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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Isoprene photochemistry over the Amazon rainforest

Isoprene photooxidation is a major driver of atmospheric chemistry over forested regions. Isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals (ISOPOO). These radicals can react with hydroperoxyl radicals (HO2) to domi-nantly produce hydroxyhydroperoxides (ISOPOOH). They can also react with nitric oxide (NO) to largely produce methyl vinyl ketone (MVK) and methacrolein (MACR). Unimolecular isomerization and bimolecular reactions with organic peroxy radicals are also possible. There is uncertainty about the relative importance of each of these pathways in the atmosphere and possible changes because of anthropogenic pollution. Herein, measurements of ISOPOOH and MVK + MACR concentrations are reported over the central region of the Amazon basin during the wet season. The research site, downwind of an urban region, intercepted both background and polluted air masses during the GoAmazon2014/5 Experiment. Under background conditions, the confidence interval for the ratio of the ISOPOOH concentration to that of MVK + MACR spanned 0.4–0.6. This result implies a ratio of the reaction rate of ISOPOO with HO2 to that with NO of approximately unity. A value of unity is significantly smaller than simulated at present by global chemical transport models for this important, nominally low-NO, forested region of Earth. Under polluted conditions, when the concentrations of reactive nitrogen compounds were high (>1 ppb), ISOPOOH concentrations dropped below the instrumental detection limit (<60 ppt). This abrupt shift in isoprene photooxidation, sparked by human activities, speaks to ongoing and possible future changes in the photochemistry active over the Amazon rainforest.

Liu, Yingjun↗
DOE OSTI2022

Oxygen-atom transfer photochemistry of a molecular copper bromate complex

Here, we report the synthesis and oxygen-atom transfer (OAT) photochemistry of [Cu(tpa)BrO 3 ]ClO 4 . In situ spectroscopy and in crystallo experiments indicate OAT proceeds from a Cu–O fragment generated by sequential Cu–O bond cleavage and OAT from BrO x to [Cu(tpa)] + . Furthermore, these results highlight synthetic opportunities in M–O photochemistry and demonstrate the utility of in crystallo experiments to evaluating photochemical reaction mechanisms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

PHOTOCHEMISTRY IN TERRESTRIAL EXOPLANET ATMOSPHERES. I. PHOTOCHEMISTRY MODEL AND BENCHMARK CASES

We present a comprehensive photochemistry model for exploration of the chemical composition of terrestrial exoplanet atmospheres. The photochemistry model is designed from the ground up to have the capacity to treat all types of terrestrial planet atmospheres, ranging from oxidizing through reducing, which makes the code suitable for applications for the wide range of anticipated terrestrial exoplanet compositions. The one-dimensional chemical transport model treats up to 800 chemical reactions, photochemical processes, dry and wet deposition, surface emission, and thermal escape of O, H, C, N, and S bearing species, as well as formation and deposition of elemental sulfur and sulfuric acid aerosols. We validate the model by computing the atmospheric composition of current Earth and Mars and find agreement with observations of major trace gases in Earth's and Mars' atmospheres. We simulate several plausible atmospheric scenarios of terrestrial exoplanets and choose three benchmark cases for atmospheres from reducing to oxidizing. The most interesting finding is that atomic hydrogen is always a more abundant reactive radical than the hydroxyl radical in anoxic atmospheres. Whether atomic hydrogen is the most important removal path for a molecule of interest also depends on the relevant reaction rates. We also find that volcanic carbon compounds (i.e., CH{sub 4} and CO{sub 2}) are chemically long-lived and tend to be well mixed in both reducing and oxidizing atmospheres, and their dry deposition velocities to the surface control the atmospheric oxidation states. Furthermore, we revisit whether photochemically produced oxygen can cause false positives for detecting oxygenic photosynthesis, and find that in 1 bar CO{sub 2}-rich atmospheres oxygen and ozone may build up to levels that have conventionally been accepted as signatures of life, if there is no surface emission of reducing gases. The atmospheric scenarios presented in this paper can serve as the benchmark atmospheres for quickly assessing the lifetime of trace gases in reducing, weakly oxidizing, and highly oxidizing atmospheres on terrestrial exoplanets for the exploration of possible biosignature gases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

