Absorption spectra polarized light

Taking into account the fact that the absorption spectrum in light polarized parallel to planar groups contains only a very weak absorption band in the visible range, it must be concluded that all the new intense bands of the crystalline state have a characteristic polarization perpendicular to the planes which produces the dichroism. 

Circular dichroism (c.d.) spectroscopy measures the difference in absorption between left- and right-circularly polarized light by an asymmetric molecule. The spectrum results from the interaction between neighboring groups, and is thus extremely sensitive to the conformation of a molecule. Because the method may be applied to molecules in solution, it has become popular for monitoring the structure of biological molecules as a function of solvent conditions. 

In applying RAIRS to CO adsorption, the contribution from CO molecules in the gas phase to the absorption spectrum at CO pressures above 10-3 mbar completely obscures the weak absorption signal of surface adsorbed CO. Beitel et al. found it possible to subtract out the gas phase absorption by coding the surface absorption signal by means of the polarization modulation (PM) technique applied to a conventional RAIRS spectrometer, p-polarised light produces a net surface electric field which can interact with adsorbed molecules, whereas both polarization states are equally sensitive to gas phase absorption because gas phase molecules are randomly oriented. By electronic filtering a differential spectrum is computed which does not show contributions from the gas phase and which has much higher surface sensitivity than a conventional RAIRS setup. 

Fig.27. )P and absorption spectra of an MOPPV LB film. Open and solid circles show value of x(3) of the MOPPV LB film in the direction parallel and perpendicular to the dipping direction. Open triangles show x of an MOPPV cast film. Solid and broken lines show the absorption spectra measured by the polarized light parallel and perpendicular to the dipping direction, respectively. Dotted line shows an absorption spectrum of the MOPPV cast film.

Figure 1,17 Absorption spectrum of a forsteritic olivine under polarized light. Ordinate axis represents optical density (relative absorption intensity, ///q). From R. G. Burns (1970), Mineralogical Applications of Crystal Field Theory. Reprinted with the permission of Cambridge University Press.

Figure 8. Absorption onset experiments for photoinduced conversion of compound 10 NH-tautomers in EPIP at 77 K (A) Initial spectrum (B,C) Monitored spectra after irradiation (At 30 min) by the polarized light (Xg = 638 nm). Conditions for polarized excitation and polarized monitoring are shown by arrows. The displacement of inner protons in tautomers conforms to experimental data. The direction of X-axis is chosen for tautomer 1 to be parallel to vector E of exciting light. (Reproduced with permission from Ref. 15. Copyright 1985 North-Holland Physics Publishing Company.)...

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). 

Fig. 11. Overall polarized absorption spectrum of 3EHS Ir, i.e. Ir4+ doped in (ethyl3NH)2[SnCl6], at 15 K. Incident light perpendicular to the (10T) crystal plane (see Ref. [26])...

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