Low-temperature dynamics of confined methyl iodide

The rotational tunneling and librational excitations of the one-dimensional quantum rotor, methyl iodide, confined in porous glass monoliths (mean pore diameter of 58 Å) have been investigated using incoherent inelastic neutron scattering. When the pores are completely filled with the solid at low temperatures $(5\mathrm{}\mathrm{K}<T<\mathrm{}35\mathrm{}\mathrm{K}),$ the system exhibits a tunneling spectrum composed of two sets of inelastic peaks: one set at ±2.5 μeV and the other at ±4 μeV. Tunneling spectra measured with partially filled pores display peaks at ±4.0 μeV corresponding to tunneling of molecules near the pore walls and show that the peaks at ±2.5 μeV in the filled pores, which are similar to bulk solid peaks, can be attributed to molecules in the center of the pore. The lowest-lying librational excitation of the methyl group exhibits behavior consistent with a two-component system with a bulk solidlike librational excitation and a lower-energy librational peak due to the surface molecules. The methyl dynamics are well-described as one-dimensional rotationally hindered rotors, with the core molecules under the influence of a single hindering barrier while the surface molecules experience a distribution of barrier heights. Quantitative estimates of this distribution are presented. Finally, the temperature dependence of the tunneling spectra for both the completely filled and partially filled pores is discussed.