Solomon Bililign

Laser Spectroscopy for Atmospheric and Environmental Sensing

Lasers and laser spectroscopic techniques have been extensively used in several applications since their advent, and the subject has been reviewed extensively in the last several decades. This review is focused on three areas of laser spectroscopic applications in atmospheric and environmental sensing; namely laser-induced fluorescence (LIF), cavity ring-down spectroscopy (CRDS), and photoluminescence (PL) techniques used in the detection of solids, liquids, aerosols, trace gases, and volatile organic compounds (VOCs).

Far-wing scattering studies on the reaction Li*(2p,3p)+H2→LiH(v″=1,2,J″)+H

We did laser pump–probe far-wing scattering experiments to study the photochemical reactions Li*(2p,3p)+H2→LiH(v″=1,2,J″)+H. We show in this work that the Li(2p)+H2⇒LiH+H reaction takes place when the kinetic energy of the reactants are high enough. We think that the Li(2p)+H2 collisions take place preferentially in bent near-C2v geometry and that the LiH2 2A′ intermediate plays a major role where wide-amplitude internal vibrations eventually result in losing a hydrogen atom from the LiH2 complex. We also have observed the Li(3p)+H2⇒LiH+H reaction. The rotational state distribution of the LiH product for excitation in the red wing is identical to that found in the blue wing region, it does not show any preference to high or low rotational states, and no asymmetry is observed in the reactive to nonreactive branching ratio. We show different possibilities for the reactive and nonreactive collisions and provide a tentative explanation of the reaction mechanism using the highly accurate ab initio potential en...

Nanochemistry – Chemical Reactions of Iron and Benzene Within Molecular Clusters

Molecular clusters represent a nanoscale test tube where chemical reactions can be examined in a unique way for the effects of the local environment and the possibility of size-dependent reactions. Previous experiments have shown that the ionization/dissociation of iron pentacarbonyl clusters can lead to the formation of iron ions and iron cluster ions and that these species can further react with dopant molecules to yield chemically rearranged products. The present experiments characterize similar reactions with benzene molecules and clusters. Heteroclusters of the form [Fe(CO)5]m(C6H6)nArp are created in an expanding supersonic jet of the component molecules. Following ionization by a 30 ps, 266 nm laser pulse, extensive dissociation, aggregation, and chemical rearrangement occurs leading to ionic products which are characterized by mass spectrometry. Cluster ions of the type Fem(C6H6)n+ are observed as products. The stability of the sandwich form of the ion, Fe(benzene)2+, is inferred from the data. Evidence for a similar special stability for the double-decker, Fe2(benzene)3+, is presented.

Error Analysis and Uncertainty in the Determination of Aerosol Optical Properties Using Cavity Ring-Down Spectroscopy, Integrating Nephelometry, and the Extinction-Minus-Scattering Method

Despite the substantial improvements in the measurements of aerosol physical and chemical properties and in the direct and indirect radiative effects of aerosols, there is still a need for studying the properties of aerosols under controlled laboratory conditions to develop a mechanistic and quantitative understanding of aerosol formation, chemistry, and dynamics. In this work, we present the factors that affect measurement accuracy and the resulting uncertainties of the extinction-minus-scattering method using a combination of cavity ring-down spectroscopy (CRDS) and integrating nephelometry at a wider range of optical wavelengths than previously attempted. Purely scattering polystyrene latex (PSL) spheres with diameters from 107–303 nm and absorbing polystyrene spheres (APSL) with 390 nm diameter were used to determine the consistency and agreement, within experimental uncertainties, of CRDS and nephelometer values with theoretical calculations derived from Mie theory for non-absorbing spheres. Overall uncertainties for extinction cross-section were largely 10%–11% and dominated by condensation particle counter (CPC) measurement error. Two methods for determining σext error are described, and they were found to produce equivalent results. Systematic uncertainties due to particle losses, RD cell geometry (RL), CPC counting efficiency, ring-down regression fitting, blank drift, optical tweezing, and recapturing of forward scattered light are also investigated. The random error observed in this work for absorbing spheres is comparable to previous reported measurements. For both absorbing and non-absorbing spheres, a statistical framework is developed for including the contributions to random error due to CPC measurement uncertainty, RL, statistical fluctuations in particle counts, fluctuations in the blank, and mass flow controller flow error. Copyright 2014 American Association for Aerosol Research

Airborne-mercury detection by resonant UV laser pumping.

Optical pumping of the Hg(0) (6s (1)S(0) --> 6p (3)P(1)) transition at 253.7 nm (in air) leads to extremely fast energy transfer and strong laser-induced-fluorescence (LIF) from the Hg(0) (7s(3)S(1) --> 6p (3)P(2)) green transition at 546.2 nm, which is not directly populated by the laser. Ionization occurs simultaneously and becomes particularly strong at reduced background pressures. These observations are consistent with the existence of a multiphoton process followed by electron collisional excitation. Preliminary studies are made to evaluate these phenomena for detecting elemental airborne mercury by LIF and point monitoring with an ionization detector. Measured sensitivities of 2 and 10 parts in 10(9) (ppb), respectively, at 0.1-Torr air pressure are projected to increase to 1 x 10(-4) and 1 x 10(-5) ppb after relevant system optimization.

