Infrared radiation, polarised

The experiment involves the measurement of the transmittance or absorbance of a sample, usually in the form of a thin film, for infrared radiation polarised parallel or perpendicular to the draw direction for a particular... 

For any molecular vibration that leads to infrared absorption, there is a periodic change in electric dipole moment. In case the direction of this change is parallel to component of the electric vector of the infrared radiation, absorption takes place otherwise it does not. In oriented bulk polymers, the dipole-moment change can be confined to specified directions. The use of polarised infrared radiation in such a case leads to absorption which is a function of the orientation of the plane of polarisation. The... 

As explained in section 2.6.1, any infrared-absorption peak is associated with a particular vibrational mode of the polymer chain. For each of these modes there is a particular direction within the polymer chain, called the infrared-transition dipole or transition-moment axis, which is the direction of the absorption dipole p. Infrared radiation is absorbed by a particular chain of the polymer only if two conditions are satisfied the frequency of the radiation must correspond to the frequency of the vibration and there must be a component of the electric vector E of the incident radiation parallel to the transition-dipole axis. If the molecules become oriented, so do the dipole axes hence the absorption of the sample will depend on the polarisation of the radiation. 

A uniaxially oriented polymer sample transmits 21% or 90% of infrared radiation corresponding to a particular absorption for radiation polarised parallel or perpendicular to the draw direction, respectively. If the transition dipole is parallel to the chain axis, calculate the value of P2(cos0)) for the chains. 

The molecular chains in a particular sample of polymer film are almost completely aligned in the simplest type of uniaxial orientation. The fractional transmissions of this sample for radiation of a particular infrared wavelength incident normally are found to be 0.20 and 0.95 for radiation polarised parallel and perpendicular to the alignment axis, respectively. For a second film of the same polymer of lower orientation the eorresponding fractional transmissions are 0.30 and 0.80. Making suitable assumptions, what can be deduced about the degree of orientation of the chains in the second film ... 

It is known that the dipole moment associated with a particular infrared absorption in a polymer makes an angle of 15° with the chain axis. A particular uniaxially drawn sample of the polymer is found to transmit 25% of incident infrared radiation at the wavelength corresponding to this absorption when the incident beam is polarised parallel to the draw direction. If the sample has a birefringence 0.85 times that of a perfectly oriented sample, estimate the corresponding percentage infrared transmission for an incident beam polarised perpendicular to the draw direction. State clearly the assumptions made. 

A highly uniaxially oriented polymer sample in the form of a thin film has mean optical polarisabilities of 2.3 x 10 and 1.7 x 10 F m per struetural unit for light polarised parallel and perpendicular to the draw direction, respectively it transmits 50% of infrared radiation at a particular wavelength 2 when the radiation is polarised parallel to the draw direction and transmits almost all... 

The infrared spectra of optically active peptide enantiomers are identical (e.g. D, D and L, L isomers) but there may be differences in the spectra of isomers where the optical activity differs at different asymmetric carbons (e.g. D, L and L, D isomers). The use of polarised infrared radiation can be helpful in peptide studies - for example, to determine the orientation of... 

Crystal orientation In general, the infrared radiation incident on a sample is partially polarised so that the relative intensities of absorption bands may alter as a crystalline sample is rotated. In an orientated crystalline sample, a functional group may be fixed within its lattice in such a position that it will not interact with the incident radiation. These crystalline orientation effects can be dramatic, especially for thin crystalline films or single crystals. 

Interestingly, the work of Blum et ai (1986) showed that a surface selection rule operates in X-ray reflection absorption. Thus the synchrotron radiation employed in their experiments was polarised in the plane of reflection and the authors noted that bonds perpendicular to the plane of reflection do not contribute to the SEXAFS (cf. the infrared SSR, discussed above). 

The effects of using polarised radiation on the spectra measured after exposure of the crystal to ethene at 373K are shown in Figure 16. Polarisation parallel to the crystal c axis enhances the contribution of the CH3 symmetric and asymmetric modes to the spectrum relative to the unpolarised spectrum or that obtained with polarisation perpendicular to the c axis. Similar polarisation effects were found for crystals exposed to ethene at room temperature or 473K, regardless of which crystal face the infrared beam was incident upon. After heating... 

The principle of the method is similar to that of the infrared method except that the polymer is illuminated by polarised radiation of a suitable wavelength to induce fluorescence and the emitted radiation is examined using an analyser. Information is thus obtained about molecular orientation with respect to two directions, the transmission directions of the polarizer and analyser. 

The information provided by the Raman spectrum of an oriented polymer differs from its infrared counterpart because of the fundamentally different processes involved in the generation of the spectra. In the infrared absorption process, as already noted, the absorption intensity is dependent on the angle between the electric vector and the direction of the dipole moment change. The Raman spectrum results from inelastic photon scattering details of which are determined by changes in the polarizability of the chemical bonds involved. Polarizability is a tensor quantity, which results in complications but, in principle, provides additional information. As we have seen, infi ared spectroscopy involves only one beam of polarized radiation, and the fraction of the nufotion absorbed by a molecule depends only on the orientation of the molecule with respect to the polarisation vector of the radiation. However, Raman scattering involves two beams of radiation, those of illumination and collection, and the scattered intensity depends on the orientation of the molecule with respect to the polarisation vectors of both beams, whidi may, of course, be different. This necessitates more detailed measurements in order to obtain the relevant information. 

The intensities of bands in a spectrum may also be affected due to radiation being optically polarised. In spectral characterisation nowadays, the use of polarised radiation in both infrared and Raman is extensive. When a polarised beam of radiation is incident on a molecule, the induced oscillations are in the same plane as the electric vector of the incident electromagnetic wave so the resultant emitted radiation tends to be polarised in the same plane. 

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