Polarization spectrum

Measurement of steady-state emission anisotropy. Polarization spectra 

6) The letter G comes from grating . In fact, the majority of the polarization effects arise from the polarization dependence of the monochromator grating, but the detector response and the optics can contribute to a lesser extent. 

7) For the sake of simplicity, this relation is often written as 

The excitation polarization spectrum (which represents the variation of r as a function of the excitation wavelength see Chapter 5) is obtained by recording the variations of fyv(AE), Ivh( e), Ihv e), and fHH(AE) and by calculating r(AE) by means of Eq. (6.8) with G(AE) = -Tvh eVIhh e)- The correction factors for wavelength dependence can be ignored because they cancel out in the ratio. 

In some cases, filters are used instead of the emission monochromator. In principle, no G factor is then considered, but in practice, effects may be due to the sensitivity of the photomultipliers to polarization (in particular, photomultipliers with side-on photocathodes). 

We have seen that the transition dipole moment occurring upon excitation of a molecule has a distinct orientation with regard to the molecular axis. This orientation can be determined by measuring the absorption of polarized light (oscillating in only one plane) by oriented single crystals, 

The values ofP range from + to - 05 = 0 or 90°). This equation would be considerably simpler if only those molecules with their transition moments parallel to the electric vector were capable of absorption or if the molecules were perfectly aligned, that is, 6 = 0. Then the angle between the two transition moments could be directly determined from the observed degree of polarization P. 

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Resonance Raman reflection spectroscopy of monolayers is possible, as illustrated in Fig. IV-14 for cetyl orange [157]. The polarized spectra obtained with an Ar ion laser allowed estimates of orientational changes in the cetyl orange molecules with a. 

Clearly, if a situation were achieved such that exceeded Np, the excess energy could be absorbed by the rf field and this would appear as an emission signal in the n.m.r. spectrum. On the other hand, if Np could be made to exceed by more than the Boltzmann factor, then enhanced absorption would be observed. N.m.r. spectra showing such effects are referred to as polarized spectra because they arise from polarization of nuclear spins. The effects are transient because, once the perturbing influence which gives rise to the non-Boltzmann distribution (and which can be either physical or chemical) ceases, the thermal equilibrium distribution of nuclear spin states is re-established within a few seconds. 

Accepting all three of the basic postulates, the origin of CIDNP can now be described qualitatively. Figure 2 illustrates a variety of different radical reaction pathways known to produce polarized spectra. 

Some of the uncertainties and ambiguities of qualitative rules such as Kaptein s and Muller s (1972) can be cleared up by computer simulation of polarized spectra. Alternatively, such quantitative approaches can be used to deduce a- and Jgr-values. 

For thermal reactions a variable temperature probe is necessary since optimum polarized spectra are usually obtained in reactions having a half-life for radical formation in the range 1-5 minutes. Reactant concentrations are usually in the range normally used in n.m.r. spectroscopy, although the enhancement of intensity in the polarized spectrum means that CIDNP can be detected at much lower concentrations. Accumulation of spectra from rapid repetitive scans can sometimes be valuable in detecting weak signals. 

Fig. 12 FTIR absorbance spectra of the stretching vibrations in single crystals of a-Sg. (a) Polarized spectra of Vg and v,o in a- Sg (resolution < 0.05 cm )> after [105, 109], (b) unpolarized spectrum of the strong absorbing manifold of components of v, V5, and Vi, respectively, in natural a-Sg (resolution 0.5 cm ), after [109]...

Four polarized ATR spectra can be recorded to characterize the three-dimensional (3D) orientation of a sample, p- and s-polarized spectra are recorded with the sample clamped with its Z- and X-axes sequentially aligned perpendicular to the incidence plane (that is, parallel to the s-polarized electric field). The absorbance measured in these different configurations is related to the anisotropic absorption indices of the sample, kj, as... 

The parameters K1/ K2/ and K3 are defined by the refractive indices of the crystal and sample and by the incidence angle [32]. If the sample has uniaxial symmetry, only two polarized spectra are necessary to characterize the orientation. If the optical axis is along the plane of the sample, such as for stretched polymer films, only the two s-polarized spectra are needed to determine kz and kx. These are then used to calculate a dichroic ratio or a P2) value with Equation (25) (replacing absorbance with absorption index). In contrast, a uniaxial sample with its optical axis perpendicular to the crystal surface requires the acquisition of spectra with both p- and s-polarizations, but the Z- and X-axes are now equivalent. This approach was used, through dichroic ratio measurements, to monitor the orientation of polymer chains at various depths during the drying of latex [33]. This type of symmetry is often encountered in non-polymeric samples, for instance, in ultrathin films of lipids or self-assembled monolayers. 

