Emissive-absorptive polarization

The SNOM image depicts chemical (or material) contrast between different materials. It depends on various factors including Rayleigh, Raman, and fluorescence emissions, absorption, polarization, and also on specific shear-force influences. Even different polymorphs are differentiated by the material contrast. ... 

Collisional redistribution of radiation. A system A + B of two atoms /molecules may be excited by absorption of an off-resonant photon, in the far wing of the (collisionally) broadened resonance line of species A. One may then study the radiation that has been redistributed into the resonance line - a process that may be considered the inverse of pressure-broadened emission. Interesting polarization studies provide additional insights into the intermolecular interactions [118, 388]. 

Figure 5 (a) Absorption (a) and emission (PL) polarized spectra of tetracene b and Lb Davydov splitting components of a tetracene single crystal seen in the absorption spectrum of a polycrystalline tetracene layer (upper full curve) as double features at 505 and =520 nm the PL spectrum (1) as measured, the PL spectrum (2) corrected for the spatial distribution of excitons in the crystal as shown in part (b). (b) The spatial distribution of singlet excitons [fix)] in a 4.7 pm-thick tetracene single crystal, obtained according to the procedure described elsewhere [53] (see also Sec. 3.1). 

The energies of the triplet states of sensitizers TX (ITX) and of Pis TPO (BAPO) are close to each other ( 260 kJ/mol), allowing for slightly exothermic or thermoneutral T-T energy transfer from sensitizer to PI. Direct photolysis of phosphine oxides results in a well-documented initial strong absorptive (A) pattern of ESR spectra (see Fig. 12.2). Sensitization by TX or ITX of the photolysis of phosphine oxides leads evidently to the same radicals, but an initial polarization pattern is quite different, namely, emission/absorption (E/A) pattern (see Fig. 12.3). 

A final noteworthy feature of these spectra is the lack of RPM polarization, which for these radicals would appear as low-field emissive, high-field absorptive transitions. It is curious that such polarization never develops at any delay time, even out to 20 xs where we have observed only TM polarization. The creation of RPM polarization requires reencounters of radicals on a suitable timescale and modulation of the exchange interaction between the unpaired electrons. This is normally accomplished by diffusion of the radicals between weak and strong exchange regions. That it never develops indicates that either these radicals do not make a significant number of reencounters, or perhaps it is due to the fast spin relaxation in the oxo-acyl radical. It may also be that the TM is simply so dominant that the RPM intensity is always much weaker and is never observed. At lower temperatures (cf. Fig. 14.1, top spectrum at 25°C), there does appear to be a slight superposition of an EIA pattern on top of the emissive TM polarization, but it has a very small effect. 

Fig. 10-8 shows the observed CIDEP spectra for the reaction of triplet eosin Y (FlBr/ ) with duroquinone. In this figure, CIDEP spectra of the duroquinone radical anion were only observed. The spectra of Xn" were not observed because of its fast spin relaxation. As clearly shown in Fig. 10-8, the initial spectrum measured at 60 ns after the laser excitation showed an emissive polarization, which was due to the usual p-type TM. This polarization was found to change as the delay time was increased. The spectrum measured at 200 ns after the excitation showed a strong absorptive polarization, which was proposed to be due to the d-type TM. Similar polarization changes were also observed for such dyes as erthrosin B (FlLi ) and dibromofluorescein (FlBr2 ) which contain heavy atoms. On the other hand, an emissive polarization was only observed for the reaction of fluoresein (Fl ), which contain no heavy atom. From these results, Tero-Kubota et al. concluded that the strong absorptive... 

Fig. 10-9 shows the time profiles of CIDEP signals of the central HF line for DQ observed from the photoexcitation of DQ in the presence of DMA and Br-substituted DMAs. As seen from curve a in this figure, an emissive polarization was found to grow for the reaction of DMA and to approach the absorptive signal due to the thermal equilibrium. This time profile can be explained by CIDEP due to the p-type TM. As seen from curve d in Fig. 10-9, an absorptive polarization was found to grow for the reaction of 2BrDMA and to approach the absorptive signal due to the thermal equilibrium. This time profile can be explained by CIDEP due to the d-type TM. In the reactions of 4BrDMA and 3BrDMA, their time profiles were found to be initial emissive polarization due to the p-type TM followed by absorptive polarization due to the d-type TM. Tero-Kubota et al. also found that the time profile observed for the reaction of 4IDMA was similar to curve d in Fig. 10-9 and that the profile for 4C1DMA to curve c in Fig. 10-9. These results show that the order of SOC in the present reactions can be expressed as follows ...

A normal emission/absorption/emission/absorption pattern that did not vary in the time interval 0.7-5 ps was observed for the radical pairs from 35 and 36, while the spectrum from 34 was initially totally absorptive (0.4-0.6ps), then rapidly changed to emissive/absorptive (1.9-2.1 ps), and eventually became totally emissive (4.6-5 ps).37 It is suggested that for ZP4V, due to the smaller spacer chain, the time elapsed between laser excitation and radical-pair formation was shorter than for ZP6V and ZP8V and the spin polarization in the porphyrin triplet was retained to a larger extent before electron transfer took place.37... 

Figure 2. Polarized absorption and fluorescence emission P, polarizer A, analyzer.

The timescale of fluorescence emission is comparable to that of rotational diffusion of proteins and the timescale of segmental motions of protein domains or individual amino acid residues. The polarization or anisotropy of the emission provides a measure of these processes. Suppose a sample is excited with vertically polarized light (Fig. 11), and that the sample is viscous so that the fluorophores do not rotate during the lifetime of the excited state. Then the emission is polarized, usually also in the vertical direction. This polarization occurs because the polarized excitation selectively excites those fluorophores in the isotropic solution whose absorption... 

Figure 3.11 Absorption, emission, and polarization spectra as well as excitation and emission polarization spectra (points) of DMS in propanol [47]. (Reproduced with permission from Elsevier.)...

The nonequilibrium polarization is not directly observable at the instant after SCRP formation if the radical pair is formed from a singlet precursor (formally diamagnetic pair with S = 0) or from a triplet precursor in which all the triplet sublevels T i, To, and T+i were equally populated. In these cases there are as many molecules with emissively polarized transitions in the ensemble as there are molecules with absorptively polarized transitions. 

The photoionization of the amino acid tyrosine in alkaline solution was studied by CW TR EPR. The photoionization of deprotonated tyrosine leads to a spin-polarized emissive/absorptive CIDEP spectrum produced by the radical-pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction J is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of the photoionization of deprotonated tyrosine is only seen upon the addition of the electron acceptor 2-bromo-2-methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic add). This channel is also found to be monophotonic. 

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