Methane tunneling in disordered solid argon–nitrogen phases

Abstract

Methane has been matrix isolated in solid Arc(N2)1−c, 0≤c≤1. Neutron spectra and diffractograms from identical samples are reported. In addition vapor-deposited argon and its mixtures with methane and nitrogen were studied by low temperature x-ray powder diffraction. The diffractograms and spectra were interpreted in terms of five different types of sites (I–V) in the vapor-deposited matrix after deposition at temperatures of 7–25 K. Shifts and broadenings of the methane tunneling spectra are caused by these sites: (I) Methane in regular fcc substitutional sites of solid pure argon or pure nitrogen in thermodynamic equilibrium shows nearly free rotation. The effective hindrance potential in nitrogen is lower than that in argon. In earlier IR-absorption work a site splitting of the vibrations was assigned as a splitting of the J=1 level of methane. INS shows that the latter is smaller than 0.030 meV. (II) Due to the nonequilibrium conditions during vapor-deposition, stacking faults and grains with hcp symmetry are formed both in pure argon and krypton, but not in pure nitrogen. Rather sharp lines in the methane spectra at 0.6 meV neutron energy transfer are assigned to such sites. (III) Argon and nitrogen form solid mixtures with an fcc structure in the argon rich phase (c≥0.45). By the addition of nitrogen the concentration of stacking faults and grains with hcp structure is enhanced. Both the peaks at 0.9 and at 0.6 meV are seen without major shift or broadening compared to the spectra of methane in pure argon. (IV) In nitrogen rich mixtures (c≤0.45) broad features in the energy range below 1 meV are observed. They are fitted by a simple mean field model. The fit shows that replacing of nitrogen next neighbors of a methane molecule by argon atoms strongly enhances the hindrance potential in this concentration range. (V) A major amount of the argon and krypton matrices is strongly distorted and has a very small coherence length. It is speculated that distorted zones are present as small clusters or on surfaces and small angle grain boundaries. The corresponding trapping sites for methane have a very low symmetry. A broad distribution of tunnel transitions around the elastic line occurs in samples which are, to a significant amount, composed of distorted lattices.