Michael Prager

Rotational states of the NH+4 ion in (NH4)2SnCl6 by inelastic neutron scattering

The tunnel splitting of the librational ground state of a NH+4 ion in (NH4)2 SnCl6 has been analyzed by ’’incoherent’’ inelastic neutron scattering. Ground state splittings of 2.96±0.04 and 1.51±0.03 μeV have been observed at T=6 K and are interpreted as transitions between A?T and T?E spin symmetry states, respectively. With increasing temperature the observed tunnel splitting shifts towards smaller energies but fails to show significant lifetime broadening. The first excited librational state is observed at 13.4±0.5 meV. An excitation with E?30 meV probably corresponds to the second excited librational state. Above 70 K classical reorientational motion gives rise to quasielastic scattering. From the temperature dependence of the quasielastic linewidth an activation energy EA=590±30 K is obtained. A three dimensional rotational potential is discussed in view of the above data. The temperature dependence of the tunneling states is compared with current theories.

The crystal structure of methane phase III

Methane is the simplest organic molecule, and like many supposedly simple molecular materials it has a rich phase diagram. While crystal structures could be determined for two of the solid phases, that of the low temperature phase III remained unsolved. Using high-resolution neutron powder diffraction and a direct-space Monte Carlo simulated annealing approach, this fundamental structure has now finally been solved. It is orthorhombic with space group Cmca, and 16 molecules in the unit cell. The structure is closely related to that of phase II, yet is no subgroup of it.

Rotational excitations and orientational disorder in solid mixtures of CH4 and Kr

The orientational disorder in a solid solution of methane and krypton (27% Kr) has been studied with inelastic neutron scattering. The spectra display a transition from diffusive high temperature rotations to rotational excitations of molecules with frozen‐in orientational disorder at low temperatures. Of particular interest is a relatively sharp feature at an energy transfer of 0.78 meV which both broadens and shifts with increasing temperature. A simple model is based on a statistical distribution of local potentials due to the presence of Kr atoms and the state of excitation of the molecules. Finally, the isotope effect of the rotational excitations has been investigated.

The pressure dependence of rotational states in (NH4)2 PdCl6

The pressure dependence of the ground‐state tunnel splitting and the energy of transition to the first excited librational state of the NH+4 ion in (NH4)2 PdCl6 has been measured with inelastic neutron scattering. A Gruneisen parameter for the tunneling transitions γtun=−13 has been obtained. The relation between the rotational states and the intermolecular interaction is based on a model theory of Huller and Raich. The influence of additional higher order terms in the rotational potential is discussed. An exponent n=11±2 of the r‐dependent interaction V∼r−n is derived.

SPHERES, Jülich's high-flux neutron backscattering spectrometer at FRM II

SPHERES is a third-generation neutron backscattering spectrometer, located at the 20 MW German neutron source FRM II and operated by the Jülich Centre for Neutron Science. It offers an energy resolution (fwhm) better than 0.65 μeV, a dynamic range of ±u200931 μeV, and a signal-to-noise ratio of up to 1750:1.

Rotational excitations in CH4 II: Effect of Kr impurities

The perturbation of the partial orientational order in CH4II caused by dilution with Kr(xKr≤0.20) has been studied with inelastic incoherent neutron scattering. Both, the lines assigned as rotational tunneling and almost free molecular rotation, display a characteristic asymmetric broadening which gets more pronounced with increasing Kr concentration. At low Kr concentrations (xKr≤0.05) the tunneling spectra are well explained by a model which is based on a statistical distribution of local rotational potentials. In this model only the number of Kr atoms replacing orientationally ordered CH4 nearest neighbors is relevant for the local potential at the sites of tunneling molecules. The different local symmetries are not considered in the model. It was not possible to formulate a quantitative model for the scattering from molecules at nearly free rotor sites.The perturbation of the partial orientational order in CH4II caused by dilution with Kr(xKr≤0.20) has been studied with inelastic incoherent neutron scattering. Both, the lines assigned as rotational tunneling and almost free molecular rotation, display a characteristic asymmetric broadening which gets more pronounced with increasing Kr concentration. At low Kr concentrations (xKr≤0.05) the tunneling spectra are well explained by a model which is based on a statistical distribution of local rotational potentials. In this model only the number of Kr atoms replacing orientationally ordered CH4 nearest neighbors is relevant for the local potential at the sites of tunneling molecules. The different local symmetries are not considered in the model. It was not possible to formulate a quantitative model for the scattering from molecules at nearly free rotor sites.

Rotational tunneling in phase III of CD4

High resolution inelastic neutron scattering has been used to study rotational tunneling in CD4 III. Measurements have been performed with an energy resolution of δE = 0.36 μeV and energy transfers E of up to EMAX = 36 μeV. The complex spectrum consists of at least eight lines with energies Ei?8 μeV. The data can only be explained assuming a superposition of several single molecule spectra. Four different models with up to five inequivalent sites describe the data almost equally well. These models have been used in an attempt to explain the tunneling spectra of CH4 in a CD4 matrix.

Rotational tunneling in CH4 under pressure (CH4 III)

High resolution neutron inelastic scattering techniques were used to measure tunneling spectra of methane (CH4) in its phase III. By increasing the pressure from 0.6 to 1.8 kbar the tunneling lines shift by about a factor of 1.5. The rather complicated spectra can be equally well described by two simple structural models. Both contain two inequivalent molecular sites of low symmetry. They already have provided a rather good fit of the spectrum of CD4 III. An exponent n = 8±2 of an effective intermolecular potential V∼r−n is obtained by analyzing the pressure dependence of the tunneling frequencies.

Rotational tunneling in methane (CD4): Isotope effect

Well‐defined tunneling lines in the μeV range have been observed in CD4 at T=4 K by high resolution neutron scattering. The observed change of tunneling energies by about a factor 50 upon deuteration is in good accord with a theoretical estimate. The symmetry of the neutron scattering operator implies a selection rule.

Temperature-dependent crystal structure analysis of methyl iodide by high-resolution neutron powder diffraction

High resolution neutron powder diffraction data recorded at 4.5 K confirm the orthorhombic structure of methyl iodide reported previously by Kawaguchi et al. (1973) from an X-ray single-crystal study at 193 K. The present study, using neutron powder diffraction, provides more precise structural parameters and reliable atomic co-ordinates for the hydrogen atoms enabling the methyl group libration to be analysed as a function of temperature up to 193 K.