Isotropic absorber

FIGURE 3.3 Variation of the absorbance. O.D., of a film of DR I -doped PMMA (2.5% w/w) with (A) low, 0.28 mW/cm and (B) high, 28 mW/cm irradiation (488 nm Ar" laser) intensity. The analysis light was the 514 nm from the same Ar laser, and both A// and are measured. A represents the isotropic absorbance (A//+2AjJ/3. (C) Variation of the absorbance—parallel, perpendicular, and isotropic—with the irradiation light intensity.The data were taken 3 minutes after the irradiation was turned on. At low irradiation intensity, the cis population is small and Aj increases as a consequence of the orientational distribution. At high irradiation intensity, saturation of the orientational hole-burning process tends to equalize A// and A. (After reference 20, by permission.)... 

This model alleviates the concept of the somewhat ambiguous molecular anisometry that is based on an arbitrary choice of fixed molecular axes. So, for each of the A and B isomers, the isotropic absorbance Aa,b = Abs// + lAhsf yS, the anisotropy AA g = Absj - Absf and the optical order parameter g = AA ySA gare given by ... 

Equation 3.10. The slopes of the early time evolution of the changes of the normalized isotropic absorbance, p(An), and the anisotropy, p(AAn), are given by Equations 3.16 through 3.18. These equations hold for an analysis light that is absorbed only by the B isomer and describe the orientational distribution of that isomer. Equations 3.16 through 3.18 are rigorously given by ... 

FIGURE 3.12 The fitted slopes P(A) and P(AA) of the observed early time evolution of the isotropic absorbance (A) and anisotropy (B). respectively. The full lines are linear theoretical fits to Equations 3.12 and 3.13. X = F A. ... 

To evaluate photoisomerization and photo-orientation parameters, and should be known, was calculated from the absorption spectrum of the polymer solution before irradiation, assuming the same extinction coefficient in the film and in solution bq was determined by the Fisher s method, modified by Rau, which holds not only for isotropic but also for anisotropic samples when the isotropic absorbance is considered (vide infra). For this determination, the isotropic absorbance change was recorded versus the irradiating light intensity, and the sample absorbance change was extracted for an irradiation flux extrapolated to infinity for three drbierent combinations of irradiation and analysis wavelengths 488-488, 532-488, and 532-532 nm, irradiation and analysis, respectively. These experiments... 

FIGUitE 3.17 Real-time evolution of the isotropic absorbance (left) and the anisotropy (right) of Spin PMMA upon linearly polarized UV irradiation for several irradiation intensities.The numbers from I to 5 indicate the value of the irradiation intensity in units of Einstein.s. cm with the coiresponding sample absorbance (value in parentheses) at the irradiation wavelength (365 nm). The moments of turning the irradiation light on and off are indicated. After reference 26, redrawn by permission. 

It was shown that the transition moment of the )3-carotene molecule is isotropically distributed in the plane of the SC film. As regards the normal direction ofthe film, the observed angular dependence ofboth reflectance and transmittance was in excellent agreement with the theoretical curves calculated assuming the isotropic absorbing medium. This means that the refractive index ofthe SC film is isotropic in the normal ofthe film. Since the refractive index can be directly related to the polarizability of a molecule using Clausius-Mosotti s law and a transition moment linearly depends on the polarizability, the present investigation reveals that the transition moment of the /3-carotene molecule is isotropically distributed both in the plane and in the normal of the SC film. The transition moment lies parallel to the molecular axis of /3-carotene, hence we can conclude that the /3-carotene molecules are randomly oriented in the SC film. 

PtGLIRE 3.21 Slopes. (A) p(A) and p(AA), and (B) p(Aiv) and (AA ), of the observed change of the early time evolution of the isotropic absorbance and the anisotropy, respectively, on the irradiation intensity (UV for Figure A and green for Figure B).The full lines are linear fits. F and AJ are as defined earlier. After reference 42, redrawn by permission of ACS,... 

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