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The frequency-domain sum frequency generation (SFG) spectroscopy and the time-domain SFG-free induction decay technique are combined for characterization of more complex molecular surfaces under ambient conditions. Relative amplitudes change and additional transitions emerge at lower surface pressure indicating changes in molecular orientation and onset of disorder.

The effect of a positively charged dodecyl trimethylammonium bromide (DTAB) surfactant on the aqueous solvation of coumarin 314 (C314), a solvatochromic molecule, at the air/water interface was investigated. Steady-state and femtosecond time-resolved second harmonic generation (SHG) spectroscopy was employed as a surface-selective tool to characterize the solvation of C314.

A study was conducted to investigate the effect of the negatively charged carboxylate group of the stearic acid surfactant CH(sub 3)(CH(sub 2))(sub 16)COO(super -) on the solvation at the surfactant air/water interface. It was found that the static solvation energetics appears to be relatively insensitive to the ionization state of the stearic acid surfactant.

The effect of a Langmuir monolayer of fatty acid on static solvation properties as well as femtosecond solvation dynamics at aplanar air-water interface is investigated using second harmonic generation as a surface-specific probe. The results demonstrate that the stearic acid monolayer induces significant changes in both static and dynamic solvation at the air-water interface.

Wepresent a general response function formalism describing thecontribution of orientational dynamics of molecules at interfacesto spectroscopic line shapes in vibrational sum frequency generation(SFG). When reorientation occurs on the time scale comparable to vibrationaldephasing, its dynamics can be extracted from polarization-selectedSFG spectral line shapes. Unique features of orientational motionat interfaces are (1) the anisotropic case-specific equilibrium orientationaldistribution and (2) possible dynamic anisotropy (e.g., differentin-plane versus out-of-plane relaxation rates), both of which mustbe taken into account. Within the small-step rotational diffusionmodel, we present solutions for two cases, the weak-confinement model,applicable when the deviations from the isotropic case are not severe,and the wobbling-in-a-cone model, which considers a hard-wall orientingpotential. SFG line shapes are calculated for a rod-like chromophoreas a function of the rotational diffusion rate. For certain equilibriumorientational distributions, orientational dynamics may result inanomalous bi-Lorentzian line shapes (two Lorentzians of differentwidths centered and the same frequency). [ABSTRACT FROM AUTHOR]