Y. Finkelstein

Tilt of N2 molecules physintercalated into C24K and C24Rb

Abstract The intercalation of molecular N 2 into C 24 X ( X = K , Rb ) has been studied using the nuclear resonance photon scattering (NRPS) technique. We found that, in the physintercalated state, the nitrogen resides in the alkali planes, in a molecular form, with the molecular symmetry axis almost parallel to the graphene basal planes. In C 24 K, the N 2 molecules are only intercalated below 200 K, while in the case of C 24 Rb, the physintercalation process starts at room temperature and the amount of intercalated N 2 increases with decreasing T , reaching saturation at around 130K. These samples were also studied using n-diffraction and the0 N 2 “small” diameter, at 120K, was found to be 0.290 nm in the ( C 24 K + N 2 ) system and slightly smaller, 0.279 nm, in the ( C 24 Rb + N 2 ) system. This difference in the measured diameters could also be attributed to a smaller tilt angle of the N 2 molecule in C 24 Rb as compared to that in C 24 K.

Study of the anisotropy in the atomic momentum distributions in a Kapton film

The molecular anisotropy of polymide layers occurring in Kapton (C22H10N2O5) films was studied using neutron Compton scattering (NCS) by employing the pulsed neutron source (ISIS) at the Rutherford Appleton Laboratory (UK). The widths of atomic momentum distributions along and normal to the Kapton film surface, σp and σc of the H and C atoms were obtained together with the corresponding effective temperatures Tp and Tc and the anisotropy ratio Tp/Tc. The results are interpreted in terms of the in-plane and out-of-plane zero-point motions of the H and C atoms relative to the film surface. The data reveal an anisotropy ratio of Tp/Tc~1.1 which is far lower than that deduced when assuming a rigid planar Kapton molecule, where the ratio is ~1.7. The results are in qualitative agreement with calculations based on the AM1 semi-empirical method included in the GAUSSIAN98 package.

Study of the Papyex structure using neutron Compton scattering

Abstract The neutron Compton scattering technique (NCS) was utilized for a precise determination of the randomly oriented crystallites fraction in partially oriented graphite. This was done by measuring the effective temperatures of the C-atom along and perpendicular to the graphite planes in a sample of Papyex and comparing the results with those obtained from a highly oriented pyrolytic graphite (HOPG) sample. The half-width at half-max of the oriented fraction of crystallites (mosaic spread) was found to be (11.0±0.6)°, while the randomly oriented fraction was found to be (26.2±1.6)%.

Out-of-plane orientation of multilayer N2 films adsorbed on Grafoil at 20 K

A study of the out-of-plane tilt angle of adsorbed 15 N 2 molecules on Grafoil using the nuclear resonance photon scattering (NRPS) technique is presented. Molecular coverages between n=0.6 and 5.0 commensurate monolayers (CML) at 20 K were studied. The tilts were determined by measuring the anisotropy ratio of resonantly scattered intensities from the 6324 keV level in 15 N with the photon beam parallel and perpendicular to the Grafoil planes. The results at n≤1 CML are in good agreement with calculations based on molecular dynamic simulations (MDS). At n>1 CML, it was possible to deduce only the average tilt angle of N 2 molecules over all layers. However, by relying on the reported structural properties of N 2 multilayers obtained by n-diffraction (ND), the tilt of N 2 molecules in the various layers was deduced and compared with theoretical expectations. The measured anisotropy ratios seem to rule out the occurrence of a pinwheel (PW) structure at n<2.7 CML predicted by MDS and Monte Carlo (MC) calculations.

A practical all-metal flange-seal for high and low temperatures

A simple, general purpose, all-metal flange-seal is described, operable in the temperature range from liquid helium to around 600 K. The design is based on cheap commercial Al foils and can replace the more expensive rubber, indium O rings, and copper gaskets, commonly used in small cells for diffraction and scattering measurements of neutrons and γ rays. Such a seal can also be used for connecting high-vacuum tubes and also in high-pressure work.

Testing the lattice modes of NaCN by nuclear resonance photon scattering

A nuclear resonance photon scattering (NRPS) study of sodium cyanide (NaC15N) between 10 and 295 K is described. This temperature range covers the fully ordered phase I, the intermediate antiferroelectric phase II and the high temperature disordered cubic phase III. The scattering intensity from the 6324 keV level in 15N was measured versus temperature and a very good agreement with the calculated values has been obtained. In the calculations, one has to account for the zero-point kinetic energy of the internal vibration of the CN- ion and the lattice modes of NaCN taken from infrared (IR), Raman and vibrational density of states (VDOS). In another approach, the external modes of motion of 15N in NaCN were accounted for by using an effective Debye temperature of Θ0 = (310±5) K.

Use of n-capture γ-rays for studies in surface and solid state physics

A review of recent studies using the nuclear resonance photon scattering (NRPS) from 15N occurring in several gases is presented. In particular, new results on the adsorption and molecular orientation of N2, N2O and NO2 on graphite are given and compared with theoretical calculations using molecular dynamics simulations (MDS). In another application, we studied the scattering of photons from NO2 occurring in solid NaNO2. The results were compared with those of adsorbed NO2 on graphite. A huge difference in the scattering cross section of the two systems was observed and explained.

Second Stage Physintercalation of N2 Molecules into C24Rb

Abstract Using neutron diffraction, a second stage physintercalation of molecular N2 into C24Rb has been observed in which the N2 molecules reside inside every second Rb layer. The new phase, C48Rb2(N2)x, coexists below 175 K with pure C24Rb and the 1st stage C24Rb(N2)x. The tilt of the N2 molecular axes relative to the graphene planes was determined versus T using the nuclear resonance photon scattering (NRPS) technique and found to be almost parallel to the graphene planes of the C24Rb sample.