PHOTOCHEMISTRY IN TERRESTRIAL EXOPLANET ATMOSPHERES. II. H{sub 2}S AND SO{sub 2} PHOTOCHEMISTRY IN ANOXIC ATMOSPHERES

Sulfur gases are common components in the volcanic and biological emission on Earth, and are expected to be important input gases for atmospheres on terrestrial exoplanets. We study the atmospheric composition and the spectra of terrestrial exoplanets with sulfur compounds (i.e., H{sub 2}S and SO{sub 2}) emitted from their surfaces. We use a comprehensive one-dimensional photochemistry model and radiative transfer model to investigate the sulfur chemistry in atmospheres ranging from reducing to oxidizing. The most important finding is that both H{sub 2}S and SO{sub 2} are chemically short-lived in virtually all types of atmospheres on terrestrial exoplanets, based on models of H{sub 2}, N{sub 2}, and CO{sub 2} atmospheres. This implies that direct detection of surface sulfur emission is unlikely, as their surface emission rates need to be extremely high (>1000 times Earth's volcanic sulfur emission) for these gases to build up to a detectable level. We also find that sulfur compounds emitted from the surface lead to photochemical formation of elemental sulfur and sulfuric acid in the atmosphere, which would condense to form aerosols if saturated. For terrestrial exoplanets in the habitable zone of Sun-like stars or M stars, Earth-like sulfur emission rates result in optically thick haze composed of elemental sulfur in reducing H{sub 2}-dominated atmospheres for a wide range of particle diameters (0.1-1 {mu}m), which is assumed as a free parameter in our simulations. In oxidized atmospheres composed of N{sub 2} and CO{sub 2}, optically thick haze, composed of elemental sulfur aerosols (S{sub 8}) or sulfuric acid aerosols (H{sub 2}SO{sub 4}), will form if the surface sulfur emission is two orders of magnitude more than the volcanic sulfur emission of Earth. Although direct detection of H{sub 2}S and SO{sub 2} by their spectral features is unlikely, their emission might be inferred by observing aerosol-related features in reflected light with future generation space telescopes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Photochemistry in terrestrial exoplanet atmospheres. III. Photochemistry and thermochemistry in thick atmospheres on super Earths and mini Neptunes

Some super Earths and mini Neptunes will likely have thick atmospheres that are not H{sub 2}-dominated. We have developed a photochemistry-thermochemistry kinetic-transport model for exploring the compositions of thick atmospheres on super Earths and mini Neptunes, applicable for both H{sub 2}-dominated atmospheres and non-H{sub 2}-dominated atmospheres. Using this model to study thick atmospheres for wide ranges of temperatures and elemental abundances, we classify them into hydrogen-rich atmospheres, water-rich atmospheres, oxygen-rich atmospheres, and hydrocarbon-rich atmospheres. We find that carbon has to be in the form of CO{sub 2} rather than CH{sub 4} or CO in a H{sub 2}-depleted water-dominated thick atmosphere and that the preferred loss of light elements from an oxygen-poor carbon-rich atmosphere leads to the formation of unsaturated hydrocarbons (C{sub 2}H{sub 2} and C{sub 2}H{sub 4}). We apply our self-consistent atmosphere models to compute spectra and diagnostic features for known transiting low-mass exoplanets GJ 1214 b, HD 97658 b, and 55 Cnc e. For GJ 1214 b, we find that (1) C{sub 2}H{sub 2} features at 1.0 and 1.5 μm in transmission and C{sub 2}H{sub 2} and C{sub 2}H{sub 4} features at 9-14 μm in thermal emission are diagnostic for hydrocarbon-rich atmospheres; (2) a detection of water-vapor features and a confirmation of the nonexistence of methane features would provide sufficient evidence for a water-dominated atmosphere. In general, our simulations show that chemical stability has to be taken into account when interpreting the spectrum of a super Earth/mini Neptune. Water-dominated atmospheres only exist for carbon to oxygen ratios much lower than the solar ratio, suggesting that this kind of atmospheres could be rare.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Photochemistry in a dense manifold of electronic states: Photodissociation of CH{sub 2}ClBr

We report electronically nonadiabatic dynamics calculations including spin-orbit coupling for the photodissociation of CH{sub 2}ClBr to yield Cl({sup 2}P{sub 3/2}), Cl({sup 2}P{sub 1/2}), Br({sup 2}P{sub 3/2}), and Br({sup 2}P{sub 1/2}). The potential energy is a 24 Multiplication-Sign 24 matrix (divided up here into four 6 Multiplication-Sign 6 blocks in a first approximation to the problem), in a spin-coupled fully diabatic representation obtained by combining the spin-free fourfold way with single-center spin-orbit coupling constants. The spin-free calculations are carried out by multiconfiguration quasidegenerate perturbation theory, and the fully diabatic potentials including spin-orbit coupling are fit to a matrix reactive force field. The dynamics are carried out by the coherent switches with decay of mixing method in the diabatic representation. The results show qualitative agreement with experiment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Synthesis of a novel strontium-based wide-bandgap semiconductor via X-ray photochemistry under extreme conditions

