Time-resolved spectroscopic technique could trace ultrafast dynamics to probe molecular couplings. Hence, using femtosecond sum frequency generation (fs-SFG) vibrational spectroscopy to study SAMs of benzyl mercaptan (BMP) or 2-phenylethanethiol (PET) on gold (Au) substrates, we observed an apparent three-segmental free-induction decay (FID) behavior of SFG signal, which can be attributed to damping of the signal contributed by free electron, electron-coupled and uncoupled phenyl C-H stretching vibrational modes. Furthermore, FIDs of thiophenol (TP) SAM on Au and trichloro(phenyl)silane (TPS) SAM on silica were also detected. The former exhibits an apparent two-segmental FID behavior with damping of only the coupled phenyl C-H stretching vibrational mode and the latter exhibits an apparent two-segmental FID behavior with damping of only the uncoupled mode, respectively. The FID data can also be explained on account of the quantum beating effect. This study demonstrates that the bridged methylene group(s) can substantially influence the coupling between the phenyl C-H stretching vibration and the surface free electron.

Molecular coupling between organic molecules and metal

255th National Meeting (2018)

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