On the electron-induced isotope fractionation in low temperature ³²O₂/³⁶O₂ ices—ozone as a case study

The formation of six ozone isotopomers and isotopologues, 16O16O16O, 18O18O18O, 16O16O18O, 18O18O16O, 16O18O16O, and 18O16O18O, has been studied in electron-irradiated solid oxygen 16O2 and 18O2 (1 : 1) ices at 11 K. Significant isotope effects were found to exist which involved enrichment of 18O-bearing ozone molecules. The heavy 18O18O18O species is formed with a factor of about six higher than the corresponding 16O16O16O isotopologue. Likewise, the heavy 18O18O16O species is formed with abundances of a factor of three higher than the lighter 16O16O18O counterpart. No isotope effect was observed in the production of 16O18O16O versus 18O16O18O. Such studies on the formation of distinct ozone isotopomers and isotopologues involving non-thermal, non-equilibrium chemistry by irradiation of oxygen ices with high energy electrons, as present in the magnetosphere of the giant planets Jupiter and Saturn, may suggest that similar mechanisms may contribute to the 18O enrichment on the icy satellites of Jupiter and Saturn such as Ganymede, Rhea, and Dione. In such a Solar System environment, energetic particles from the magnetospheres of the giant planets may induce non-equilibrium reactions of suprathermal and/or electronically excited atoms under conditions, which are quite distinct from isotopic enrichments found in classical, thermal gas phase reactions

Functional Relationships between Kinetic, Flow, and Geometrical Parameters in a High-Temperature Chemical Microreactor.

Computational fluid dynamics (CFD) simulations and isothermal approximation were applied for the interpretation of experimental measurements of the C10H7Br pyrolysis efficiency in the high-temperature microreactor and of the pressure drop in the flow tube of the reactor. Applying isothermal approximation allows the derivation of analytical relationships between the kinetic, gas flow, and geometrical parameters of the microreactor, which, along with CFD simulations, accurately predict the experimental observations. On the basis of the obtained analytical relationships, a clear strategy for measuring rate coefficients of (pseudo) first-order bimolecular and unimolecular reactions using the microreactor was proposed. The pressure- and temperature-dependent rate coefficients for the C10H7Br pyrolysis calculated using variable reaction coordinate transition state theory were invoked to interpret the experimental data on the pyrolysis efficiency

Lightning Does Not Strike Twice: Robust MDPs with Coupled Uncertainty

We consider Markov decision processes under parameter uncertainty. Previous studies all restrict to the case that uncertainties among different states are uncoupled, which leads to conservative solutions. In contrast, we introduce an intuitive concept, termed "Lightning Does not Strike Twice," to model coupled uncertain parameters. Specifically, we require that the system can deviate from its nominal parameters only a bounded number of times. We give probabilistic guarantees indicating that this model represents real life situations and devise tractable algorithms for computing optimal control policies using this concept.Comment: ICML201

Low-temperature formation of polycyclic aromatic hydrocarbons in Titan’s atmosphere

The detection of benzene in Titan’s atmosphere led to the emergence of polycyclic aromatic hydrocarbons (PAHs) as potential nucleation agents triggering the growth of Titan’s orange-brownish haze layers. However, the fundamental mechanisms leading to the formation of PAHs in Titan’s low-temperature atmosphere have remained elusive. We provide persuasive evidence through laboratory experiments and computations that prototype PAHs like anthracene and phenanthrene (C14H10) are synthesized via barrierless reactions involving naphthyl radicals (C10H7•) with vinylacetylene (CH2=CH–C≡CH) in low-temperature environments. These elementary reactions are rapid, have no entrance barriers, and synthesize anthracene and phenanthrene via van der Waals complexes and submerged barriers. This facile route to anthracene and phenanthrene—potential building blocks to complex PAHs and aerosols in Titan—signifies a critical shift in the perception that PAHs can only be formed under high-temperature conditions, providing a detailed understanding of the chemistry of Titan’s atmosphere by untangling elementary reactions on the most fundamental level

Cosmic-ray-mediated Formation of Benzene on the Surface of Saturn's Moon Titan

The aromatic benzene molecule (C_6H_6)—a central building block of polycyclic aromatic hydrocarbon molecules—is of crucial importance for the understanding of the organic chemistry of Saturn's largest moon, Titan. Here, we show via laboratory experiments and electronic structure calculations that the benzene molecule can be formed on Titan's surface in situ via non-equilibrium chemistry by cosmic-ray processing of low-temperature acetylene (C_2H_2) ices. The actual yield of benzene depends strongly on the surface coverage. We suggest that the cosmic-ray-mediated chemistry on Titan's surface could be the dominant source of benzene, i.e., a factor of at least two orders of magnitude higher compared to previously modeled precipitation rates, in those regions of the surface which have a high surface coverage of acetylene

A Unified Mechanism on the Formation of Acenes, Helicenes, and Phenacenes in the Gas Phase.

