Structure and dynamics of ultrathin organic films

Existing and newly developed nonlinear optical methods have been used to characterize molecular structure, orientation and dynamics at interfaces. Optical second harmonic generation (SHG) studies have been conducted on molecular films of the organic dyes, rhodamine 6G and malachite green, adsorbed at the fused silica/air interface. The second harmonic response from the organic dye films was found to be a non-monotonic function of the surface concentration with discontinuities corresponding to completion of successive monolayers. This behavior reflects ordering of molecular adlayers, which extends for several layers. Polarized SHG studies in conjunction with optical absorbance and fluorescence measurements were used to determine the type of interactions responsible for the adsorption in the ordered layers. The results indicate that the well-defined order within the first layer is due to the specific adsorbate-substrate interactions. Orientation in subsequent layers is teed by adsorbate-adsorbate interactions, dependent on the nature of the adsorbate and consequently on its orientation within the first layer. These results provide a direct measure of the extent of the interfacial ordering and molecular interactions at the solid/air interface and can be used to determine an adsorbate's filling factor.; Time and frequency resolved vibrational spectra of a monolayer of amphiphilic molecules at the CaF2/air interface were investigated using infrared-visible sum frequency generation. Vibrational wave packet dynamics following coherent excitation of CH3 vibrational modes with broad bandwidth infrared pulses is reported. The induced macroscopic polarization displays quantum interference effects (quantum beats) and decays on a time scale dependent on the nature of the interfacial environment. These observations provide a link between the degree of structural order of the monolayer and the vibrational coherence lifetime. Monitoring interfacial wave packet dynamics represents a new method for characterizing these important chemical regions.; Surface-specific five wave mixing spectroscopy (5WM) was used to examine intermolecular motions in organic molecular films at the fused silica/air interface. This new surface-specific nonlinear spectroscopic technique probes elements of the fourth-order susceptibility tensor, chi(4). Four-beam 5WM experiments carried out with various polarization combinations provide the relative magnitudes of the non-vanishing elements of chi (4). Impulsively Stimulated Raman Sag (SRS) from molecular adsorbates is reported. The two-beam temporal response is modulated by low frequency inter- and intramolecular motions driven through stimulated Raglan processes. Both, in-phase and out-of-phase Raman induced quantum beats have been observed in the two-beam SRS temporal profiles. The Fourier-transform analysis of the Raman induced quantum beats yields the adsorbate Raman active mode frequencies: Possible mechanisms of surface-specific SRS and their analogy with the bulk phase, chi(3)-based spectroscopies have been discussed.