Refractive index measurements of poly(methyl methacrylate) (PMMA) from 0.4-1.6 µm.

Using a transmission-spectrum-based method, the refractive index of a 50 µm thick sample of poly(methyl methacrylate) (PMMA) was measured as a function of wavelength. To mitigate the effects of nonplane-parallel surfaces, the sample was measured at 16 different locations. The technique resulted in the measurement of index at several thousand independent wavelengths from 0.42 to 1.62 µm, with a relative RMS accuracy <0.5×10(-4) and absolute accuracy <2×10(-4).

Thin sample refractive index by transmission spectroscopy.

Transmission spectroscopy and a small number of refractometer index measurements are combined to provide refractive index measurements of transparent samples ~50 um thick at hundreds of wavelengths with absolute accuracies <1 x 10(-4). Key to the technique is the use of independent index measurements to circumvent the need for an independent thickness measurement of the sample. The method was demonstrated on glass samples where fits to Cauchy curves had RMS accuracies <3 x 10(-5) from 415 to 1610 nm. Issues that must be addressed to reach this level of accuracy are discussed.

n-Alkane adsorption to polar silica surfaces.

The structures of medium-length n-alkane species (C(8)-C(11)) adsorbed to a hydrophilic silica/vapor interface were examined using vibrational sum frequency spectroscopy. Experiments sampling out-of-plane orientation show a clear pattern in vibrational band intensities that implies chains having primarily all-trans conformations lying flat along the interface. Further analysis shows that the methylene groups of the alkane chains have their local symmetry axes directed into and away from the surface. Spectra acquired under different polarization conditions interlock to reinforce this picture of interfacial structure and organization. Variation in signal intensities with chain length suggests that correlation between adsorbed monomers weakens with increasing chain length. This result stands in contrast with alkane behavior at neat liquid/vapor interfaces where longer length alkanes show considerably more surface induced ordering than short chain alkanes.

Ti:sapphire, broadband vibrational sum-frequency generation spectrometer with a counter-propagating geometry.

We report the development of a counter-propagating, broadband vibrational sum-frequency generation spectrometer based on a Ti:sapphire regenerative amplifier. We present simple procedures for aligning the spectrometer and for setting the timing of the IR and visible pulses. We demonstrate that the use of this geometry offers a number of important advantages over a co-propagating geometry, including a high dynamic range, reduced nonresonant background signal at buried interfaces, and minimal beam deviation upon changing samples.

Hydrogen-bonding molecular ruler surfactants as probes of specific solvation at liquid/liquid interfaces.

Resonance-enhanced, second harmonic generation (SHG) is used to measure the electronic structure of solutes sensitive to specific solvation adsorbed to liquid/liquid and liquid/solid interfaces. Here, specific solvation refers to solvent-solute interactions that are directional and localized. N-methyl-p-methoxyaniline (NMMA) is a solute whose first allowed electronic transition wavelength remains almost constant (approximately 315 nm) in non-hydrogen-bonding solvents regardless of solvent polarity. However, in hydrogen-bond-accepting solvents such as dimethylsulfoxide, NMMA's absorbance shifts to longer wavelengths (320 nm), whereas in hydrogen-bond-donating solvents (e.g., water), the absorbance shifts to shorter wavelengths (approximately 300 nm). SHG experiments show that at alkane/silica interfaces, surface silanol groups serve as moderately strong hydrogen-bond donors as evidenced by NMMA's absorbance of 307 nm. At the carbon tetrachloride/water interface, NMMA absorbance also shifts to slightly shorter wavelengths (298 nm) implying that water molecules at this liquid/liquid interface are donating strong hydrogen bonds to the adsorbed NMMA solutes. In contrast, experiments using newly developed molecular ruler surfactants with NMMA as a model hydrophobic solute and a hydrophilic, cationic headgroup imply that, as NMMA migrates across an aqueous/alkane interface, it carries with it water that functions as a hydrogen-bond-accepting partner.

Differentiating solvation mechanisms at polar solid/liquid interfaces.

Resonance enhanced second harmonic generation (SHG) has been used to identify solvation mechanisms at different solid/liquid interfaces. Solvation interactions are characterized as being either nonspecific and averaged over the entire solute cavity or specific, referring to localized, directional interactions between a solute and its surroundings. SHG spectra report the electronic structure of solutes adsorbed to silica/organic solvent interfaces, and different solutes are chosen to probe either interfacial polarity or interfacial hydrogen bond donating/accepting opportunities. SHG results show that interfacial polarity probed by p-nitroanisole depends sensitively on solvent structure, whereas hydrogen bonding interactions probed by indoline are insensitive to solvent identity and instead are dominated by the hydrogen bond donating properties of the polar silica substrate. The bulk solvation interactions were modeled with a series of ab initio calculations that characterized solute electronic structure within a dielectric continuum and in the presence of explicit, individual solvent molecules. Collectively, these measurements and calculations create a comprehensive picture of how solvation mechanisms vary at different polar, solid surfaces.

Solvation of p-nitrophenol at a water/alkane interface: the role of ionic strength and salt identity.

Second harmonic generation (SHG), a surface specific, nonlinear optical spectroscopy, was used to study the interfacial solvation of a neutral surfactant, p-nitrophenol (PNP), adsorbed to the water/cyclohexane interface in the presence of simple salts at varying salt concentrations. The purpose of this work was to determine what relationship (if any) exists between interfacial polarity and bulk solution ionic strength. Data show an apparent red shift in SHG spectra with an increase in salt anion size from fluoride to chloride to bromide at 1 M salt concentrations. A spectral red shift of the PNP electronic excitation implies an increase in local polarity. Within experimental limits, however, these observed interfacial spectral shifts mimic shifts in absorbance spectra observed for PNP in bulk electrolyte solutions. Given the similarities between bulk and surface behavior, we conclude that observed shifts in SHG spectra may be attributed to effects similar to those found in bulk solution. Additionally, the surface adsorption of PNP to the water/cyclohexane interface was studied to determine the surface distribution of PNP and the conjugate base, p-nitrophenoxide (PNP(-)), for a 10 mM PNP solution. PNP adsorption is favored over PNP(-) adsorption by a factor of 10, giving rise to an equilibrium surface distribution that is an order of magnitude greater than that found in bulk solution. These findings indicate that the amount of PNP(-) at the surface in an aqueous solution of 10 mM PNP is negligible.