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

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.