Determination by neutron spectroscopy of the external frequencies of a CH4 molecule adsorbed in an NaA zeolite cavity

Abstract

The external vibration frequencies of a CH₄ molecule inside an NaA zeolite cavity have been measured by neutron spectroscopy. The experiments were carried out at 4, 10, 60 and 150 K. At 4 K two broad peaks were found, centred at 69 and 178 cm^–1. Comparison between experiments and calculation on the CH₄–NaA system suggests that the low-frequency peak corresponds to the external vibrations in the adsorption site. The 178 cm^–1 peak must be attributed to the libration of the trapped molecule.