Waldeck et al. [145] and Rulliere et al. [146]. Other well-studied polar excited state isomerization examples include pinacyanol, l,l -diethyl-4,4 -cyanine, and crystal violet, which have been studied by Sundstrom, Gilbro and their coworkers [148] and Ben-Amotz and Harris [148] and others who are referenced in these papers [148,149],... 

As early as 1964, Corey had suggested the intermediacy of an oriented pi complex in the cycloaddition of enones (43) and soon thereafter Hammond and coworkers, on the basis of arene fluorescence quenching by dienes, suggested the possible involvement of a polar excited state complex with substantial charge transfer character (44). Since then the possibility of cycloadditions occurring through the intervention of exciplex or excimer intermediates per se or as precursors for radical ions pairs, eq. 12,... 

The enantiomeric excesses In chiral products reported from circularly polarized Irradiation of achiral reactants In achiral solvents are usually less than 0.2-0.3% (77, 79). The largest enantiomeric excess reported Is 1.6% for an unrelated rearrangement (80). Furthermore, It Is known that circularly polarized Irradiation of JBN In dlchloromethane yields very low (<0.2%) atroplsomerlc excesses (81). Therefore, the 1.1% atroplsomerlc excess of BN observed upon Irradiation of solutions In mixture k cannot be ascribed to circularly polarized excitations. 

In similar work, Sipe, Moss and Van Driel [84] determined a functional form of the rotational anisotropy for cubic centrosymmetric media. Their derived expressions for the total reflected p- and s-polarized SH fields from perfectly terminated (111) and (100) crystals under p- and s-polarized excitation take the form... 

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