Motional effect on the sum-frequency vibrational spectra from methanol surface

The surface of methanol was investigated by surface-specific sum-frequency vibrational spectroscopy. The spectrum in the C-H stretch-vibration range did not show any peak from antisymmetric-stretch normal mode of the methyl group, which is clearly visible in Raman and infrared absorption spectra. Fast rotational motion of the surface methanol molecules was proposed to explain the absence of this peak, and it was shown that motional averaging effect caused the signal reduction from this specific mode when the timescale of the motion was faster than the vibrational-mode dephasing time of the terminal methyl group.