Incident angle-dependent polarized absorption

In relation to the molecular alignment, incident angle-dependent polarized absorption spectroscopy [31, 32] of thin films gives important information on the electronic states of the molecules and their interaction in the solid disordered system (i.e. thin films). This method is based on measuring the polarized... 

Figure 11.7 Incident angle-dependent polarized absorption spectra of DMQtT thin film. From 5. Hotta, Y. Ichino and Y. Yoshida, Molecular alignments and spectroscopy in thin films of thiophene-based oligomers, in Electronic and Optical Properties of Conjugated Molecular Systems in Condensed Phases, ed. S. Hotta, Research Signpost, Trivandrum, 2003, pp. 615-635. Reprinted with permission from Research Signpost, Copyright (2003) Research Signpost...

Figure 11.8 Incident angle-dependent polarized absorption spectra of BP1T thin film. Reprinted with permission from S. Hotta, Y. Ichino, Y. Yoshida and M. Yoshida, Spectroscopic features of thin films of thiopheneYphenylene co-oligomers with vertical molecular alignment, J. Phys. Chem., B 104(44), 10316-10320 (2000). Copyright 2000 American Chemical Society...

In LB films not only the interaction of chromophores but also their orientation can be controlled at the molecular level. Molecular orientation of chromophores has been determined by several methods including polarized UV/vis or IR absorption, second harmonic generation (SHG), Electron Spin Resonance (ESR), or resonance Raman scattering. We have measured the incident angle and polarization angle dependencies of polarized UV/vis absorption to study the molecular orientation of alloxazine, porphyrin, and carbazolyl chromophores, or 4,4 -bipyridinium radical cations in LB films[3-12]. Usually in-plane components of transition dipoles of chromophores are... 

The out-of-plane orientation of chromophores can be more easily controlled in LB films as compared with the in-plane orientation. Many chromophores are known to show anisotropic orientation in the surface normal direction. The molecular structure of chromophores and their position in amphiphile molecules, the surface pressure, the subphase conditions are among those affect their out-of-plane orientation. The out-of-plane orientation has been studied by dichroic ratio at 45° incidence, absorbance ratio at normal and 45° incidence, and incident angle dependence of p-polarized absorption [3,4,27,33-41]. The evaluation of the out-of-plane orientation in LB films is given below using amphipathic porphyrin (AMP) as an example [5,10,12]. 

Fig. 2.16 Experimental geometries for the measurements of the polarization angle dependence (a) and the incident angle dependence (b) of IR absorption. 4>in is the polarization angle, which is defined by the angle between the rubbing direction and the analyzer direction. Oin is the incident angle of the IR light. The polarization angle and the incident angle were varied by rotating the sample around the Z and the Y axes, respectively. Reproduction by permission from [50].

Figure 5 The incident (5 and polarization angle a dependence of polarized absorption. The y and z axes indicate the dipping direction and the surface normal. The transition dipole M and its xy projection makes and 0 from the z and y axes, respectively.

The angular dependence of the polarized absorption spectra of LB films containing AMP deposited at lower (20 mN m1) and higher (43 or 50 mN m 1) surface pressures was studied to determine the molecular orientation of porphyrins as schematically shown in Figure 5. No polarization angle (a) dependence was observed at normal incidence. This indicates that the projections of the transition dipole moments of the porphyrins are statistically... 

To quantitatively analyze the molecular distribution within the rubbed polymer surfaces we have recorded absorption spectra for a series of electric field vector orientations within the two principal planes perpendicular to the film surface. These are the previously introduced planes parallel x-z) and perpendicular (y-z) to the rubbing direction. The polarization dependence is summarized by the intensity of the tt resonance in the normalized AEY spectra, which are plotted versus the photon incidence angle a as soUd and open symbols in Fig. 6.7. The solid lines are the result of a fit to the data using (6.3). 

Figure 2.40. Angle-of-incidence dependence of absorption depth AR at 2075 cm" in p-polarized IRRAS spectra for 0.2-nm film with optical constants as for Fig. 2.39 in ZnSe-water layer-film-Pt system for water layer thicknesses (1) 0.25, (2) 0.5, (3) 1, (4) 2, and (5) 5 pm.

Figure 3.25. Thickness dependence of absorption depth (AT = 1 - T) for (a) vjox. P) vroy> and (c) VLQz bands in transmission spectra of standing film of anisotropic inorganic material (Table 3.3) at several angles of incidence (a, c) for p-polarization p) for s-polarization. Dashed lines are calculated according to thin-film approximation Equation (1.98). Reprinted, by permission, from K. Yamamoto and FI. Ishida, Appl. Opt. 34, 4177 (1995), p. 4181, Fig. 6. Copyright 1995 Optical Society of America.

Figure 8.11 Dependence of (a) effective polarizer angle cp, and (b) dark-state light leakage of crossed polarizers, on the viewing azimuthal angle 4>q and polar angle 0q. The absorption axes of the crossed polarizers are set at 45° and -45°, respectively, and the incident light wavelength is /l = 550nm. Zhu 2006. Reproduced with permission from IEEE.

In this section we derive an approximate expression for the absorption cross section of a large weakly absorbing sphere. We assume that the incident plane wave can be subdivided into a large number of rays the behavior of which at interfaces is governed by the Fresnel equations and Snell s law (Section 2.7). A representative ray incident on the sphere at an angle 0, is shown in Fig. 7.1. At point 1 on the surface of the sphere the incident ray is divided into externally reflected and internally transmitted rays these lie in the plane of incidence, which is determined by the normal to the sphere and the direction of the incident ray. If the polar coordinates of point 1 are (a, 0f, ), the plane of incidence is the plane = constant. At point 2 the transmitted ray encounters another boundary and therefore is partially reflected and partially transmitted. In a like manner we can follow the path of the rays within the sphere, a path that does not deviate outside the plane of incidence. At each point where a ray encounters a boundary it is partially reflected internally and partially transmitted into the surrounding medium. On physical grounds we know that the absorption cross section cannot depend on the polarization of the incident... 

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