Optical Constants of HNO3/H2O and H2SO4/HNO3/H2O at Low Temperatures in the Infrared Region

Acidic layer-enhanced nanoconfinement of anions in cylindrical pore of single-walled carbon nanotube

The adsorption of the nitrate ion by the cylindrical pore of single-walled carbon nanotubes (SWCNT) was found to be aided by an acidic adsorbed layer. Adsorbed water in the vicinity of the pore wall can supply protons through ionization, forming the acidic layer, according to Raman spectra and results of solution pH fluctuations caused by ion species adsorption. Such an acidic adsorbed layer leads to surplus adsorption of anionic species where the adsorbed amount of nitrate ions is much larger than that of cations. Also, we could observe the Raman bands being assignable to the symmetrical stretching mode at an extremely high-frequency region for nano-restricted nitrate ions compared to any other bulk phases. The abnormal band shift of adsorbed nitrate ions indicates that the nitrate ions are confined in the pore under the effects of nanoconfinement by the pore and the strong interaction with the acidic layer in the pore. Our results warn that we have to construct the adsorption model of aqueous electrolytes confined in carbon pores by deliberating the acid layer formed by the adsorbed water.

Infrared Optical Constants from Pressed Pellets of Powders: I. Improved n and k Values of (NH4)2SO4 from Single-Angle Reflectance

In combination with other parameters, the real, n(ṽ∼), and imaginary, k(ṽ∼), components of the complex refractive index, n^ = n + ik, can be used to simulate the optical properties of a material in different forms, e.g., its infrared spectra. Ultimately, such n/k values can be used to generate a database of synthetic reflectance spectra for the different morphologies to which experimental data can be compared. But obtaining reliable values of the optical constants n/k for solid materials is challenging due to the lack of optical quality specimens, usually crystals, large enough to measure. An alternative to crystals is to press the powder into a uniform disk. We have produced pellets from ammonium sulfate, (NH₄)₂SO₄, powder and derived the pellets' n and k values via single-angle reflectance using a specular reflectance device in combination with a Fourier transform infrared spectrometer. The single-angle technique measures amplitude of light reflected from the material as a function of wavelength over a wide spectral domain; the optical constants are determined from the reflectance data using the Kramers-Kronig relationship. We investigate several parameters associated with the pellets and pellet formation and their effects upon delivering the most reliable n/k values. Parameters studied include pellet diameter, mass, and density (void space), drying, grinding, sieving, and particle size in the pellet formation, as well as pressing pressure and duration. Of these parameters, using size-selected mixtures of dried, small (<50 µm) particles and pressing at ≥10 tons for at least 30 min were found key to forming highly reflective samples. Comparison of two sets of previous literature n(ṽ∼) and k(ṽ∼) values obtained from crystalline (NH₄)₂SO₄ both as crystal reflectance as well as extinction spectra of aerosols measured in a flow tube shows reasonable agreement, but suggests the present values, as confirmed from two independent techniques, represent a substantial improvement for n/k values for (NH₄)₂SO₄, also demonstrating promise to measure the optical constants of other materials.

Optical and Chemical Characterization of Uranium Dioxide (UO2) and Uraninite Mineral: Calculation of the Fundamental Optical Constants

Uranium dioxide (UO₂) is a material with historical and emerging applications in numerous areas such as photonics, nuclear energy, and aerospace electronics. While often grown synthetically as single-crystal UO₂, the mineralogical form of UO₂ called uraninite is of interest as a precursor to various chemical processes involving uranium-bearing chemicals. Here, we investigate the optical and chemical properties of a series of three UO₂ specimens: synthetic single-crystal UO₂, uraninite ore of relatively high purity, and massive uraninite mineral containing numerous impurities. An optical technique called single-angle reflectance spectroscopy was used to derive the optical constants n and k of these uranium specimens by measuring the specular reflectance spectra of a polished surface across the mid- and far-infrared spectral domains (ca. 7000-50 cm^-1). X-ray diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were further used to analyze the surface composition of the mineralogical forms of UO₂. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO₂ (as well as other metal oxides and radioactive decay products). Knowledge of the infrared optical constants for this series of uranium chemicals facilitates nondestructive, noncontact detection of UO₂ under a variety of conditions.

Single-angle reflectance spectroscopy to determine the optical constants n and k: considerations in the far-infrared domain

Single-angle infrared (IR) reflectance spectroscopy is a proven and effective method for determining the complex optical constants n and k of condensed matter. The modern method uses a Fourier transform IR spectrometer to record the quantitative reflectance R(ν) spectra followed by application of the Kramers-Kronig transform (KKT) to obtain the complex optical constants. In order to carry out the KKT, it is essential to measure the reflectance spectra to as high and low a frequency (wavenumber) as possible. Traditionally, the reflectance spectra of solid specimens consist of large (typically>10  mm diameter) polished single-crystal faces free of defects or voids. The requirement of a large polished face, however, is not a realistic expectation for many synthetic, geologic, or rare specimens where the size is usually small and the morphology can vary. In this paper we discuss several improvements and considerations to both the hardware and far-IR measurement protocols that lead to more accurate R(ν) values and thus to more accurate n/k values, especially for small (millimeter-sized) specimens where the R(ν) spectrum is concatenated from multiple independent R(ν) spectra from overlapping hardware/spectral domains. Specifically, the improved hardware and analyses introduced here include the following: (1) providing a set of far-IR calibration standards; (2) custom-designing and manufacturing low reflectivity, stray-light reducing sample masks for small specimens; (3) minimizing stray light interaction between the sample mask, the interferometer Jacquinot stop, and the detector; (4) optimizing the methods to "splice" together the spectra from independent domains; (5) discussing what methods one can use to obtain or calculate the important R(0  cm^-1) value; (6) using a quartic relationship to extrapolate from the measured R data to R(0); and (7) accounting for the limiting effects of diffraction for the spot size at the sample mask and detector for millimeter-sized specimens, especially at the very long wavelengths. These seven considerations are all highly interconnected and are discussed in turn, as well as their strong interdependencies. This paper presents a holistic approach for determining reliable n/k values of millimeter-sized samples using single-angle reflectance in the mid- and far-IR.

