Effective Refractive Index Values and Single Scattering Albedo Implications for Dry-Generated Clays As Retrieved from Cavity Ring-Down Spectroscopy.

Despite the widespread atmospheric presence of mineral dust and clay aerosol, there is still much uncertainty associated with their contribution to climate forcing especially as related to their nonspherical shape and rough surface. This paper provides a method for retrieving effective refractive indices for a series of clay aerosols produced via dry-generation, which more closely captures natural emission mechanisms. Aerosols were generated using a wrist-action shaker, size determined via tandem differential mobility analysis and optically interrogated with a cavity ring-down spectrometer. The three specific aerosols studied are montmorillonite, Na-rich montmorillonite, and illite. The retrieved refractive indices at 532 nm, room temperature, and <2% relative humidity are m = 1.55(3) + 0.15(6) i, m = 1.60(2) + 0.00(4) i, and m = 1.27(4) + 0.31(7) i, respectively. The effective and bulk refractive index values were then used to calculate single scattering albedo, which shows the changed warming potential of montmorillonite and illite based on effective values. These results may be used to refine the inclusion of dust components in climate models.

Color Richness in Cephalopod Chromatophores Originating from High Refractive Index Biomolecules.

Cephalopods are arguably one of the most photonically sophisticated marine animals, as they can rapidly adapt their dermal color and texture to their surroundings using both structural and pigmentary coloration. Their chromatophore organs facilitate this process, but the molecular mechanism potentiating color change is not well understood. We hypothesize that the pigments, which are localized within nanostructured granules in the chromatophore, enhance the scattering of light within the dermal tissue. To test this, we extracted the phenoxazone-based pigments from the chromatophore and extrapolated their complex refractive index (RI) from experimentally determined real and approximated imaginary portions of the RI. Mie theory was used to calculate the absorbance and scattering cross sections (cm2/particle) across a broad diameter range at λ = 589 nm. We observed that the pigments were more likely to scatter attenuated light than absorb it and that these characteristics may contribute to the color richness of cephalopods.

Deliquescence behavior of internally mixed clay and salt aerosols by optical extinction measurements.

Internal mixtures of montmorillonite, a clay component of mineral dust, with sodium chloride or ammonium sulfate were studied optically using cavity ring down spectroscopy. The effects of the addition of the clay to the optically observed deliquescence relative humidity (DRH) and water uptake of these salts were considered by investigating a series of different salt mass fractions. In most cases, montmorillonite alters the hygroscopic properties, lowering the DRH in comparison to the pure salt, and causes the particles to transition from solid to liquid at a lower relative humidity than is expected based on the salt alone. Predictions based on volume-weighted mixing rules were not accurate for most measurements around the DRH. We attribute deviations from theory to changes in the Gibbs free energy of the system caused by disturbances in the ion-ion interactions and lattice structure allowing water uptake prior to the DRH of the salt. Our optical results contradict some current measurements in the literature that suggest little change in the hygroscopic behavior of salts when insoluble mineral dust components are added.

(2+1) resonance-enhanced ionization spectroscopy of a state-selected beam of OH radicals.

A state-selected beam of hydroxyl radicals is generated using a pulsed discharge source and hexapole field. The OH radicals are characterized by resonance-enhanced multiphoton ionization (REMPI) spectroscopy via the nested D 2Sigma- and 3 2Sigma- Rydberg states. Simplified spectra are observed from the selected |MJ|=3/2 component of the upper Lambda-doublet level of the lowest rotational state (J=32) in ground (v"=0) and excited (v"=1-3) vibrational levels of the OH X 2Pi3/2 state. Two-photon transitions are observed to the D 2Sigma-(v'=0-3) and 3 2Sigma-(v'=0,1) vibronic levels, extending previous studies to higher vibrational levels of the Rydberg states. Spectroscopic constants are derived for the Rydberg states and compared with prior experimental studies. Complementary first-principle theoretical studies of the properties of the D 2Sigma- and 3 2Sigma- Rydberg states [see M. P. J. van der Loo and G. C. Groenenboom, J. Chem. Phys. 123, 074310 (2005), following paper] are used to interpret the experimental findings and examine the utility of the (2+1) REMPI scheme for sensitive detection of OH radicals.

Fluorescence-dip infrared spectroscopy and predissociation dynamics of OH A 2Sigma+ (v = 4) radicals.

Fluorescence-dip infrared spectroscopy, an UV-IR double-resonance technique, is employed to characterize the line positions, linewidths, and corresponding lifetimes of highly predissociative rovibrational levels of the excited A (2)Sigma(+) electronic state of the OH radical. Various lines of the 4 <--2 overtone transition in the excited A (2)Sigma(+) state are observed, from which the rotational, centrifugal distortion, and spin-rotation constants for the A (2)Sigma(+) (v = 4) state are determined, along with the vibrational frequency for the overtone transition. Homogeneous linewidths of 0.23-0.31 cm(-1) full width at half maximum are extracted from the line profiles, demonstrating that the N = 0 to 7 rotational levels of the OH A (2)Sigma(+) (v = 4) state undergo rapid predissociation with lifetimes of < or =23 ps. The experimental linewidths are in near quantitative agreement with first-principles theoretical predictions.

Evidence for partial quenching of orbital angular momentum upon complex formation in the infrared spectrum of OH-acetylene.

The entrance channel leading to the addition reaction between the hydroxyl radical and acetylene has been examined by spectroscopic characterization of the asymmetric CH stretching band of the pi-hydrogen bonded OH-acetylene reactant complex. The infrared action spectrum observed at 3278.6 cm(-1) (origin) consists of seven peaks of various intensities and widths, and is very different from those previously reported for closed-shell HF/HCl-acetylene complexes. The unusual spectrum arises from a partial quenching of the OH orbital angular momentum in the complex, which in turn is caused by a significant splitting of the OH monomer orbital degeneracy into (2)A(') and (2)A(") electronic states. The magnitude of the (2)A(')-(2)A(") splitting as well as the A rotational constant for the OH-acetylene complex are determined from the analysis of this b-type infrared band. The most populated OH product rotational state, j(OH)=9/2, is consistent with intramolecular vibrational energy transfer to the nu2 C triple bonded C stretching mode of the departing acetylene fragment. The lifting of the OH orbital degeneracy and partial quenching of its electronic orbital angular momentum indicate that the electronic changes accompanying the evolution of reactants into products have begun to occur in the reactant complex.

Infrared spectrum and stability of a pi-type hydrogen-bonded complex between the OH and C2H2 reactants.

A hydrogen-bonded complex between the hydroxyl radical and acetylene has been stabilized in the reactant channel well leading to the addition reaction and characterized by infrared action spectroscopy in the OH overtone region. Analysis of the rotational band structure associated with the a-type transition observed at 6885.53(1) cm(-1) (origin) reveals a T-shaped structure with a 3.327(5) A separation between the centers of mass of the monomer constituents. The OH (v = 1) product states populated following vibrational predissociation show that dissociation proceeds by two mechanisms: intramolecular vibrational to rotational energy transfer and intermolecular vibrational energy transfer. The highest observed OH product state establishes an upper limit of 956 cm(-1) for the stability of the pi-type hydrogen-bonded complex. The experimental results are in good accord with the intermolecular distance and well depth at the T-shaped minimum energy configuration obtained from complementary ab initio calculations, which were carried out at the restricted coupled cluster singles, doubles, noniterative triples level of theory with extrapolation to the complete basis set limit.