Neutron scattering and proton spin conversion in solid CH4

Abstract

The total neutron cross section of pure and O2-doped condensed CH4 has been measured in the temperature range 0.75 < T < 100 K. The neutron wave length was sufficiently long (4.7 Å) so that changes in cross section could be directly related to changes in [Formula: see text], the mean squared proton nuclear angular momentum per molecule, to a sensitivity of about 1%. The temperature dependences of [Formula: see text] for the pure and doped specimens differ considerably in solid phase II (T < 20.4 K). For the former specimen, the change in cross section is consistent with conversion occurring between the nuclear spin symmetry species on the orientationally disordered sublattices only. The addition of oxygen enhances the rate of conversion such that the value of [Formula: see text] corresponds to conversion on both the disordered and ordered sublattices. The characteristic lifetimes of the catalyzed and uncatalyzed conversion processes have been estimated.