Facet shapes and thermo-stabilities of H₂SO₄•HNO₃ hydrates involved in polar stratospheric clouds

The nucleation, ice crystal shapes and thermodynamic stability of polar stratospheric clouds particles are interesting concerns owing to their implication in the ozone layer destruction. Some of these particles are formed by conformers of H2O, HNO3, and H2SO4. We carried out calculations using density functional theory (DFT) to obtain optimized structures. Several stable trimers are achieved -divided in two groups, one with HNO3 moiety, second with H2SO4 moiety- after pre-optimization at B3LYP/6-31G and subsequently optimization at B3LYP/aug-cc-pVTZ level of theory. For both most stable conformers five H2O molecules are added to their optimized trimers to calculate hydrated geometries. The OH stretching harmonic frequencies are provided for all aggregates. The zero-point energy correction (ZEPC), relative electronic energies (∆E), relative reaction Gibbs free energies ∆(∆G)k-relative, and cooling constant (K cooling ) are reported at three temperatures: 188 K, 195 K, and 210 K. Shapes given in our calculations are compared with various experimental shapes as well as comparisons with their thermo-stabilities.

Infrared optical constants of crystalline sodium chloride dihydrate: application to study the crystallization of aqueous sodium chloride solution droplets at low temperatures

Complex refractive indices of sodium chloride dihydrate, NaCl·2H(2)O, have been retrieved in the 6000-800 cm(-1) wavenumber regime from the infrared extinction spectra of crystallized aqueous NaCl solution droplets. The data set is valid in the temperature range from 235 to 216 K and was inferred from crystallization experiments with airborne particles performed in the large coolable aerosol and cloud chamber AIDA at the Karlsruhe Institute of Technology. The retrieval concept was based on the Kramers-Kronig relationship for a complex function of the optical constants n and k whose imaginary part is proportional to the optical depth of a small particle absorption spectrum in the Rayleigh approximation. The appropriate proportionality factor was inferred from a fitting algorithm applied to the extinction spectra of about 1 μm sized particles, which, apart from absorption, also featured a pronounced scattering contribution. NaCl·2H(2)O is the thermodynamically stable crystalline solid in the sodium chloride-water system below the peritectic at 273.3 K; above 273.3 K, the anhydrous NaCl is more stable. In contrast to anhydrous NaCl crystals, the dihydrate particles reveal prominent absorption signatures at mid-infrared wavelengths due to the hydration water molecules. Formation of NaCl·2H(2)O was only detected at temperatures clearly below the peritectic and was first evidenced in a crystallization experiment conducted at 235 K. We have employed the retrieved refractive indices of NaCl·2H(2)O to quantify the temperature dependent partitioning between anhydrous and dihydrate NaCl particles upon crystallization of aqueous NaCl solution droplets. It was found that the temperature range from 235 to 216 K represents the transition regime where the composition of the crystallized particle ensemble changes from almost only NaCl to almost only NaCl·2H(2)O particles. Compared to the findings on the NaCl/NaCl·2H(2)O partitioning from a recent study conducted with micron-sized NaCl particles deposited onto a surface, the transition regime from NaCl to NaCl·2H(2)O is shifted by about 13 K to lower temperatures in our study. This is obviously related to the different experimental conditions of the two studies. The partitioning between the two solid phases of NaCl is essential for predicting the deliquescence and ice nucleation behavior of a crystalline aerosol population which is subjected to an increasing relative humidity.

Nonlinear Vibrational Spectroscopic Studies of the Adsorption and Speciation of Nitric Acid at the Vapor/Acid Solution Interface

Nitric acid plays an important role in the heterogeneous chemistry of the atmosphere. Reactions involving HNO(3) at aqueous interfaces in the stratosphere and troposphere depend on the state of nitric acid at these surfaces. The vapor/liquid interface of HNO(3)-H2O binary solutions and HNO(3)-H(2)SO(4)-H2O ternary solutions are examined here using vibrational sum frequency spectroscopy (VSFS). Spectra of the NO2 group at different HNO(3) mole fractions and under different polarization combinations are used to develop a detailed picture of these atmospherically important systems. Consistent with surface tension and spectroscopic measurements from other laboratories, molecular nitric acid is identified at the surface of concentrated solutions. However, the data here reveal the adsorption of two different hydrogen-bonded species of undissociated HNO(3) in the interfacial region that differ in their degree of solvation of the nitro group. The adsorption of these undissociated nitric acid species is shown to be sensitive to the H2O:HNO(3) ratio as well as to the concentration of sulfuric acid.