In this work, the synthesis and characterization of a novel, low cost, amorphous wide-bandgap semiconductor via X-ray induced decomposition of strontium oxalate at high pressure have been demonstrated. By means of IR spectroscopy, the final product is identified as a mixture of strontium carbonate, strontium oxalate and CO-derived materials. Band gap measurements demonstrate that the final product exhibits a much lower band gap (2.45 eV) than the initial strontium oxalate powder (4.07 eV), suggesting that the synthesized material can be highly useful in electronic and optical applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Model space diabatization for quantum photochemistry

Diabatization is a procedure that transforms multiple adiabatic electronic states to a new representation in which the potential energy surfaces and the couplings between states due to the electronic Hamiltonian operator are smooth, and the couplings due to nuclear momentum are negligible. In this work, we propose a simple and general diabatization strategy, called model space diabatization, that is applicable to multi-configuration quasidegenerate perturbation theory (MC-QDPT) or its extended version (XMC-QDPT). An advantage over previous diabatization schemes is that dynamical correlation calculations are based on standard post-multi-configurational self-consistent field (MCSCF) multi-state methods even though the diabatization is based on state-averaged MCSCF results. The strategy is illustrated here by applications to LiH, LiF, and thioanisole, with the fourfold-way diabatization and XMC-QDPT, and the results illustrate its validity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

High-Resolution Mass Spectrometry and Molecular Characterization of Aqueous Photochemistry Products of Common Types of Secondary Organic Aerosols

A significant fraction of atmospheric organic compounds is predominantly found in condensed phases, such as aerosol particles and cloud droplets. Many of these compounds are photolabile and can degrade through direct photolysis or indirect photooxidation processes on time scales that are comparable to the typical lifetimes of aqueous droplets (hours) and particles (days). This paper presents a systematic investigation of the molecular level composition and the extent of aqueous photochemical processing in different types of secondary organic aerosol (SOA) from biogenic and anthropogenic precursors including α-pinene, β-pinene, β-myrcene, d- limonene, α-humulene, 1,3,5-trimethylbenzene, and guaiacol, oxidized by ozone (to simulate a remote atmosphere) or by OH in the presence of NOx (to simulate an urban atmosphere). Chamber- and flow tube-generated SOA samples were collected, extracted in a methanol/water solution, and photolyzed for 1 h under identical irradiation conditions. In these experiments, the irradiation was equivalent to about 3-8 h of exposure to the sun in its zenith. The molecular level composition of the dissolved SOA was probed before and after photolysis with direct-infusion electrospray ionization high-resolution mass spectrometry (ESI-HR-MS). The mass spectra of unphotolyzed SOA generated by ozone oxidation of monoterpenes showed qualitatively similar features, and contained largely overlapping subsets of identified compounds. The mass spectra of OH/NOx generated SOA had more unique visual appearance, and indicated a lower extent of products overlap. Furthermore, the fraction of nitrogen containing species (organonitrates and nitroaromatics) was highly sensitive to the SOA precursor. These observations suggest that attribution of high-resolution mass spectra in field SOA samples to specific SOA precursors should be more straightforward under OH/NOx oxidation conditions compared to the ozone driven oxidation. Comparison of the SOA constituents before and after photolysis showed the tendency to reduce the average number of atoms in the SOA compounds without a significant effect on the overall O/C and H/C ratios. SOA prepared by OH/NOx photooxidation of 1,3,5-trimethylbenzene and guaiacol were relatively resilient to photolysis despite being the most light-absorbing. The composition of SOA prepared by ozonolysis of monoterpenes changed more significantly as a result of the photolysis. The results indicate that aqueous photolysis of dissolved SOA compounds in cloud/fog water can occur in a number of SOA, and on atmospherically relevant time scales. However, the extent of change depends on the specific type of SOA.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2012

Toward photochemistry of integrated heterogeneous systems

This paper begins with describing the excitation mechanisms in surface photochemistry and nuclear dynamics of adsorbate induced by electronic excitation. An illustrative example is Cs adsorbate on a Cu(111) surface. This adsorption system shows drastic changes in the electronic structure with coverage; this allows us to examine different types of electronic excitations that stimulate nuclear motions of Cs. Remarks are made on challenges in photoinduced processes at well-defined surfaces: direct observations of adsorbate-substrate vibrational modes and photoinduced reactions between adsorbates. Then, the paper addresses some issues in more complex systems: metal-liquid interfaces and powdered photocatalysts of metal oxides. Photochemistry and photoinduced nuclear dynamics at metal-liquid interfaces have not been well explored. Studies on this subject may make it possible to bridge the gap between surface photochemistry and electrochemistry. Photocatalysis with powdered catalysts has been extensively studied and is still an active area, but our understanding of the mechanism of photocatalysis is far from satisfactory. Although complicated, the highly integrated systems provide an opportunity to extend our knowledge of surface photochemistry.

