Light polarised

W. C. McCrone, L. C. McCrone, and J. G. DeUy, Polarised Light Microscopy, Ann Arbor Science PubHshers, Inc., Ann Arbor, Mich., 1979. 

Most treatments of polarised light in transmission are to be found in the mineralogical literature, but a fine book presenting the subject in relation to crystal identification and structure analysis is by Bunn (1945). 

Figure 6.2. Sloichiomclric CuPl, ordered at 550°C for 157 hours. Viewed under polarised light in reflection. Shows growth of ordered domains, heterogeneously nucleated at grain boundaries and surface scratches (after Irani and Cahn 197.5).

When we compare the VV with the VH configurations for the polarised light, the Raman intensity shows anisotropic behaviour. Most importantly, the A g mode at 165 cm is suppressed in the VH configuration, while the lower frequeney E g and E2g modes are not suppressed. This anisotropy is due to the... 

The polarimeter is an instrument with which the essential oil chemist cannot possibly dispense. The hypothesis, first seriously enunciated by Le Bel and van t Hoff, that substances which contained an asymmetric carbon atom i.e. a carbon atom directly united to four different atoms or radicles) were capable of rotating the plane of polarisation of a beam of polarised light, has now become a fundamental theory of organic chemistry-. The majority of essential oils contain one or more components containing such a carbon atom, and so possess the power of effecting this rotation. In general, the extent to which a given oil can produce this effect is fairly constant, so that it can be used, within limits, as a criterion of the purity or otherwise of the oil. 

The angle of rotation is, of course, directly proportional to the thickness of the layer of active substance through which the polarised light passes. The expression optical rotation or rotatory power is universally understood to be the observed angle of rotation produced by a column of 100 mm. of the optically active substance. If tubes of other lengths be... 

Ellipsometry measurement of the characteristics of polarised light after reflection from a surface. 

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. 

Enantiomer A single form of an optically active compound. Optically active compounds usually (but not exclusively) contain one or more chiral centres. Enantiomers are defined by their ability to rotate the plane of beam of polarised light one way or the other and these are referred to as either D or L , or alternatively + or, depending on whether the polarised light is rotated to the right (Dextro) or the left (Levo). 

Meso compound A symmetrical compound containing two chiral centres configured so that the chirality of one of the centres is equal and opposite to the other. Such internal compensation means that these compounds have no overall effect on polarised light (e.g., meso tartaric acid). 

The question also arises as to where the chiral molecules came from. Were the L-amino acids or the D-sugars selected on the primeval Earth, or are exuaterresuial sources responsible for the homochirality This second possibility is dealt with by hypotheses on the effect of circularly polarised light, of extraterrestrial origin, on chiral molecules in the molecular clouds from which the solar system was formed. One such hypothesis was proposed by Rubenstein et al. (1983) and developed further by others, particularly A. W. Bonner (Bonner and Rubenstein, 1987) both scientists worked at Stanford University. The authors believe that the actual radiation source was synchrotron radiation from supernovae. The excess of one enantiomeric form generated by this irradiation process would have needed to be transported to Earth by comets and meteorites, probably during the bombardment phase around 4.2-3.8 billion years ago. 

A further complication is that for a carbon, which has four different groups attached to it in a tetrahedral arrangement, there are two optical isomers called laevo (l-) and dextro (d-) from their different abilities to rotate the plane of polarised light. They are structural mirror images which cannot be superimposed ... 

Figure 2.41 The direction of the electric vector for light polarised in the S- and P-directions before being incident at a planar surface at an angle 6, and the resolution of the electric vector of the P-polarised light into its components parallel and perpendicular to the reflective surface.

P-polarised light can be reduced to two components Px polarised parallel to the surface and Ps polarised perpendicular to the surface. The Px component also suffers a 180° phase change on reflection for all 9 and is thus blind to any surface species. However, the phase shift for the Pz component changes rapidly with 9. This results in the ratio of the standing wave/incident ray mean electric held strength, <( >/<( ,2 >, varying with 9 as shown in Figure 2.42. The above discussion has two important implications ... 

S-polarised light is effectively blind to adsorbed and near-surface species at all angles of incidence. 

The surface selection rule operates in addition to the normal IR selection rules in determining which vibrational modes are observed. As a result of the SSR the relative intensities of the fundamental IR adsorption bands of an adsorbed species can be used to give information on the orientation of the species with respect to the surface. Both S- and P-polarised light interact equally with the randomly oriented solution species. 

Consider a linearly polarised monochromatic light ray incident on a metal surface. Such a ray can always be resolved into two orthogonal components and if the plane of reflectance at the metal is chosen as the reference, then these components correspond to S- and P-polarised light, as discussed in the previous section. 

Figure 2.58 Schematic representation of the generation of elliptically polarised tight riu the reflect ion of plane-polarised light from a reflective surface.

Figure 2.59 The path followed by the tip of the electric vector of elliptically polarised light. ES, (1 and ,. ref show the directions of the planes of the S-potarised and P-polarised light. For...

Figure 3.93 Reflectance spectra (8cm 1 resolution, 750 scans) of SSBipy adsorbed at an Au electrode immersed in N2 saturated 0.1 M NaH2PO + 0.1 M NaCIO + NaOH at pH 7.1. The spectra were collected at + 0.3 V vs. SCE and normalised to the reference taken at —0.2 V using P- and S-polarised light as indicated on the figure. From Christensen et al. (1991).

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