Methane isotopomers adsorbed on KBr(100): An exploration of adsorbate orientation and rotation

Abstract

The adsorption of CH4, CH2D2, and CHD3 on the single crystal (100) surfaces of KBr in the temperature range 5–42 K was studied using polarized infrared spectroscopy. The spectra of CH4 at 42 K show two features in the ν3 C–H stretching region: the higher frequency feature with a transition dipole tilted 18° away from the surface normal and the lower frequency component at 85° from the surface normal. These data are consistent with a slightly tilted tripod for CH4 on KBr(100). With decreasing temperature a splitting occurs in the lower frequency component. A similar splitting is seen in the ν4 C–H stretch mode of CHD3 where the three features are assigned to a tilted H-up tripod and two types of tilted H-down tripods. The CH2D2 spectra are also consistent with the tripod assignment. Selection rules indicate that observation of free-rotation is allowed as vibrator transitions but none were seen. The temperature dependent splitting observed in the CH4 spectrum is consistent with hindered rotation that is quenched at low temperatures to allow detection of molecules with transition dipoles in slightly different environments.