36 MATERIALS SCIENCE↗
DOE OSTI2012

CHEMICAL PROCESSES IN PROTOPLANETARY DISKS. II. ON THE IMPORTANCE OF PHOTOCHEMISTRY AND X-RAY IONIZATION

We investigate the impact of photochemistry and X-ray ionization on the molecular composition of, and ionization fraction in, a protoplanetary disk surrounding a typical T Tauri star. We use a sophisticated physical model, which includes a robust treatment of the radiative transfer of UV and X-ray radiation, and calculate the time-dependent chemical structure using a comprehensive chemical network. In previous work, we approximated the photochemistry and X-ray ionization; here, we recalculate the photoreaction rates using the explicit UV wavelength spectrum and wavelength-dependent reaction cross sections. We recalculate the X-ray ionization rate using our explicit elemental composition and X-ray energy spectrum. We find that photochemistry has a larger influence on the molecular composition than X-ray ionization. Observable molecules sensitive to the photorates include OH, HCO{sup +}, N{sub 2}H{sup +}, H{sub 2}O, CO{sub 2}, and CH{sub 3}OH. The only molecule significantly affected by the X-ray ionization is N{sub 2}H{sup +}, indicating that it is safe to adopt existing approximations of the X-ray ionization rate in typical T Tauri star-disk systems. The recalculation of the photorates increases the abundances of neutral molecules in the outer disk, highlighting the importance of taking into account the shape of the UV spectrum in protoplanetary disks. A recalculation of the photoreaction rates also affects the gas-phase chemistry due to the adjustment of the H/H{sub 2} and C{sup +}/C ratios. The disk ionization fraction is not significantly affected by the methods adopted to calculate the photochemistry and X-ray ionization. We determine that there is a probable 'dead zone' where accretion is suppressed, present in a layer, Z/R {approx}< 0.1-0.2, in the disk midplane, within R Almost-Equal-To 200 AU.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Photoactive High Explosives: Substituents Effects on Tetrazine Photochemistry and Photophysics

High explosives that are photoactive, i.e., can be initiated with light, offer significant advantages in reduced potential for accidental electrical initiation. In this study, we examined a series of structurally related tetrazine based photoactive high explosive materials to detail their photochemical and photophysical properties. Using photobleaching infrared absorption, we determined quantum yields of photochemistry for nanosecond pulsed excitation at 355 and 532 nm. Changes in mass spectrometry during laser irradiation in vacuum measured the evolution of gaseous products. Fluorescence spectra, quantum yields, and lifetimes were measured to observe radiative channels of energy decay that compete with photochemistry. For the 6 materials studied, quantum yields of photochemistry ranged from <10 –5 to 0.03 and quantum yield of fluorescence ranged from <10 –3 to 0.33. In all cases, the photoexcitation nonradiatively relaxed primarily to heat, appropriate for supporting photothermal initiation processes. Lastly, the photochemistry observed was dominated by ring scission of the tetrazine, but there was evidence of more extensive multistep reactions as well.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Role of Water in Methanol Photochemistry on Rutile TiO2(110)

Photochemistry of the molecularly and dissociatively adsorbed forms of methanol on the vacuum-annealed rutile TiO2(110) surface was explored using temperature programmed desorption (TPD), both with and without coadsorbed water. Methoxy, and not methanol, was confirmed as the photochemically active form of adsorbed methanol on this surface. UV irradiation of methoxy-covered TiO2(110) lead to depletion of the methoxy coverage and formation of formaldehyde and a surface OH group. Coadsorbed water did not promote either molecular methanol photochemistry or thermal decomposition of methanol to methoxy. However, terminal OH groups (OHt), prepared by coadsorption of water and oxygen atoms, thermally converted molecularly adsorbed methanol to methoxy at 120 K, thus enabling photoactivity. While chemisorbed water molecules had no influence on methoxy photochemistry, water molecules hydrogen-bonded in the second layer to bridging oxygen (Obr) sites inhibited the methoxy photodecomposition to formaldehyde. From this we conclude that Obr sites accept protons from the hole-mediated conversion of methoxy to formaldehyde. These results provide new fundamental understanding of the hole-scavenging role of methanol in photochemical processes on TiO2-based materials and how water influences this photochemistry. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle under contract DEAC05-76RL01830. The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

Environmental Molecular Sciences Laboratory↗