Polarimeters, design

Figure 8.10 Polarimeter design for the measurement of circular dichroism. The intensity measured at the detector is...

The polarimeter designed and constructed by the University of Hertfordshire, consists of a module containing an aduomatic half (or quarter) wave retarder, which can be stepped to set angular positions. The module is placed upstream of IRIS, the observatory near infrared camera, in which a magnesium fluoride Wollaston prism is used as an analyser (see Figure 1). A focal plane mask, within IRIS, serves to blank out half the fleld so that the e-and o-rays from the Wollaston prism do not overlap when imaging extended objects. 

The strategy in the design of any optical polarimeter is the use of the appropriate optical elements comprising the PSG and PSA to retrieve the desired information from the sample. Symbolically, this can be understood by considering the following expression for the intensity measured at the detector,... 

The design of a full Mueller matrix polarimeter based on a combination of a (P/RQ) psG and a (RQ/P)psa is shown in Figure 8.11. Here two rotary quarter wave plate... 

A full Mueller matrix polarimeter can also be fashioned from the (P/PEM1/PEM2)/,SG and the (PEM3/PEM4/P)/, since these designs, like the rotating... 

Whether a sample is dextrorotatory (abbreviated "(+) ) or levorotatory (abbreviated (-)") is determined experimentally by a polarimeter. Except for the molecule glyceraldehyde, there is no direct, universal correlation between direction of optical rotation ((-i-) and (-)) and designation of configuration (R and 5). In other words, one dextrorotatory compound might have R configuration while a different dextrorotatory compound might have S configuration. 

Each member of a pair of stereoisomers will rotate plane-polarized light in different directions. A polarimeter is used to measure the direction of rotation of plane-polarized light. Compounds that rotate light in a clockwise direction are termed dextrorotatory and are designated by a plus sign (+). Compounds that rotate light in a counterclockwise direction are called levorotatory and are indicated by a minus sign (—). 

Fig. 3.17 One enantiomer of a chiral compound rotates the plane of polarized light through a characteristic angle, a° the instrument used to measure this rotation is called a polarimeter. The direction indicated (a clockwise rotation as we view the light as it emerges from the polarimeter) is designated as +a°. The other enantiomer of the same compound would rotate the plane of polarized light through an angle —a°.

Ans. There is no relationship between the direction of optical rotation and the D- or L-configuration. D- or L-refers to the actual configuration at the stereocenter and can be determined only by x-ray crystallography. Only after a determination of the configuration by X-ray crystallography can we use the convention described in Problem 17.18 to designate one enantiomer as d- and the other as L-. Optical rotation is determined by using a polarimeter and its direction is unrelated to the d- or L-nomenclature. 

An important aim of the LIRTS design is to retain maximum flexibility for the accommodation of a wide range of focal plane Instruments. To establish a representative set of requirements for the Phase A telescope design the science team has considered three model Instruments compatible with the scientific objectives outlined above. These are a multiband photometer/polarimeter, a Michelson interferometer (X/AX 10 ) and a submillimetre heterodyne line receiver (X/AX > lO ). 

The ORD detector is similar in design and function to the PDR chiral polarimeter however, the light source for this detector is an Xe-Hg lamp, which is readily available and utilizes the strong line emissions of Hg at 365 nm. This detector utilizes the lowest wavelength of the polarimeters (365 nm vs. 426 and 670 nm) and therefore one would expect that this detector would give the strongest signal based on Drude s equation however, it also runs the risk of interference from the Cotton effects. 

---