Understanding the behavior of water at hydrophobic surfaces has been a topic of much interest for many decades. In most areas of biological, environmental, or technological relevance, the aqueous phase is not pure water, but comprises a host of ions including those associated with the acidity or basicity of the solution. The notion that ions, including hydroxide and/or hydronium, accrue at hydrophobic interfaces is increasingly invoked as a possible explanation for the behavior of water adjacent to soft hydrophobic interfaces such as liquids and monolayers. The focus of this study is on exploring the behavior of aqueous solutions of salts, acids and bases in contact with hydrocarbon and fluorocarbon self-assembled monolayers (SAMs) using vibrational sum frequency spectroscopy (VSFS). The studies take a systematic approach to understanding how each component of the SAMs' interfaces contribute to the overall observed behavior of ions and water in the overall boundary region. To achieve this, the spectroscopy of the SAM/water interface in the presence and absence of aqueous phase ions, acids and bases is compared with similar measurements taken at the substrate (SiO 2 )/water interface and the hydrophobic liquid/ water interface. The results show that the behavior of water and ions at the SAM/aqueous interface is significantly influenced by the substrate surface for both hydrocarbon and fluorocarbon SAM systems. Conditions where water and ions near a SAM interface mimic that of a liquid hydrophobic surface are identified.

Calcium fluoride is a slightly soluble compound commonly extracted from ores via flotation at elevated pH, where surfactant molecules bind with hydroxylated surface sites. The addition of F-(aq) suppresses surfactant adsorption by replacing these sites. In this paper, we look at the effects of aqueous Cl-, Br-, F-, and SO4(2-) on the water structure at the CaF2/H2O interface at a pH where surface hydroxylation has not yet occurred. Using static and time-resolved vibrational sum-frequency spectroscopy (VSFS), we find that aqueous Cl- and Br- have only electrostatic screening effects on the interface and do not perturb the interfacial water or surface structure. Sulfate, which we find to be strongly attracted to the interface, affects the interfacial water more than Cl- or Br-. This is in contrast to F- ions that directly interact with the surface and alter the water structure and bonding at the CaF2 surface in addition to screening the surface charge.

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Purcell enhanced Raman scattering (PERS) by means of a doubly resonant Fabry-Perot microcavity (mode volume ≈ 100 μm3 and finesse ≈ 30 000) has been investigated as a technique for isotopic ratio gas analysis. At the pump frequency, the resonant cavity supports a buildup of circulating power while simultaneously enabling Purcell spontaneous emission rate enhancement at the resonant Stokes frequency. The three most common isotopologues of CO2 gas were quantified, and a signal was obtained from 13C16O2 down to a partial pressure of 2 Torr. Due to its small size and low pump power needed (∼10 mW) PERS lends itself to miniaturization. Furthermore, since the cavity is resonant with the emission frequency, future improvements could allow it to serve as its own spectral analyzer and no separate spectroscopic device would be needed.

Alakai Defense Systems has recently developed a man-portable ultraviolet Raman spectrometer system. The portable Raman improvised explosives detector was designed to provide rapid, standoff detection of chemicals of interest to the end user, including, but not limited to explosives, narcotics, toxic industrial chemicals, and toxic industrial materials. In this paper, we discuss general aspects of the system design and user interface. Spectral and instrument performance data are shown for several common materials involved in narcotics manufacture, as well as cocaine and heroin, with comparisons to currently marketed handheld Raman instruments.