Wintertime overnight NOx removal in a southeastern United States coal‐fired power plant plume: A model for understanding winter NOx processing and its implications

The authors would like to thank theNSF-NCAR Research Aircraft Facility staff. Data are available from NCAR at http://data.eol.ucar.edu/master_list/?project=WINTER. The model algorithm used was developed in IGOR Pro and is available at https://esrl.noaa.gov/csd/groups/csd7/measurements/2015win-ter/pubs/. Funding for Fibiger was sup-ported by NSF award 1433358

Experimental and theoretical studies of the quenching of Li(3p,4p) by N2

Quenching mechanisms of the Li3p and Li4p states in collision with the nitrogen molecule are studied by laser-induced fluorescence spectroscopy and by a quantum chemical calculation. The Li3p state is observed to be efficiently quenched to the Li3s state detected as intense 3s-->2p emission. The Li4p state is efficiently quenched to the Li4s and Li3d states detected as 4s-2p and 3d-2p emissions, respectively. The potential-energy surfaces for the Li(2s-4p)N2 states show a large number of conical intersections and avoided crossings resulting from the couplings between the ionic [Li+(N2)-] and covalent configurations. There are a large number of stable excited states, and we give here the spectroscopic constants for the lowest two stable isomers correlating to Li2p+N2.

Flight Deployment of a High‐Resolution Time‐of‐Flight Chemical Ionization Mass Spectrometer: Observations of Reactive Halogen and Nitrogen Oxide Species

We describe the University of Washington airborne high-resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS) and evaluate its performance aboard the NCAR-NSF C-130 aircraft during the recent Wintertime INvestigation of Transport, Emissions and Reactivity (WINTER) experiment in February–March of 2015. New features include (i) a computer-controlled dynamic pinhole that maintains constant mass flow-rate into the instrument independent of altitude changes to minimize variations in instrument response times; (ii) continuous addition of low flow-rate humidified ultrahigh purity nitrogen to minimize the difference in water vapor pressure, hence instrument sensitivity, between ambient and background determinations; (iii) deployment of a calibration source continuously generating isotopically labeled dinitrogen pentoxide (N2O5) for in-flight delivery; and (iv) frequent instrument background determinations to account for memory effects resulting from the interaction between sticky compounds and instrument surface following encounters with concentrated air parcels. The resulting improvements to precision and accuracy, along with the simultaneous acquisition of these species and the full set of their isotopologues, allow for more reliable identification, source attribution, and budget accounting, for example, by speciating the individual constituents of nocturnal reactive nitrogen oxides (NOz = ClNO2 + 2 × N2O5 + HNO3 + etc.). We report on an expanded set of species quantified using iodide-adduct ionization such as sulfur dioxide (SO2), hydrogen chloride (HCl), and other inorganic reactive halogen species including hypochlorous acid, nitryl chloride, chlorine, nitryl bromide, bromine, and bromine chloride (HOCl, ClNO2, Cl2, BrNO2, Br2, and BrCl, respectively).

Measurement of the Fourth O−H Overtone Absorption Cross Section in Acetic Acid Using Cavity Ring-Down Spectroscopy

We report the absolute absorption cross sections of the fourth vibrational O-H (5ν(OH)) overtone in acetic acid using cavity ring-down spectroscopy. For compounds that undergo photodissociation via overtone excitation, such intensity information is required to calculate atmospheric photolysis rates. The fourth vibrational overtone of acetic acid is insufficiently energetic to effect dissociation, but measurement of its cross section provides a model for other overtone transitions that can affect atmospheric photochemistry. Though gas-phase acetic acid exists in equilibrium with its dimer, this work shows that only the monomeric species contributes to the acetic acid overtone spectrum. The absorption of acetic acid monomer peaks at ∼615 nm and has a peak cross section of 1.84 × 10(-24) cm(2)·molecule(-1). Between 612 and 620 nm, the integrated cross section for the acetic acid monomer is (5.23 ± 0.73) × 10(-24) cm(2)·nm·molecule(-1) or (1.38 ± 0.19) × 10(-22) cm(2)·molecule(-1)·cm(-1). This is commensurate with the integrated cross section values for the fourth O-H overtone of other species. Theoretical calculations show that there is sufficient energy for hydrogen to transition between the two oxygen atoms, which results in an overtone-induced conformational change.

Energy transfer in Li(4p)+(Ar,H2,CH4) collisions

The direct collisional energy transfer processes of the excited states of Li(4p) by several gases are investigated under gas cell conditions. The nonreactive absorption profiles of the collision complex are monitored as a function of laser detuning from the Li(2s-4p) resonances. Pronounced structures in the absorption spectra along with high level ab initio calculations of the relevant potential energy surfaces are used to understand the experimental results.