Ultrafast vibrational dynamics and spectroscopy of a siloxane self-assembled monolayer

Time and frequency domain sum-frequency generation (SFG) were combined to study the dynamics and structure of self-assembled monolayers (SAMs) on a fused silica surface. SFG-free induction decay (SFG-FID) of octadecylsilane SAM in the CH stretching region shows a relatively long time scale oscillation that reveals that six vibrational modes are involved in the response of the system. Five of the modes have commonly been used for the fitting of SFG spectra in the CH stretching region, namely the symmetric stretch and Fermi resonance of the methyl group, the antisymmetric stretch of the methyl, as well as the symmetric and antisymmetric stretches of the methylene group. The assignment of the sixth mode to the terminal CH(2) group was confirmed by performing a density function theory calculation. The SFG-FID measures the vibrational dephasing time (T(2)) of each of the modes, including a specific CH(2) group within the SAM, the terminal CH(2), which had never been measured before. The relatively long (∼1.3 ps) dephasing of the terminal CH(2) suggests that alkyl monolayer structure is close to that of the liquid condensed phase of Langmuir Blodgett films.