It is important to stress that ATR absorbance is strongly affected by the sample/crystal contact. Quantitative results are thus difficult to obtain even if the contact is maintained during the sample rotation that is required to record all four polarized spectra. A reference band that does not show significant dichroism is thus most often used to normalize the polarized absorbances in order to obtain quantitative data. For instance, the 1,410 cm-1 band of PET has often been chosen for that purpose, not only for ATR studies but also for specular reflectance (see below) and even transmission studies when the sample thickness is not uniform. It was shown that an appropriate normalization is possible even if no such reference band is available, by using a combination of two bands with orthogonal dichroism [34]. When performing ATR experiments, one should also make certain that the applied pressure does not create artifacts by affecting the structure of the sample. 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

For oriented systems, the determination of molecular conformation is a complex problem because Raman spectra contain signals inherently due to both molecular conformation and orientation. To extract only the information relative to the conformation, one has to calculate a spectrum that is independent of orientation, in a similar way to the A0 structural absorbance of IR spectroscopy (Section 4). Frisk et al. [57] have shown that for a uniaxial sample aligned along the Z-axis, a spectrum independent of orientation (so-called isotropic spectrum), fso, can be calculated from the following linear combination of four polarized spectra [57]... 

Figure 10 shows polarized spectra of two types of silks recorded by Raman spectromicroscopy the dragline silk (the lifeline) of the spider Nephila edulis and the cocoon silk of a wild silkworm Sarnia cynthia ricini. The position of the amide I band at 1,668-1,669 cm-1 for both threads is characteristic of the /i-sheet... 

Figure 10 Polarized spectra obtained by Raman microspectroscopy of (A) the dragline silk of the spider Nephila edulis and (B) the cocoon silk of the silkworm Sarnia cynthia ricini. Adapted with permission from Rousseau et al. [63]. Copyright 2004 American Chemical Society.

Grabowski, J., and Gantt, E. (1978) Photophysical properties of phycobiliproteins from phycobilisomes Fluorescence lifetimes, quantum yields, and polarization spectra. Photochem. Pbotobiol. 28, 39-45. 

Spectroscopy Excitation polarization spectra distinction between excited states... 

Fig. B9.3.1. A absorption spectrum of the B excitation polarization spectra of the model multi-chromophoric cyclodextrin CD7(6) and compound NAEt and CD7(6). Solvent mixture variations in the emission maximum as a func- (9 1 v/v) of propylene glycol and 1,4-dioxane at tion of the excitation wavelength (broken line). 200 K (adapted from Berberan-Santos et al.a)).

Absorption spectra, emission spectra and excitation polarization spectra were recorded in a propylene glycol-dioxane glass at 200 K. Comparison was made with the reference chromophore 2-ethylnaphthoate (NAEt). 

Fig. 15. Tm-cTi absorption and polarization spectra of naphthalene-dg in 3-methylpentane at 77 K (From Pavlopoulos, Ref. ))...

Fig. 5 a. Spin-polarized spectra of 1 photolysis of diphenyldiazomethane in toluene. The chemical shift scale is in hertz downfield from the toluene methyl resonance 86)... 

Figure 7. Cross polarization spectra of cured, carhon-black-filled polyisoprene using (a) spin lock of 20 msec, contact time 10 msec and (b) spin lock of 20 msec,...

Fig. 5.12. a-c Laser-induced polarized spectra of synthetic zircon artificially activated by Eu... 

Emerald, Cr " doped beryl, has a beryl structure with the Cr " impurity ions in highly distorted octahedron sites. The discovery of lasing action in emerald stimulated investigation of its luminescence properties. It was established that its tuning range is approximately 730-810 nm, while luminescence consists of a narrow line at 684 nm and a band peaking at 715 nm with similar decay times of 62 ps. The relative intensities of those line and band are different in a- and 7T-polarized spectra (Fabeni et al. 1991). 

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