A modified molecular beam instrument for the imaging of radicals interacting with surfaces during plasma processing

A new instrument employing molecular beam techniques and laser induced fluorescence(LIF) for measuring the reactivity of gas phase radicals at the surface of a depositing film has been designed and characterized. The instrument uses an inductively coupled plasma source to create a molecular beam containing essentially all plasma species. A tunable excimer pumped dye laser is used to excite a single species in this complex molecular beam.LIF signals are imaged onto a gated, intensified charge coupled device (ICCD) to provide spatial resolution. ICCD images depict the fluorescence from molecules both in the molecular beam and scattering from the surface of a depositing film. Data collected with and without a substrate in the path of the molecular beam provide information about the surface reactivity of the species of interest. Here, we report the first measurements using the third generation imaging of radicals interacting with surfaces apparatus. We have measured the surface reactivity of SiH molecules formed in a 100% SiH_4 plasma during deposition of an amorphous hydrogenated silicon film. On a 300 K Si (100) substrate, the reactivity of SiH is near unity. The substrate temperature dependence (300–673 K) of the reactivity is also reported. In addition, reactivity measurements for OH molecules formed in a water plasma are presented. In contrast to the SiH molecule, the reactivity of OH radicals is 0.55±0.05 on the surface of a Si (100) substrate

Plasma enhanced chemical vapor deposition of SiO_2 using novel alkoxysilane precursors

This communication describes our results using these novel alkoxysilane precursors for PECVD of SiO_2 films in an inductively coupled rf plasma reactor. The effects of deposition time, rf power, and organosilane pressure on the films’ characteristics are described

Reaction of Zn^+ with NO_2. The gas-phase thermochemistry of ZnO

The homolytic bond dissociation energies of ZnO and ZnO^+ have been determined by using guided ion‐beam mass spectrometry to measure the kinetic‐energy dependence of the endothermic reactions of Zn^+ with nitrogen dioxide. The data are interpreted to yield the bond energy for ZnO, D^0_0=1.61±0.04 eV, a value considerably lower than previous experimental values, but in much better agreement with theoretical calculations. We also obtain D^0_0(ZnO^+)=1.67±0.05 eV, in good agreement with previous results. Other thermochemistry derived in this study is D^0_0(Zn^+–NO)=0.79±0.10 eV and the ionization energies, IE(ZnO)=9.34±0.02 eV and IE(NO_2)=9.57±0.04 eV

Reaction of O_2 ^+(X ^2Π_g) with H_2 , D_2 , and HD: Guided ion beam studies, MO correlations, and statistical theory calculations

Absolute cross sections are measured for the reactions of O_2 ^+(X ^2Π_g) with H_2 , D_2 , and HD from thermal energies to over 4 eV. The OH^+ + OH, HO_2^+ + H, O^+ + H_2O, and H_2O^+ + O product channels (and the corresponding isotopic analogs) are observed, although H_2^+ + O_2 is not. While the first three products appear at their thermodynamic thresholds, formation of H_2O^+ + O, the least endothermic channel, exhibits a barrier to reaction. In the HD system, the DO_2^ + product ion is strongly favored over the HO^2^+ product. Results for internally excited O_2^+ reactants, probably the a  ^4Π_u state, are also presented. Analysis of the excitation functions, molecular orbital arguments, and statistical kinetic theories are used to understand the mechanisms and dynamics of this reaction. It is shown that the inefficiency of the O^+ product channel is due to spin and symmetry constraints. The other three product channels proceed through a long‐lived intermediate, but formation of this intermediate from reactants requires surmounting a barrier measured to be 1.1±0.1 eV. The intramolecular isotope effects are shown to be due to statistical and dynamic effects

Chemical Composition of Gas- and Aerosol-Phase Products from the Photooxidation of Naphthalene

The current work focuses on the detailed evolution of the chemical composition of both the gas- and aerosol-phase constituents produced from the OH-initiated photooxidation of naphthalene under low- and high-NO_x conditions. Under high-NO_x conditions ring-opening products are the primary gas-phase products, suggesting that the mechanism involves dissociation of alkoxy radicals (RO) formed through an RO_2 + NO pathway, or a bicyclic peroxy mechanism. In contrast to the high-NO_x chemistry, ring-retaining compounds appear to dominate the low-NO_x gas-phase products owing to the RO_2 + HO_2 pathway. We are able to chemically characterize 53−68% of the secondary organic aerosol (SOA) mass. Atomic oxygen-to-carbon (O/C), hydrogen-to-carbon (H/C), and nitrogen-to-carbon (N/C) ratios measured in bulk samples by high-resolution electrospray ionization time-of-flight mass spectrometry (HR-ESI-TOFMS) are the same as the ratios observed with online high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS), suggesting that the chemical compositions and oxidation levels found in the chemically-characterized fraction of the particle phase are representative of the bulk aerosol. Oligomers, organosulfates (R-OSO_3), and other high-molecular-weight (MW) products are not observed in either the low- or high-NO_x SOA; however, in the presence of neutral ammonium sulfate seed aerosol, an organic sulfonic acid (R-SO_3), characterized as hydroxybenzene sulfonic acid, is observed in naphthalene SOA produced under both high- and low-NO_x conditions. Acidic compounds and organic peroxides are found to account for a large fraction of the chemically characterized high- and low-NO_x SOA. We propose that the major gas- and aerosol-phase products observed are generated through the formation and further reaction of 2-formylcinnamaldehyde or a bicyclic peroxy intermediate. The chemical similarity between the laboratory SOA and ambient aerosol collected from Birmingham, Alabama (AL) and Pasadena, California (CA) confirm the importance of PAH oxidation in the formation of aerosol within the urban atmosphere

