SEPARATION OF THE COHERENT AND INCOHERENT SCATTERING OF C2Cl6 BY POLARIZATION ANALYSIS

In neutron scattering by C2CI6 the incoherent contribution to the quasielastic linewidth gives information on the reorientational motion of the molecules by scattering on individual atoms. The coherent scattering contains in contrast also interference effects. With polarisation analysis it is possible to distinguish between coherent and spin incoherent scattering. If it is possible to modulate the polarisation of the incoming beam one can perform high resolution TOF experiments simultaneously with polarisation analysis. On the diffractometer D7 we use 8 detectors with supermirror analysers of 1 0 x 5 cm cross-section and a supermirror polariser. A Mezei flipper supplied with a pseudostatistical pulse sequence and a second polariser generate the modulated neutron beam of one spin state. The results show that it is possible to separate the coherent and incoherent scattering peak. An analysis of the TOF data gives the relaxation times of the molecular motion. Introduction The diffuse scattering of neutrons provides important information on the dynamics of condensed matter. In the plastic phase of molecular crystals a large contribution to this scattering is elastic and quasielastic. This is due to the reorientational motion of individual molecules and to molecule-molecule correlations. Under the assumption of rigid molecules it is the incoherent scattering which is easier accessible to an analysis because we can deduce the density distribution (at t -* °°) of an individual incoherently scattering atom in the molecule from the elastic incoherent part. The quasielastic incoherent contribution on the other hand gives hints to the kind of motion of the molecule. The energy integrated incoherent scattering the scattering by a single atom at t = 0 is independent of the momentum transfer Q. The integration over elastic and quasielastic incoherent scattering only gives an intensity proportional to the Debye Waller factor. Coherent elastic contributions enter completely into the Bragg scattering as long as the orientational disorder is of a dynamical origin. From the coherent quasielastic part one gets again details of the molecular motion. The modulation in Q-space, however, is in general due to the interference terms and therefore different from Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982723 C7-152 JOURNAL DE PHYSIQUE that of the incoherent scattering. The quasielastic coherent line widths may differ from the quasielastic incoherent widths also. Most importantly the energy integrated quasielastic coherent scattering can be compared with the diffuse X-ray scattering. During the last few years a large amount of hydrogeneous organic molecules have been examined [ I ] . Due to the very large incoherent cross section of hydrogen, one gets in these substances only the incoherent scattering part. Coherent scattering in a plastic crystal has been studied only once until now on a purely coherent scatterer CBr4 [2]. In the present work we investigate the diffuse scattering in an orientationally disordered crystal for which the incoherent cross section is comparable to the coherent one. A theoretical model must describe the coherent and incoherent parts simultaneously. Because it is difficult to fit such a complex formalism to the experimental data, it appeared desirable to separate the different scattering parts by an experiment. Experimental Aspects The behaviour of the neutron spin in coherent and incoherent scattering processes is well known ancwas demonstrated for the first time experimentally by Moon, Riste and Koehler m. In Table 1 are summarized the main features for spin flip (++) and non-spin flip scattering (4.4).