Anisotropy of light absorption

This can partly explain the differences found for the two electrodes. The somewhat preliminary results shown here demonstrate that large effects of this kind can be found and one has to take into account the anisotropy of light absorption in the study of photoeffects at semiconductor electrodes. 

The absorption spectrum at 0° has a maximum at 600 nm. Upon rotation of the direction of the incident polarized light by as much as 90°, the absorption intensity decreases. The anisotropy of the absorption spectra reflects the regular orientation of the photogenerated closed-ring isomers and indicates that the photochromic reaction occurred in the single-crystalline phase. The blue color disappeared by irradiation with visible light a > 480 nm). Anthracene-substituted derivatives also showed photochromic properties (01JPC(A)1741). 

Photo-induced anisotropy (PIA) is quantitatively described by Eqs. (5-1) and (5-2), by An in terms of the induced birefringence and by the parameter S in terms of light absorption behavior. 

The anisotropy of the absorption of linearly polarized light by an oriented substance gives information about the electronic states involved, which is not present in the spectroscopic studies carried out in isotropic solvents. 

Fluorescent chemical sensors occupy nowadays a prominent place among the optical devices due to its superb sensitivity (just a single photon sometimes suffices for quantifying luminescence compared to detecting the intensity difference between two beams of light in absorption techniques), combined with the required selectivity that photo- or chemi-luminescence impart to the electronic excitation. This is due to the fact that the excitation and emission wavelengths can be selected from those of the absorption and luminescence bands of the luminophore molecule in addition, the emission kinetics and anisotropy features of the latter add specificity to luminescent measurements8 10. 

The degree of CP absorption is usually expressed by the difference in absorption of left-and right-handed CP light (circular dichroism, A A) or by the so-called anisotropy factor g ... 

Experiments involving anisotropy of phosphorescence or of the absorption of the triplet state rely upon the same principles as the measurement of fluorescence anisotropy. All are based upon the photoselection of molecules by polarized light and the randomization of polarization due to Brownian motion occurring on the time scale of the excited state. Anisotropy is defined as... 

Nevertheless, in order to obtain reliable results with lignins, fluorescence, light absorption, and anisotropy must be taken into account. Fluorescence is easily eliminated by the use of an adequate interference filter. It is well known that lignins exhibit an absorption spectrum with a maximum in the near ultra-violet tailing all the way into the visible. 

As it was pointed out earlier, the optical and photoelectrical properties of the polydiacetylenes may be explained in the framework of ID crystal model in which the interaction between the electrons in the chain is much stronger than between the chains. The interaction energy differs by 100 fold. Considering the anisotropy of the optical and photoelectrical properties one can expect this fact. This was actually observed for monocrystals and films of the polydiacetylene. Absorption spectra of the monocrystal films are presented in Fig. 21 for varying light polarization [141]. 

The mean diffusion length of the minorities is drastically dependent on crystal imperfections. It may also be dependent on surface orientation, if they have an anisotropic mobility. Optical anisotropy is necessary for seeing an influence of crystal orientation on the penetration depth of the light while crystal imperfections will only affect the penetration depth in a range of wave lengths where light absorption in the crystal is very weak. The photocurrents are very small in this case which will not be discussed here. 

Thus, in this section we have described the manner in which absorption of light by a molecule leads to polarization of the angular momenta of the absorbing level. We have also shown how to calculate the multipole moments created on the lower level. It is important to stress that the adopted model of description enables us to obtain precise analytical expressions for the multipole moments, including both cases, namely those for arbitrary values of angular momenta and those for the classic limit J — oo. Our subsequent discussion will concern problems connected with the manifestation of ground state angular momenta anisotropy in experimentally observable quantities. 

Chiral-specific photochemistry using CPL depends on the circular dichroism (A = r l) of the reactants, i.e., the differences in the absorption coefficients or right and left CPL. The rate of the reaction depends on the amount of light absorbed so if A > 0, there will be a bias towards one of the enantiomers leading either to its preferential enhancement or destruction and an ee The enantiomeric purity of the chiral product is determined by the anisotropy factor, g, where g - A / and = 0.5( r+ l). For optically active compounds g values are fairly small, 0.01. 

The absorption of light is proportional to the scalar product of the incident electric field and of a molecular vector named the transition moment. Thus, excitation of an isotropic population of fluorescent species by polarized light generally creates a temporary anisotropic population of excited molecules. Molecular motions progressively destroy this anisotropy, and affect the polarization of the reemitted fluorescence light which can be studied using pulse fluorometry techniques. 

The rotational diffusion of molecules is investigated by exciting their fluorescence with short pulses of polarized light and by observing the time dependence of the polarized emission. For a symmetric body the anisotropy of the emission (Fig. 9) is characterized by 3 rotational relaxation times each of which is a function of the rotational diffusion constants around the main axes of rotation. The corresponding amplitudes a depend on the position of the absorption vector and the emission vector within the coordinates of the rotating unit. 

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