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being nearly four times greater than that for C2H2.1,4 Other
models either include only the diacetylene product2 or do not
include photolysis of vinylacetylene at all.3 Absorption cross
sections of vinylacetylene in the region of 160–260 nm were
recently measured as a function of temperature.16 These
results have been incorporated into photochemical models,
although the lowest temperatures studied were still well above
atmospheric conditions on Titan.4
(using 10.5 eV light) does not allow for detection of C2H2 due
to its high ionization potential (11.4 eV22). This deficiency will
be addressed by a theoretical study of the dissociation of
vinylacetylene under the conditions of our reaction tube. A
detailed ground state Potential Energy Surface (PES) for C4H4
has been calculated in Paper I.23 In the simulations presented
here, we have assumed that vinylacetylene, like 1,3-butadiene,
undergoes rapid internal conversion following ultraviolet ex-
citation, and that dissociation occurs subsequently on the
ground state singlet PES. A simulation of the system using
this PES will allow us to predict the importance of the
acetylene product channel.
Even under the cold conditions of the supersonic expansion,
vinylacetylene has a broad, unstructured absorption beginning
around 222 nm and increasing to the blue past the range of our
photoexcitation laser system. 1,3-Butadiene has a similar ab-
sorption spectrum, the breadth of which is explained by rapid
internal conversion to the ground state via two conical inter-
sections.14 Diacetylene, by contrast, has broadened but vibro-
nically-resolved transitions beginning around 250 nm, and the
photophysics is dominated by intersystem crossing rather than
internal conversion.17,18 Furthermore, in the case of diacetylene,
the lowest bond dissociation energy is above the wavelength
range of its major ultraviolet absorption, so even if internal
conversion occurs it does not lead to free radical products.17
Because of the similarities between the spectra of vinylacetylene
and 1,3-butadiene, it is likely that they undergo similar photo-
physical processes. However, further experimental and compu-
tational exploration of this point is warranted.
Furthermore, this theoretical approach allows us to assess
the differences between the conditions of our reaction tube,
Titan’s atmosphere, and a shock tube pyrolysis experiment.
Typical temperatures in the shock tube studies are between
1000–2500 K, while Titan’s atmosphere at an altitude of
100 km has a temperature of approximately 150 K.24 The role
of collisional cooling is also very different between the shock
tube experiments, which are often carried out near 1 bar, and
Titan’s atmosphere, which at 100 km has a pressure of
approximately 10 mbar.24 Rate constants derived at high
temperature are frequently extrapolated to the much colder
temperatures of Titan’s atmosphere because of the lack of any
other data. A recent study of the mechanisms of benzene
formation in Titan’s atmosphere demonstrated that the ben-
zene mole fraction was quite sensitive to the rate constants
used, but frequently room temperature rate constants were
used due to the lack of low temperature results.6 An under-
standing of the appropriate rate constants is crucial to the
correct modeling of Titan’s atmosphere. As will be shown
below, the conditions of the reaction tube in the present
experiment are very similar to those of Titan’s atmosphere.
The combination of experiment and theory will also serve the
larger goal of understanding the differences between high
temperature thermal dissociation and low temperature ultra-
violet photodissociation.
Although the photochemistry of other, less stable, C4H4
isomers has been studied in an argon matrix, vinylacetylene
was not the primary reactant in these studies.19,20 The only
studies of the unimolecular dissociation of vinylacetylene have
been under the high-temperature conditions of shock tube
pyrolysis. One comprehensive study identified three dissocia-
tion pathways and their initial reaction rate expressions.21 At a
typical temperature of 1500 K, those rates are
C4H4 ! C4H3 þ H 12:8 sꢁ1
! 2C2H2 184:3 sꢁ1
ð1Þ
! C4H2 þ H2 20:0 sꢁ1
Although the dilute gas mixture in the present experiment
ensures that primary products dominate, secondary reaction
products can also be identified. In particular, we are interested
in identifying novel routes to aromatic ring formation from
photolysis of small hydrocarbons. The role of even-carbon
hydrocarbons in the formation of aromatics in combustion
chemistry, and, by extension, planetary atmospheres, has long
been debated.25–27 One specific issue is the importance of the
C4H5 and C4H3 radicals relative to their odd-carbon counter-
part propargyl radical (C3H3). Here we present spectroscopic
evidence that the aromatic molecule phenylacetylene is a
product of the photochemistry of vinylacetylene under condi-
tions very similar to those of Titan’s atmosphere. We first
identify the C4H3 radical as a primary dissociation product,
then identify C6H4 and C8H6 as major secondary reaction
products. Finally, the C8H6 molecule is spectroscopically
identified as phenylacetylene.
The ratio of C2H2 to C4H2 was found by end product analysis
to be 7–10 : 1, while the rate constants of the three initiating
reactions heavily favored production of C2H2. These results
are in stark contrast to the behavior of C4H4 used in models of
Titan’s atmosphere, where C4H2 is assumed to be the major
product.1,2,4 In the shock tube study,21 the C2H2 and C4H2
products were followed in time by absorption at 230 nm and
by gas chromatography, but the radical reaction channel could
only be inferred by simulating the kinetics of the entire system.
One goal of the present work is to detect the primary
products of unimolecular decomposition of vinylacetylene
directly by mass spectrometry. Photolyzing vinylacetylene in-
side a small reaction tube outside the nozzle of a supersonic jet
pulsed-valve allows any reactions to proceed only as long as
the traversal time of the reaction tube, approximately 15 ms,
after which the mixture is expanded into the vacuum chamber.
The reaction products are identified by vacuum ultraviolet
laser ionization time-of-flight mass spectrometry. Acetylene is
anticipated to be a major product, based on the shock tube
studies.21 Unfortunately, our experimental detection method
II. Experimental methods
The supersonic jet time-of-flight mass spectrometry technique
used for the photochemistry experiments has been described
ꢀc
This journal is the Owner Societies 2006
5318 | Phys. Chem. Chem. Phys., 2006, 8, 5317–5327