Vibrational Sum Frequency Spectroscopic Investigation of the Azimuthal Anisotropy and Rotational Dynamics of Methyl-Terminated Silicon(111) Surfaces

Polarization-selected vibrational sum frequency generation spectroscopy (SFG) has been used to investigate the molecular orientation of methyl groups on CH_(3)-terminated Si(111) surfaces. The symmetric and asymmetric C–H stretch modes of the surface-bound methyl group were observed by SFG. Both methyl stretches showed a pronounced azimuthal anisotropy of the 3-fold symmetry in registry with the signal from the Si(111) substrate, indicating that the propeller-like rotation of the methyl groups was hindered at room temperature. The difference in the SFG line widths for the CH_3 asymmetric stretch that was observed for different polarization combinations (SPS and PPP for SFG, visible, and IR) indicated that the rotation proceeded on a 1–2 ps time scale, as compared to the 100 fs rotational dephasing of a free methyl rotor at room temperature

Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy. Nature 474

The air-water interface is perhaps the most common liquid interface. It covers more than 70 per cent of the Earth's surface and strongly affects atmospheric, aerosol and environmental chemistry. The airwater interface has also attracted much interest as a model system that allows rigorous tests of theory, with one fundamental question being just how thin it is. Theoretical studies have suggested a surprisingly short 'healing length' of about 3 ångströms (1 Å 5 0.1 nm), with the bulk-phase properties of water recovered within the top few monolayers The vibrational spectroscopy of aqueous interfaces has progressed significantly in recent years with the development of surface-selective spectroscopic techniques The intramolecular and intermolecular vibrational coupling between the OH transition dipoles of the same molecule or between neighbours affect the spectral lineshapes of the water OH-stretch band We overcome these challenges by using the heterodyne-detected SFG technique 10 , which uses interference of the signal with a reference beam to (1) amplify the SFG signal, considerably enhancing the sensitivity, (2) make it linear with the number of chromophores, an