Mary K. Gilles

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

Nanoscale chemical imaging of a working catalyst by scanning transmission X-ray microscopy

The modern chemical industry uses heterogeneous catalysts in almost every production process. They commonly consist of nanometre-size active components (typically metals or metal oxides) dispersed on a high-surface-area solid support, with performance depending on the catalysts’ nanometre-size features and on interactions involving the active components, the support and the reactant and product molecules. To gain insight into the mechanisms of heterogeneous catalysts, which could guide the design of improved or novel catalysts, it is thus necessary to have a detailed characterization of the physicochemical composition of heterogeneous catalysts in their working state at the nanometre scale. Scanning probe microscopy methods have been used to study inorganic catalyst phases at subnanometre resolution, but detailed chemical information of the materials in their working state is often difficult to obtain. By contrast, optical microspectroscopic approaches offer much flexibility for in situ chemical characterization; however, this comes at the expense of limited spatial resolution. A recent development promising high spatial resolution and chemical characterization capabilities is scanning transmission X-ray microscopy, which has been used in a proof-of-principle study to characterize a solid catalyst. Here we show that when adapting a nanoreactor specially designed for high-resolution electron microscopy, scanning transmission X-ray microscopy can be used at atmospheric pressure and up to 350 °C to monitor in situ phase changes in a complex iron-based Fisher–Tropsch catalyst and the nature and location of carbon species produced. We expect that our system, which is capable of operating up to 500 °C, will open new opportunities for nanometre-resolution imaging of a range of important chemical processes taking place on solids in gaseous or liquid environments.

Ultraviolet photoelectron spectroscopy of the phenide, benzyl and phenoxide anions, with ab initio calculations

Abstract The 351 nm photoelectron spectra of the phenide, benzyl and phenoxide anions are reported. Information obtained for phenyl radical includes an adiabatic electron affinity (EA) of 1.096(6) eV, two vibrational modes at 600(10) and 968(15) cm −1 , and an excited electronic state at ⩽ 1.7 eV. For benzyl, the radical EA is 0.912(6)eV, a vibration appears at 514(15)cm −1 , and another is possibly present at 1510(25)cm −1 . Phenoxyl radical has an EA of 2.253(6)eV, exhibits a vibration at 515(15) cm −1 , and possibly another at 1490(25) cm −1 . The first excited state of phenoxyl radical appears at 1.06(5) eV above the ground state. Ab initio calculations using GAUSSIAN 88 are used to elicit geometries, normal modes of the active vibrations, and help confirm the presence of various vibrations. Combining our results with previous measurements we find gas-phase acidities for benzene and toluene of 399(2) kcal mol −1 and 380.5(1.5) kcal mol −1 respectively, and a hydrogen bond dissociation energy for phenol of 86.5(2)kcal mol −1 .

Biogenic Potassium Salt Particles as Seeds for Secondary Organic Aerosol in the Amazon

Salty Origins of Fresh Water Cloud droplets above the Amazonian rain forest form mostly around organic aerosols, but the source of the aerosols has been a mystery. Pöhlker et al. (p. 1075) report that particles rich in potassium salts emitted by Amazonian vegetation can act as the seeds for the growth of organic aerosol particles that function as condensation nuclei for water droplets. These specks of biogenic salts provide a surface for the condensation of low- or semi-volatile organic compounds formed by the atmospheric oxidation of isoprene and terpenes, molecules produced in great abundance by many kinds of Amazonian plants. Potassium salt particles account for the previously mysterious initiation sites of aerosol growth above the Amazonian rainforest. The fine particles serving as cloud condensation nuclei in pristine Amazonian rainforest air consist mostly of secondary organic aerosol. Their origin is enigmatic, however, because new particle formation in the atmosphere is not observed. Here, we show that the growth of organic aerosol particles can be initiated by potassium-salt–rich particles emitted by biota in the rainforest. These particles act as seeds for the condensation of low- or semi-volatile organic compounds from the atmospheric gas phase or multiphase oxidation of isoprene and terpenes. Our findings suggest that the primary emission of biogenic salt particles directly influences the number concentration of cloud condensation nuclei and affects the microphysics of cloud formation and precipitation over the rainforest.

Photoelectron spectroscopy of the halogen oxide anions FO−, ClO−, BrO−, IO−, OClO−, and OIO−

The 351 nm photoelectron spectra of FO−, ClO−, BrO−, IO−, OClO−, and OIO− are reported. The spectra of the halogen monoxides display transitions to both spin–orbit states of the 2Πi ground state neutrals. Anion vibrational frequencies are observed in the spectra and bond lengths are obtained for the anions from Franck–Condon simulations. Spectra of the halogen dioxides display two active vibrational modes—the symmetric stretch and the bend. Anion symmetric stretching frequencies and normal coordinate displacements from the corresponding neutral are reported. Adiabatic electron affinities found for the halogen oxides are 2.272(6) eV (FO), 2.276(6) eV (ClO), 2.353(6) eV (BrO), 2.378(6) eV (IO), 2.140(8) eV (OClO), and 2.577(8) eV (OIO). The difference between the neutral and anion dissociation energies [D0(XO)−D0(XO−)] is reported for each of the halogen monoxides. Anion heats of formation (298 K) are also determined.