Effect of NOx level on secondary organic aerosol (SOA) formation from the photooxidation of terpenes

Secondary organic aerosol (SOA) formation from the photooxidation of one monoterpene (α-pinene) and two sesquiterpenes (longifolene and aromadendrene) is investigated in the Caltech environmental chambers. The effect of NOx on SOA formation for these biogenic hydrocarbons is evaluated by performing photooxidation experiments under varying NOx conditions. The NOx dependence of α-pinene SOA formation follows the same trend as that observed previously for a number of SOA precursors, including isoprene, in which SOA yield (defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) decreases as NOx level increases. The NOx dependence of SOA yield for the sesquiterpenes, longifolene and aromadendrene, however, differs from that determined for isoprene and α-pinene; the aerosol yield under high-NOx conditions substantially exceeds that under low-NOx conditions. The reversal of the NOx dependence of SOA formation for the sesquiterpenes is consistent with formation of relatively low-volatility organic nitrates, and/or the isomerization of large alkoxy radicals leading to less volatile products. Analysis of the aerosol chemical composition for longifolene confirms the presence of organic nitrates under high-NOx conditions. Consequently the formation of SOA from certain biogenic hydrocarbons such as sesquiterpenes (and possibly large anthropogenic hydrocarbons as well) may be more efficient in polluted air

Sulfate sulfur isotopes and major ion chemistry reveal that pyrite oxidation counteracts CO₂ drawdown from silicate weathering in the Langtang-Trisuli-Narayani River system, Nepal Himalaya

Drawdown of atmospheric carbon dioxide (CO₂) due to silicate weathering in the Himalaya has previously been implicated in Cenozoic cooling. However, over timescales shorter than that of the removal of marine sulfate (SO₄²⁻), the oxidation of pyrite (FeS₂) in weathering systems can counteract the alkalinity flux of silicate weathering and modulate pCO₂. Here we present evidence from ³⁴S/³²S isotope ratios in dissolved SO₄²⁻ (δ³⁴S_(SO₄)), together with dissolved major ion concentrations, that reveals FeS₂ oxidation throughout the Langtang-Trisuli-Narayani River system of the Nepal Himalaya. River water samples were collected monthly to bimonthly throughout 2011 from 16 sites ranging from the Lirung Glacier catchment through the Narayani River floodplain. This sampling transect begins in the High Himalayan Crystalline (HHC) formation and passes through the Lesser Himalayan (LH) formation with upstream influences from the Tethyn Sedimentary Series (TSS). Average δ³⁴S_(SO₄) in the Lirung Glacier outlet is 3.6‰, increases downstream to 6.3‰ near the confluence with the Bhote Kosi, and finally declines to −2.6‰ in the lower elevation sites. Using new measurements of major ion concentrations, inversion shows 62–101% of river SO₄²⁻ is derived from the oxidation of sulfide minerals and/or organic sulfur, with the former process likely dominant. The fraction of H₂SO₄-driven weathering is seasonally variable and lower during the monsoon season, attributable to seasonal changes in the relative influence of shallow and deep flow paths with distinct residence times. Inversion results indicate that the primary control on δ³⁴S SO₄ is lithologically variable isotope composition, with the expressed δ³⁴S value for the LH and TSS formations (median values −7.0–0.0‰ in 80% of samples) lower than that in the HHC (median values −1.7–6.2‰ in 80% of samples). Overall, our analysis indicates that FeS₂ oxidation counteracts much of the alkalinity flux from silicate weathering throughout the Narayani River system such that weathering along the sampled transect exerts minimal impact on pCO₂ over timescales >5–10 kyr and <10 Myr. Moreover, reanalysis of prior datasets suggests that our findings are applicable more widely across several of the frontal Himalayan drainages

Lipid remodeling in Rhodopseudomonas palustris TIE-1 upon loss of hopanoids and hopanoid methylation

The sedimentary record of molecular fossils (biomarkers) can potentially provide important insights into the composition of ancient organisms; however, it only captures a small portion of their original lipid content. To interpret what remains, it is important to consider the potential for functional overlap between different lipids in living cells, and how the presence of one type might impact the abundance of another. Hopanoids are a diverse class of steroid analogs made by bacteria and found in soils, sediments, and sedimentary rocks. Here, we examine the trade-off between hopanoid production and that of other membrane lipids. We compare lipidomes of the metabolically versatile α-proteobacterium Rhodopseudomonas palustris TIE-1 and two hopanoid mutants, detecting native hopanoids simultaneously with other types of polar lipids by electrospray ionization mass spectrometry. In all strains, the phospholipids contain high levels of unsaturated fatty acids (often >80 %). The degree to which unsaturated fatty acids are modified to cyclopropyl fatty acids varies by phospholipid class. Deletion of the capacity for hopanoid production is accompanied by substantive changes to the lipidome, including a several-fold rise of cardiolipins. Deletion of the ability to make methylated hopanoids has a more subtle effect; however, under photoautotrophic growth conditions, tetrahymanols are upregulated twofold. Together, these results illustrate that the ‘lipid fingerprint’ produced by a micro-organism can vary depending on the growth condition or loss of single genes, reminding us that the absence of a biomarker does not necessarily imply the absence of a particular source organism