A unified low-temperature reaction mechanism on the formation of acenes, phenacenes, and helicenes-polycyclic aromatic hydrocarbons (PAHs) that are distinct via the linear, zigzag, and ortho-condensed arrangements of fused benzene rings-is revealed. This mechanism is mediated through a barrierless, vinylacetylene mediated gas-phase chemistry utilizing tetracene, [4]phenacene, and [4]helicene as benchmarks contesting established ideas that molecular mass growth processes to PAHs transpire at elevated temperatures. This mechanism opens up an isomer-selective route to aromatic structures involving submerged reaction barriers, resonantly stabilized free-radical intermediates, and systematic ring annulation potentially yielding molecular wires along with racemic mixtures of helicenes in deep space. Connecting helicene templates to the Origins of Life ultimately changes our hypothesis on interstellar carbon chemistry

Chemical dynamics of triacetylene formation and implications to the synthesis of polyynes in Titan's atmosphere

For the last four decades, the role of polyynes such as diacetylene (HCCCCH) and triacetylene (HCCCCCCH) in the chemical evolution of the atmosphere of Saturn's moon Titan has been a subject of vigorous research. These polyacetylenes are thought to serve as an UV radiation shield in planetary environments; thus, acting as prebiotic ozone, and are considered as important constituents of the visible haze layers on Titan. However, the underlying chemical processes that initiate the formation and control the growth of polyynes have been the least understood to date. Here, we present a combined experimental, theoretical, and modeling study on the synthesis of the polyyne triacetylene (HCCCCCCH) via the bimolecular gas phase reaction of the ethynyl radical (CCH) with diacetylene (HCCCCH). This elementary reaction is rapid, has no entrance barrier, and yields the triacetylene molecule via indirect scattering dynamics through complex formation in a single collision event. Photochemical models of Titan's atmosphere imply that triacetylene may serve as a building block to synthesize even more complex polyynes such as tetraacetylene (HCCCCCCCCH)

Absorption Cross Sections of NH_3, NH_2D, NHD_2, and ND_3 in the Spectral Range 140-220 nm and Implications for Planetary Isotopic Fractionation

Cross sections for photoabsorption of NH_3, NH_2D, NHD_2, and ND_3 in the spectral region 140-220 nm were determined at ~298 K using synchrotron radiation. Absorption spectra of NH_2D and NHD_2 were deduced from spectra of mixtures of NH_3 and ND_3, of which the equilibrium concentrations for all four isotopologues obey statistical distributions. Cross sections of NH_2D, NHD_2, and ND_3 are new. Oscillator strengths, an integration of absorption cross sections over the spectral lines, for both A ← X and B ← X systems of NH_3 agree satisfactorily with previous reports; values for NH_2D, NHD_2, and ND_3 agree with quantum chemical predictions. The photolysis of NH_3 provides a major source of reactive hydrogen in the lower stratosphere and upper troposphere of giant planets such as Jupiter. Incorporating the measured photoabsorption cross sections of NH_3 and NH_2D into the Caltech/JPL photochemical diffusive model for the atmosphere of Jupiter, we find that the photolysis efficiency of NH_2D is lower than that of NH_3 by as much as 30%. The D/H ratio in NH_2D/NH_3 for tracing the microphysics in the troposphere of Jupiter is also discussed

Reactivity of the Indenyl Radical (C9 H7 ) with Acetylene (C2 H2 ) and Vinylacetylene (C4 H4 ).

The reactions of the indenyl radicals with acetylene (C2 H2 ) and vinylacetylene (C4 H4 ) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C9 H7 . ) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C6 H5 . ) and acetylene forming phenylacetylene (C6 H5 CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol-1 ) and slow, contrary to the exoergic (-38 kJ mol-1 ) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces