Orientation and tunneling in monolayer methane: CHD3 physisorbed onto NaCl(100)

Abstract

Polarized infrared spectra of CHD3 adsorbed on NaCl(100) in the temperature range 5 to 33 K are reported. Doublet features are observed in the ν1 and ν4 spectral regions. Analysis of the ν1 doublet reveals one transition dipole perpendicular to the surface and another transition dipole at 70±15° from the surface normal. These features are assigned to the C–H stretching vibrations of molecules oriented in a tripod conformation with the hydrogen in the “top” position (H-up) and in a tripod with the hydrogen in one of the “base” positions (H-down), respectively. The fraction of H-up orientational isomers increases with decreasing temperature. It is shown that an energy splitting between H-up and H-down orientational isomers of 6.9±0.5 cm−1 can explain the temperature dependence. Several possible explanations of the energy splitting are explored. Tunneling between orientational isomers is shown to be the only mechanism of reorientation consistent with the observed conversion time scale.