Orientation-selective disordering

Increase in orientation selectivity in the investigation of disordered systems... 

The characterization is based on the assumption that selected orientations for the major axes of magnetic resonance tensors exist. Another assumption is the random scattering of major axes with respect to this direction according to the Gauss law. The orientation and disordering parameters are optimised by simulation of theoretical ESR spectra and comparison with experimental spectra. The mathematical algorithm developed for the simulation program is also discussed. Results of this approach were applied to experimental data for films of copper phthalocyanine and dipivaloil methanate, obtained by sedimentation on quartz plates. 

This section gives an explanation of the different terms of the static spin Hamiltonian. The concept of orientation selection by selective m.w. excitation, which is central to many pulse EPR experiments on disordered systems, is explained. 

At X-band the EPR spectrum has a width of 90 mT ( 2 GHz), and at W-band it is 400 mT ( 11 GHz) wide. The effective excitation bandwidth of the pulse, in comparison to the EPR spectrum, thus provides orientation selection in disordered samples. This orientationally selective excitation allows the magnetic interactions, with respect to the g-matrix coordinate system, to be estimated. 

In general it is not necessary to measure all possible configurations in order to identify all the phonons. However, it should be pointed out that frequently violations of the selection rules appear or that in a particular orientation forbidden phonons appear. These are believed to be caused by disorders in the single crystal. 

The crystal structure of [(2,4,6-triphenylthiopyrylium) (Cu2l3)"] has been determined at - 120°C (82MI2). The cations are disordered over three orientations, so the accuracy of the heterocycle geometrical parameters is rather small. The heterocycle is approximately planar. The planes of the phenyl substituents in the 2, 4, and 6 positions form the dihedral angles 16.6°, 15.8°, and 15.9°, respectively, with the mean plane of the heterocycle. Selected bond lengths and bond angles are reported in Fig. 2B. 

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