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The goal of the Laval
nozzle project is to gain an understanding of
reactive dynamics at low temperatures (50 –
200K), as applicable to the Earth’s atmosphere,
the interstellar medium (ISM) and other
planetary atmospheres such as Titan’s (one of
Saturn’s many moons), through chemical kinetic
measurements. These measurements were
traditionally difficult to make using cryogenic
cooling methods owing to problems with
condensation at reactor walls and low reactant
vapor pressures were difficult to overcome.
However, adaptation of rocket nozzle design
technology has allowed for the creation of low
temperature equilibrated supersonic gas flows
suitable for measuring the rates of chemical
processes in well understood and characterizable
flows. The convergent/divergent Laval nozzle
shape controls the gas expansion in such a
manner that the post nozzle flow is uniform in
temperature and density for a sufficient number
of nozzle diameters to conduct kinetic
experiments. In addition an attractive feature
of these types of isentropic expansions are the
fact that the post flow conditions are solely
determined by the pre-expansion conditions and
the nozzle contour, therefore making it easy to
create a nozzle for a certain flow regime.
The pulsed
supersonic Laval nozzle flow reactor has
recently been employed in Tucson to study the
reactions of hydroxyl (OH) and imidogen (NH)
radicals with a variety of reactant species.
Many exothermic neutral-radical reactions are
now known to occur rapidly at low temperatures,
in contrast to traditional activated complex
theory, and an understanding of these reactions
lends insight into the mechanism and reaction
pathway for systems typically neglected in the
modeling of low temperature environments.
Interest in the OH + HBr system and it’s
isotopic variants is generated in it’s
applicability to Br partitioning of the Earth’s
atmosphere, creating an active species that can
further react with other species, such as in the
oxidation of ozone forming BrO + O2.
The studies have revealed the inverse
temperature dependence of the rate coefficient
below 200 K and also elucidated information
regarding temperature dependent primary and
secondary kinetic isotope effects in a
temperature window previously unexplored.
Reactions of
NH radicals with unsaturated hydrocarbons are of
interest to the planetary atmosphere of Titan,
as the main atmospheric constituents, aside from
N2 and methane are unsaturated
hydrocarbons (ethylene, acetylene, propylene,
and diacetylene to name a few) in the
approximately parts per million to billion
concentration ranges. Study of these reactions
allows for the determination of the mechanism
for reaction, H atom abstraction, versus adduct
formation, versus addition/elimination, and in
the latter two cases provides mechanistic routes
to nitrogen incorporation into hydrocarbon
containing molecules, which are currently not
well understood. One could then potentially
envision these intermediates as a subset of the
precursors leading to the rich and diverse
chemistry found to occur in Titans atmosphere.
These
experiments and others are accomplished in the
Laval flow reactor schematically pictured below,
Figure 1. The essential components are labeled,
and consist of the nozzle coupled to a
stagnation volume where radicals are created,
and the appropriate instrumentation necessary
for laser induced fluorescence (LIF) detection
of the radical species, which includes a Nd:YAG
pumped dye laser, photomultiplier tube,
amplifier, boxcar, and oscilloscope. A more
detailed picture of the actual nozzles and
stagnation housing is seen in Figure 2. The
central nozzle shown is a Mach three version
used to generate 135 K flows. The nozzles on the
left and right show the convergent and divergent
sections of the nozzle, respectively. All
nozzles built in our laboratory share a common
throat diameter of 1 cm, shown as the distance
on the calipers for reference.
Figure 1
Figure 2
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