ORIENTATIONAL MOLECULAR DYNAMICS AT AIR/WATER INTERFACES STUDIED WITH VIBRATIONAL SFG SPECTROSCOPY

Author Institution: Department of Chemistry, Wayne State University, Detroit, MI 48202Unraveling molecular dynamics at interfaces is important for understanding surface reactivity, cell membrane dynamics, and the formation of hydrogen bonds. Up to now details of the rotational molecular dynamics were overlooked in most vibrational SFG studies. Here we investigate molecular orientational dynamics at the air/water interface on a model system, a propiolic acid rigid rod molecule. We measured SFG signals from the C$\equiv$C triple bond stretch with various input and output polarizations. The most spectrally narrowed lineshape for SSP polarization mostly represents the vibrational dynamics, while spectrally broader SPS polarization is more sensitive to the in-plane orientational dynamics. Our model system based on propiolic acid at the air/water interface demonstrates the in-plane rotation of propiolic acid within the dephasing time scale ($\sim$1 ps). In a remarkable contrast, similar carboxylate groups in bulk water show significantly longer re-orientation times (7-18 ps), presumably due to the significant difference in H-bonding interactions at interfaces and bulk for small molecules

SPECTROSCOPIC LINE SHAPES OF BROAD BAND SUM FREQUENCY GENERATION

Author Institution: Department of Chemistry, Wayne State University, Detroit, MI 48202Using a combination of theoretical modeling and experimental measurements we show how the visible pulse shape and delay can be used to simultaneously maximize the spectral resolution and signal intensity, avoiding the trade-off between the two inherent in the broad band SFG schemes. It is found that at negative time delays (visible pulse come before IR pulse), the instantaneous non resonant background is suppressed by revealing the slow-decaying resonant contribution. Symmetric CC (triple bond) vibrational mode of propiolic acid was used as a test system. Spectra with narrow, symmetric line shapes close to the true transition line width were obtained at negative time delays without sacrificing signal intensity and resolution. Model calculations were performed in order to fit data with different time delays and extract the true line width of the particular vibrational mode. Etalon and stretcher based visible pulses were used to compare the VSFG spectral characteristics. Stretcher based visible pulses were found to produce VSFG spectra with high signal to noise ratio possibly due to the smoothness of the temporal profile of the pulse where as etalon based visible pulse has a singularity at time zero