Emission anisotropy factor

In marked contrast to the behaviour of staphylococcal nuclease and azurin, which also has its emission anisotropy factor nearly constant across the emission band, the CPL properties of human serum albumin reflect a heterogeneity in the population of the molecules of this protein, different molecules having different emission spectra and different dissymmetric environments for their single tryptophan residue 381). 

This equation shows that, at time t, each anisotropy term is weighted by a factor that depends on the relative contribution to the total fluorescence intensity at that time. This is surprising at first sight, but simply results from the definition used for the emission anisotropy, which is based on the practical measurement of the overall ly and I components. A noticeable consequence is that the emission anisotropy of a mixture may not decay monotonously, depending of the values of r, and Ti for each species. Thus, r(t) should be viewed as an apparent or a technical anisotropy because it does not reflect the overall orientation relaxation after photoselection, as in the case of a single population of fluorophores. 

Using the same method that led to Eq. (5.27), it is easy to establish the rule of multiplication of depolarization factors when several processes inducing successive rotations of the transition moments (each being characterized by cos2 C,) are independent random relative azimuths, the emission anisotropy is the product of the depolarization factors (3 cos2 c, — l)/2 ... 

Fluorescence polarization is the subject of Chapter 5. Factors affecting the polarization of fluorescence are described and it is shown how the measurement of emission anisotropy can provide information on fluidity and order parameters. 

The factor R may be called a diffusion anisotropy factor for the surface. For diffusion of a W on the W (110), Tsong Casanova find a diffusion anisotropy factor of 1.88 from a set of data taken at 299 K, and of 2.14 from a set of data taken at 309 K. The average is 2.01, which agrees with the theoretical value, 2, to within 0.5%. A more detailed general analysis has since then been reported,137 and diffusion anisotropy on the W (110) surface has also been observed in field emission experiments.138 It should be noted, however, that the same ratio can be expected if an adatom jumps instead in the [001] and [110] directions with an equal frequency. Thus a measurement of surface diffusion anisotropy factor alone does not establish uniquely the atomic jump directions. The atomic jump directions can, of course, be established from a measurement of the displacement distribution function in two directions as discussed in the last section. Such measurements can only be done with atomic resolution field ion microscopy. 

The factor a is a corrective factor linked to the emission anisotropy of the filament. We determined that a = 0.96 nndemeath the source. 

Emission quantum yield (Of), anisotropy factor (r) and average number of y-CD units per nanotube i) in diphenylpolyenes with different number of double bonds N. From Ref. [75]... 

Gamma-ray anisotropy or nuclear orientation thermometry OOl-l Spatial distribution of gamma-ray emission Spatial distribution related to Boltzmann factor for nuclear spin states Useful standard forT < IK... 

The resolution of rhombic g-factors in powder spectra of apparent axial symmetry spectra at low frequency can be more easily obtained for species having larger -anisotropy as schematically shown in Fig. 4.5 using parameters for the carbon dioxide radical, C02 , with g =2.0032, gy = 2.0014, gz =1.9975 obtained from single crystal measurements. The powder spectrum at X-band shows an apparent axial symmetric spectrum shape. With a typical line-width of ca 0.2-0.4 mT, the gn and gy features are resolved already at Q-band. The anisotropic g-factors can be directly measured at the absorption-(gx) and emission-(gz) like lines and at the centre (gy) of the 1st derivative Q-band spectrum. 

The polarization characteristics of monochromators have important consequences in the measurement of fluorescence anisotropy. Such measurements must be corrected for the varying efficiencies of each optical component this correction is expressed as the G-factor (Section 10.4). However, the extreme properties of the concave gratings (Figure 2.11) can cause difficulties in the measurement of fluorescence polarization. For example, assume that the polarization is to be measured at an excitation wavelength of 450 nm. The excitation intensities will be nearly equal with the excitation polarizers in each orientation, which makes it easier to compare the relative emission intensities. If the nission is unpdarized, the relative intensities of the parallel (U) and perpendicular (X) excit ion will be nearly equal. However, suppose the excic ion is at 340 nm, in which case the intensities the... 

At first glance, it s ms easy to perform fluorescence exp ments. However, there are numerous factors which can compromise the data and invalidate the results. One needs to be constantly aware of the possibility of san le contamination or contamination of the signal by scattered or stray light. Collection of emission spectra, and examination of blank samples, is essuitial for all experimuits. One cannot reliably interpret intensity values, anisotropy, or lifetimes without carefiil examination of the emissbn spectra. 

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