Vector fields conditional symmetry

Symmetry considerations under TERS conditions have recently been considered as a method for obtaining molecular orientations [93, 94]. Berweger and Raschke considered using the polar phonon mode selection rules for TERS to determine nanocrystallographic information fi om solids [93]. TERS enables control of experimental parameters required to extract information from the Raman tensor such as the polarization- and k-vector-dependent field enhancement relative to the surface. The selection mles were demonstrated for both near-field TERS and far-field scatting from bulk and nanocrystalline LiNb03. 

It is important to make the distinction, and state the exact relation, between the measured observable and the actual hyperpolarizability tensor component(s), even if, for specific experimental conditions and molecular symmetries, their values turn out to be identical. What is also important is the fact that only one experimental condition is favorable for EFISHG, namely parallel polarizations for all optical and static fields. This leads to only one observable, resulting in only a single value to be deduced. It is not possible with EFISHG to determine more than one tensor component hence, one often contends either with the approximation that P. was determined, or with the statement that / , was obtained, in any of the above-mentioned relations to the individual tensor components. Even then, the assumption that the dipole moment vector and the vector part of the third-rank tensor along the molec-... 

Several different types of diffraction condition are used to characterise radiation damage. These are achieved by tilting the specimen with reference to the Kikuchi pattern. These include dynamical two-beam , bright-field kinematical and weak-beam conditions - see Jenkins and Kirk for a full description. Under dynamical two-beam conditions, small dislocation loops located close to foil surfaces exhibit black-white contrast (Fig. 9.3), and their symmetry can be used to determine the Burgers vectors and habit-planes. 

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