Observation of playa salts as nuclei in orographic wave clouds

During the Ice in Clouds Experiment-Layer Clouds (ICE-L), dry lakebed, or playa, salts from the Great Basin region of the United States were observed as cloud nuclei in orographic wave clouds over Wyoming. Using a counterflow virtual impactor in series with a single-particle mass spectrometer, sodium-potassium-magnesium-calcium-chloride salts were identified as residues of cloud droplets. Importantly, these salts produced similar mass spectral signatures to playa salts with elevated cloud condensation nuclei (CCN) efficiencies close to sea salt. Using a suite of chemical characterization instrumentation, the playa salts were observed to be internally mixed with oxidized organics, presumably produced by cloud processing, as well as carbonate. These salt particles were enriched as residues of large droplets (>19 μm) compared to smaller droplets (>7 μm). In addition, a small fraction of silicate-containing playa salts were hypothesized to be important in the observed heterogeneous ice nucleation processes. While the high CCN activity of sea salt has been demonstrated to play an important role in cloud formation in marine environments, this study provides direct evidence of the importance of playa salts in cloud formation in continental North America has not been shown previously. Studies are needed to model and quantify the impact of playas on climate globally, particularly because of the abundance of playas and expected increases in the frequency and intensity of dust storms in the future due to climate and land use changes.

Atmospheric fate of several alkyl nitrates Part2UV absorption cross-sectionsand photodissociation quantum yields

The UV absorption cross-sections of methyl, ethyl and isopropyl nitrate between 233 and 340 nm have been measured using a diode array spectrometer in the temperature range 240–360 K. The absorption cross-sections of these alkyl nitrates decrease with increasing wavelength and decrease with decreasing temperature for λ>280 nm. The photodissociation quantum yield for CH 3 ONO 2 to produce NO 2 and CH 3 O was found to be essentially unity at 248 nm using transient UV absorption methods. Production of O and H atoms in the photodissociation of methyl nitrate at 248 and 308 nm were found to be negligible using resonance fluorescence detection of the atoms. High quantum yields for O atoms were measured following 193 nm photolysis of methyl nitrate. The OH radical was measured to be a photoproduct with a very small quantum yield. Using the OH rate coefficients reported in the accompanying paper and the UV absorption cross-sections and the photodissociation quantum yields measured here, the first-order rate constants for atmospheric loss of methyl, ethyl and isopropyl nitrate were calculated. Photolysis was found to be the dominant atmospheric loss process for the three alkyl nitrates.

A study of the electronic structures of Pd2- and Pd2 by photoelectron spectroscopy

The ultraviolet negative ion photoelectron spectrum of Pd−2 is presented for electron binding energies up to 3.35 eV. The anion is prepared by sputtering in a flowing afterglow ion source. Multiple low‐lying electronic states of Pd2, all unidentified previously, are observed with resolved vibrational structure. The spectrum shows two strong electronic bands, each with similar vibrational progressions. Franck–Condon analyses are carried out on the two transitions and molecular constants are extracted for the anion and the two neutral electronic states. With the help of simple molecular orbital arguments and ab initio calculations, these two electronic bands are assigned as the triplet ground state (3Σ+u) and a singlet excited state (1Σ+u). The adiabatic electron affinity is E.A.(Pd2)=1.685±0.008 eV and the singlet excitation energy T0(1Σ+u) is 0.497±0.008 eV (4008±65 cm−1 ). The bonding in the palladium dimers is discussed and we find that the anion bond strength is 1.123±0.013 eV stronger than that of the...

Particle formation from pulsed laser irradiation of soot aggregates studied with a scanning mobility particle sizer, a transmission electron microscope, and a scanning transmission x-ray microscope

We investigated the physical and chemical changes induced in soot aggregates exposed to laser radiation using a scanning mobility particle sizer, a transmission electron microscope, and a scanning transmission x-ray microscope to perform near-edge x-ray absorption fine structure spectroscopy. Laser-induced nanoparticle production was observed at fluences above 0.12 J/cm(2) at 532 nm and 0.22 J/cm(2) at 1064 nm. Our results indicate that new particle formation proceeds via (1) vaporization of small carbon clusters by thermal or photolytic mechanisms, followed by homogeneous nucleation, (2) heterogeneous nucleation of vaporized carbon clusters onto material ablated from primary particles, or (3) both processes.

Correlations between optical, chemical and physical properties of biomass burn aerosols

Correlations between Optical, Chemical and Physical Properties of Biomass Burn Aerosols R. J. Hopkins, 1 K. Lewis, 2 Y. Desyaterik, 3 Z. Wang, 1,4 A. V. Tivanski, 1 W. P. Arnott, 2 A. Laskin, 3 and M. K. Gilles 1,* Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA. Department of Physics, University of Nevada, Reno, Nevada, USA. William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National College of Engineering, University of California, Berkeley, California, USA. Laboratory, Richland, Washington, USA. Abstract Aerosols generated from burning different plant fuels were characterized to determine relationships between chemical, optical and physical properties. Single scattering albedo (ω) and Angstrom absorption coefficients (α ap ) were measured using a photoacoustic technique combined with a reciprocal nephelometer. Carbon-to-oxygen atomic ratios, sp 2 hybridization, elemental composition and morphology of individual particles were measured using scanning transmission X-ray microscopy coupled with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) and scanning electron microscopy with energy dispersion of X-rays (SEM/EDX). Particles were grouped into three categories based on sp 2 hybridization and chemical composition. Measured ω (0.4 – 1.0 at 405 nm) and α ap (1.0 - 3.5) values displayed a fuel dependence. The category with sp 2 hybridization >80% had values of ω ( 0.8) and α ap (1.0 to 3.5) values, indicating increased absorption spectral selectivity.