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The key objectives of this project are to understand the formation
and growth mechanisms of unsaturated hydrocarbon molecules together with
their hydrogen deficient precursors from the ‘bottom up’ in low
temperature environments and to apply these findings to better comprehend the
hydrocarbon chemistry in the atmosphere of Saturn’s moon Titan. This
presents a major unsolved chemical problem that will yield to a focused
attack given the timely combination of emerging laboratory techniques and
theory herein proposed. The implications to the hydrocarbon chemistry of
Titan's atmosphere offer extraordinary opportunities for understanding the
origin and chemical evolution of the Solar System. First, it provides the
potential to reconstruct the scene of the primordial terrestrial atmosphere
since Titan and proto-Earth are believed to have emerged with similar
atmospheres from the Solar Nebula. Secondly, our studies assist to better
understand the origin and formation of the organic, hydrocarbon-based haze
layers on Titan. The hydrocarbon molecules present in these atmospheric
layers absorb the destructive ultraviolet radiation from the Sun and act as
‘prebiotic ozone’ to preserve astrobiologically important
molecules on Titan. Hence, due to the low surface temperature of 94 K, Titan
provides us with a unique prebiotic “atmospheric laboratory”
yielding vital clues – at a frozen stage on the chemical composition of
the atmosphere of proto-Earth a few billions of years ago. |
To achieve these objectives, we will develop a tightly
integrated collaborative network spanning the full range from fundamental
studies in electronic structure theory, photochemistry, reaction dynamics,
and kinetics to applications in planetary chemistry. First, utilizing a
recently commissioned crossed beams machine, Ralf I. Kaiser (University of Hawaii at Manoa) will investigate the collision energy dependent
dynamics of reactions leading to hydrocarbon growth under single collision
conditions. These studies, focusing on ethynyl radical reactions with
unsaturated hydrocarbons, provide information on the reaction product(s),
their branching ratios, and the intermediates involved over a broad range of
collision energies from 0.5 kJmol-1 to 40 kJmol-1. The results also yield
insight into elementary mechanisms of the reactions of ethynyl radicals with
unsaturated hydrocarbons. This reaction class is strongly believed to be the key step in the formation and growth of hydrocarbon molecules in
Titan’s atmosphere. Secondly, since the hydrocarbon rich atmospheres of
planets and moons are subjected to solar ultraviolet photons,
photodissociation of unsaturated hydrocarbons drives the chemistry as well.
Arthur G. Suits (Wayne State University) will characterize wavelength-dependent photodissociation
dynamics of a variety of hydrocarbons (121–308 nm) exploiting the power
of slice ion imaging and the recently developed reflectron multimass imaging
approach. These experiments yield wavelength-dependent photodissociation
products and their branching fractions under collisionless conditions.
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Thirdly, the experiments are complemented by kinetics studies (Ian R Sims, University of Rennes, France) Here, a detailed knowledge on the reaction rate constants of
ethynyl radical reactions at the low temperature regime from 10 to 250 K is
crucial. These data are complementary to the crossed beams experiments and
reveal rate constants of bimolecular reactions of ethynyl radicals. Fourthly,
electronic structure calculations (Alexander M. Mebel, Florida International University) are imperative to extend the experiments, which can be carried
out only at a limited number of collision energies (crossed beams),
wavelengths (photodissociation dynamics), and pressures (crossed beams,
photodissociation, kinetics). These calculations deliver absolute rate
constants, photodissociation cross sections, branching ratios, and
information on the reaction intermediates and products and their
pressure-dependence over a broad range from single collision conditions up to
three-body collision regimes prevailing in atmospheres of planets and their
moons. In those systems where non-statistical effects are observed, we share
the chemically accurate potential energy surfaces with collaborator Joe Bowman (Emory University) to carry out dynamics calculations. Finally, we incorporate our
findings into reaction networks simulating the chemical evolution of
Titan’s hydrocarbon chemistry in collaboration with Yuk Yung (Caltech).
By comparing the model predictions with publicly available data from the
Cassini-Huygens mission to Titan and with astronomical observations (Alan
Tokunaga, University of Hawaii at Manoa) the models can then be refined until
an agreement between predicted and observed concentrations of hydrocarbon molecules
in Titan’s atmosphere is reached and a coherent picture of the
underlying chemistry emerges. We can apply these findings also to the
atmospheres of the giant planets in the outer Solar System.
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We will develop the following collaborative actions integrating our studies
in educational and training activities: i) bringing teaching innovation to
the departments of the research teams and preparing novel teaching material,
ii) organizing annual scientific workshops in Reaction Dynamics &
Astrochemistry, iii) encouraging researchers, graduate and undergraduate
students to do hands-on research in chemical reaction dynamics, theoretical
chemistry, and astrochemistry, iv) broadening the participation of
underrepresented minorities in research and educational activities at the
participating institutions, and v) expanding public awareness and relaying
the latest breakthroughs from our network to pupils and teachers to enable
educators to incorporate research into school and college teaching and also
to the public. These studies promise a broad payoff both scientifically and
from a perspective of human resources development. |
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