D. E. Gragson

Tunable picosecond infrared laser system based on parametric amplification in KTP with a Ti:sapphire amplifier

A picosecond laser system that will generate high-power tunable IR pulses with bandwidths suitable for spectroscopic applications is discussed. The system is based on white-light continuum generation in ethylene glycol and optical parametric amplification in potassium titanyl phosphate. The nonlinear-optical processes are driven by a regeneratively amplified Ti:sapphire laser that produces 1.7-ps pulses at a repetition rate of 1 kHz. Energies as high as 40 and 12 microJ have been achieved over the signal (1.02-1.16-microm) and idler (2.6-3.7-microm) tuning ranges, respectively. The IR beam temporal and spatial characteristics are also presented.

Induced Changes in Solvent Structure by Phospholipid Monolayer Formation at a Liquid–Liquid Interface

Abstract Vibrational sum frequency spectroscopy has been used in conjunction with dynamic surface tension measurements to study formation of a 1,2-dilauroyl-sn-phosphatidylcholine (DLPC) monolayer at a water–carbon tetrachloride interface. Surface tension measurements show that an aqueous solution of liquid crystalline phosphocholine vesicles (4.5 μM DLPC) requires several hours to form a tightly packed, fully equilibrated monolayer of DLPC monomers. Vibrational spectra of the interfacial region at different stages in the monolayer formation process indicate that the solvent structure undergoes dramatic re-organization as the monolayer forms. Initial adsorption of DLPC monomers severely disrupts the interfacial hydrogen bonding. Intensity in the OH stretching region oscillates in a systematic fashion during the first 2 h of monolayer formation before finally settling to a level characteristic of the fully equilibrated monolayer. Frequency shifts of the OH stretching vibration show that water molecules with their C2 axes aligned parallel to the interface experience a markedly different environment than those water molecules aligned perpendicular to the interface. This difference is attributed to the effect of the adsorbed, zwitterionic DLPC head-groups which, if aligned parallel to the interface, can stabilize in-plane water molecules.

Vibrational sum frequency spectroscopy of surfactants and phospholipid monolayers at liquid-liquid interfaces

Work from our laboratory on vibrational sum frequency spectroscopic investigations of molecular ordering at the carbon tetrachloride-water interface is reviewed. Simple charged surfactants adsorbed at the liquid-liquid interface are seen to induce alignment of interfacial water molecules to a degree which is dependent on the induced surface potential. Saturation of water molecule alignment occurs at a surfactant surface concentration corresponding to a calculated surface potential of approximately 160 mV. In complementary studies, the relative degree of hydrocarbon chain ordering within monolayers of symmetric phosphatidylcholines of different chain lengths is inferred by the relative signal contributions of the methyl and methylene symmetric stretch modes. The degree of hydrocarbon chain disorder observed depends strongly on the method of monolayer preparation. By one method, a decrease in hydrocarbon chain order is seen with increasing chain length. Another method of monolayer formation yielded very well ordered hydrocarbon chains for the longest chain phosphatidylcholine studied, and showed much greater disorder in shorter chain species which was comparable to the other preparation method. These studies are a foundation for further work with this technique geared towards understanding molecular-level structural features in membrane-like assemblies and surface biochemical interactions of relevance to biomedical research.