Q dependence of rotational excitations of CH4 molecules in argon

Abstract

Rotational excitations of CH4 molecules, matrix isolated in solid argon, are observed with inelastic neutron scattering (INS). In the argon matrix the orientational crystalline field Vc(Ω) leads to weakly hindered rotation. Experimental data are compared with the predictions of the ‘‘Extended James–Keenan model’’ for solid CH4 II. Deviations from complete orientational disorder are studied via the Q dependence of the scattered intensities. The transition energies for CH4 in Ar differ considerably from the corresponding transition energies in solid CH4 II due to the stronger crystalline field, while the transition matrix elements seem to be rather similar. The parameters β4=7.5 and β6=−25 of the expansion of the crystalline field into octahedral rotor functions are estimated from the experimental data and compared with predictions, based on different empirical potentials between nonbonding atoms.