Polarised radiation studies

A very useful technique which has recently been developed for Fe work, although its application is more general, is the use of polarised radiation. It is convenient to discuss the subject here, but the reader is advised to refer to the appropriate sections in later chapters for more detailed discussion of the spectra of some of the materials mentioned. Polarisation of the emitted y-ray was first shown in 1960 [36], and can take place by the Stark and Zeeman effects already familiar in optical polarisation studies [37]. 

There are two basic approaches, the simplest of which is to use a magnetically split source matrix, such as an iron foil, and to magnetise it in a fixed direction. With iron foils it is convenient to magnetise perpendicular to the direction of emission. The emitted radiation is then not monochromatic but consists of six individual components with intensity ratios 3 4 1 1 4 3, 

In the event that the Mdssbauer absorber is unsplit or is a random polycrystal, the observed spectrum shows no new features. However, if both source and absorber are directionally polarised by a magnetic field which can be internal in origin or by an electric field such as is associated with the electric field gradient tensor in a single-crystal absorber the polarisation of each emission line becomes important. 

In principle each source emission line will come into resonance with each line in the absorber, but where the emission spectrum has a high symmetry such as in iron foil there is generally a reduction in the number of noncoincident components. One important effeet in this type of experiment is a dramatic variation of line intensity with change in the relative orientation of the principal directions in the source and absorber. Calculation of the line intensities is difficult, but the detailed theory has been outlined [38, 39]. Equation 3.53 can be simplified to give an equation for the proportional amplitude probability of a source (Ml only) emission line as 

The 6 functions are those tabulated in Table 3.4 for / = 1, and (y — y ) is the relative angle between the source and absorber polarisation directions projected onto the plane perpendicular to the y-ray axis. In the event that the absorber eigenstates are not pure eigenfunctions, e.g. for an electric field gradient tensor with an asymmetry parameter, the more complex expression 

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The spectmm from an undulator is very different, and numerous peaks result from interference effects within the undulator. When the electron acceleration is confined to the orbit plane and the emission angle very low, the radiation is strongly elliptically polarised and, in the orbit plane itself, it is to within a few per cent linearly polarised. Use of a sequence of permanent magnets with magnetisation arranged in a spiral sequence enables circularly polarised radiation to be extracted from such a helical undulator and this radiation is particularly important for magnetic studies. 

Since the parallel components of the dynamic dipole are active in RAIRS, it is possible to use the azimuthal dependence to obtain the orientation of the adsorbate at the surface. A similar technique has been applied to adsorbates on metals in HREELS measurements made off specular in order to observe parallel modes through impact or resonant scattering processes. This was first demonstrated for the Rh(CO)2 molecule on anisotropic TiO2(110) surface [72]. The results of this study also allow a test of the three layer model theory (Fig.5,6) as applied to S-polarised radiation. Fig. 11 shows the FT-RAIRS spectrum for 1/3 monolayer of Rh(CO)2 on Ti02(l 10) measured with P and S polarised radiation. 

Much attention has been given to the analysis of OH stretching bands of micas occurring in an isolated region of the spectrum ranging from about 3750 to 3550 cm (Farmer 1974). Using polarized radiation, the pleochroic scheme of IR absorption bands, measured on oriented crystal sections, makes it possible to impose defined constraints on the orientation of the OH dipoles. In a fundamental polarised IR study, Tsuboi (1950) determined the position of the H atom in the structure of muscovite. The variation of absorption intensity of the OH stretching band with the direction of the electric vector of... 

The examination of the appearance or otherwise of OH absorption bands from single crystals examined in polarised radiation has also been employed in the determination of structures — as, for example, in the study of the bonded OH groups in polyvinyl alcohol [65] and in carbohydrates [46]. As in the case of free OH groups, deuteration is often of considerable assistance in identifying OH groups and in studying bonding effects, and it has been employed for this purpose by Rowen and Plyler [66], Sheppard [116], Sutherland [125] and many others. 

These characteristic OH bands can also be used to give information on the crystal structure of soUd acids. Kuratani [15] has studied the infra-red dichroism of cinnamic, adipic and other acids crystallising as needles, over the wave-lengths 3570—2850 cm", and found that the absorption is stronger when the electric vector of the polarised radiation is parallel to the long axis of the needles, from which it follows that the O—H. .. O bonds are parallel to this axis. 

Circularly polarised 190 nm radiation has been used to study the direct photoracemisation of the enantiomers of trans cyclo-octene (Inoue et al.), and new evidence has been reported for the 90° twisted intermediate on the SI surface of stilbene (Gano et al.). Intramolecular charge-transfer excitation of 4-dimethyl-amino-4 -cyanostilbene and 4-azetidinyl-4 -cyanostilbene is considered to involve at most an intermediate with a lifetime of less than 1 ps (lUchev et al.), and a novel synthetic route to 5,6-dihydro-4H-l,2-oxazines (26) from y,5-unsaturated oximes has been described by Armesto et al. The photorearrangement of the ammonium salts of the dibenzobarrelene (27) to the semibullvalene isomers has... 

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