We report measurement of the OH librational band in nanoscopic pools of water and methanol confined within reverse micelles. The librational band, which peaks near 670 cm-1 in the bulk liquids, shifts to lower frequency as the reverse micelle size decreases. In addition, the shape of the band changes considerably as a function of decreasing size. The librational band at all compositions is well fit by a two-state model based on the relative fractions of bound and free water (or methanol) within the reverse micelles.

Binary mixtures of water with acetone, acetonitrile, and methanol over their entire range of compositions have been studied spectroscopically and by using molecular dynamics ͑MD͒ simulations. We report absorption coefficients and indices of refraction over a frequency range from 3 to 55 cm Ϫ1 , and from 400 to 1200 cm Ϫ1. The far-infrared absorption of the mixtures is substantially less than that for ideal mixtures, and Debye time constants calculated from the spectra are longer for the real than for the ideal mixtures. Significant composition dependence is observed in the high frequency librational spectra of the mixtures, and is reproduced by the MD simulations. Single dipole and angular velocity spectra are also reported, as are detailed changes in the hydrogen bonding environment in the mixtures. There is a loss of tetrahedral water structure on mixing, yet water molecules have a strong tendency to aggregate, especially in the acetone and acetonitrile mixtures. Spatial distribution functions are reported for the acetone/water system.

We report the frequency-dependent absorption coefficient and index of refraction in the far-infrared region of the spectrum for mixtures of acetonitrile and water. The mixtures do not behave ideally, and deviate from ideality most noticeably for mixtures that are between 25% and 65% acetonitrile by volume. Two implementations of the Debye model for describing the dielectric relaxation behavior of mixtures are compared, and we show that these mixtures are better treated as uniform solutions rather than as two-component systems. We find an enhanced structure in the mixtures, relative to ideal mixtures, but we do not find direct evidence for microheterogeneity. The Debye time constant for the primary relaxation process for the mixtures is up to 25% longer than that for an ideal mixture.

The first application of incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) in the near-ultraviolet for the simultaneous detection of two key atmospheric trace species, HONO and NO2, is reported. For both compounds the absorption is measured between 360 and 380 nm with a compact cavity-enhanced spectrometer employing a high power light-emitting diode. Detection limits of ∼4 ppbv for HONO and ∼14 ppbv for NO2 are reported for a static gas cell setup using a 20 s acquisition time. Based on an acquisition time of 10 min and an optical cavity length of 4.5 m detection limits of ∼0.13 ppbv and ∼0.38 ppbv were found for HONO and NO2 in a 4 m3 atmospheric simulation chamber, demonstrating the usefulness of this approach for in situ monitoring of these important species in laboratory studies or field campaigns.

We describe the application of incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) for the in situ detection of atmospheric trace gases and radicals (NO3, NO2, O3, H2O) in an atmospheric simulation chamber under realistic atmospheric conditions. The length of the optical cavity across the reaction chamber is 4.5 m, which is significantly longer than in previous studies that use high finesse optical cavities to achieve high absorption sensitivity. Using a straightforward spectrometer configuration, we show that detection limits corresponding to typical atmospheric concentrations can be achieved with a measurement time of seconds to a few minutes. In particular, with only moderate reflectivity mirrors, we report a measured sensitivity of 4 pptv to NO3 in a 1 min acquisition time. The high spatial and temporal resolution of the IBBCEAS method and its pptv sensitivity to NO3 makes it useful in laboratory studies of atmospheric processes as well as having obvious potential for field measurements.