This kinetics experiment is designed for observing various rapid
radical-radical reactions in the gas phase. The reactor consists of a quartz
flow tube (length: 40cm, ID: 1cm) through which a mixture of reactants,
precursors, and buffer gas flows. Mass flow controllers keep pressure and flow
velocity at a defined value which allows the calculation of the concentration of
each component in the reactor. The flow tube can be either heated (300K - 900K)
or cooled (240K - 300K). Typical experimental conditions for pressure and
velocity lie in the ranges of 1 to 5 Torr and 5 to 20 m/s.
Radicals are generated either by photolysis alone or by a combination of a
plasma discharge and photolysis. For this purpose the radiation of an excimer
laser can be directed through the flow tube along its axis thus creating
uniform initial concentrations for each radical species inside the reactor. The
available output energy per pulse (pulse width = 15 ns) lies between
approximately 50 mJ for laser radiation at wavelength 193 nm and 100 mJ for
l = 248 nm. Assuming the (reasonable) absorption
coefficient for the
precursor molecules of the radicals at these wavelengths is
1x10-18 cm2,
radical concentrations of up to 1014
molecules/cm3 can be generated.
The components of the gas mixture are sampled through a pinhole (diameter: 1
mm) in the wall of the flow tube 25 cm upstream from the gas inlets. A fraction
of the effusing molecules is ionized by radiation emitted from a hollow cathode
lamp and directed by a low voltage electrode assembly through a skimmer
(diameter: 1 mm) and then enters the time-of-flight mass spectrometer (TOFMS)
which is used for analysis.
The hollow cathode lamp is a cw light source for VUV photons which can be
operated with different gases. Noble gases usually deliver the highest output
intensities and, depending on the gas used, the wavelength of the most prominent
line emitted can be chosen to be 58.4 nm (He), 74.4 nm (Ne), 106.7 nm (Ar),
116.4 nm (Kr), or 113 nm (Xe).1 By changing the wavelength it is
possible to achieve a somewhat selective sensitivity for different species
according to their ionization potential. Moreover, photoionization produces
generally less cracking than electron impact and, therefore, a spectrum which
is easier to interpret.
Due to the nature of its operation, the TOFMS can only be used in a pulsed
mode allowing inos to fly through the flight tube only in seperate "bunches." In
order to combine the TOFMS with a continuously working ion source, it is
necessary to gate the inlet to the spectrometer. This is accomplished with two
grids. The latter one is used to repel the incoming ions whereas the first grid
pushes only ions that are between the two grids across the repelling grid into
the flight tube.
After each laser pulse, a train of mass spectra is initiated that probes the
time-dependent concentrations of all species. The time resolution depends only
on the time required for one complete mass spectrum, which can be chosen to be
32 ms, 64 ms, or 128 ms. For a given observation time of e.g., 32 ms,
1000, 500, or 250 complete mass spectra are taken after a single laser pulse.
From these data, continuous temporal evolution of all species can be
reconstructed from which rate coefficients can be extracted.
Signals are acquired by counting techniques consisting either of a
discriminator/oscilloscope combination or of a special computer-based TOF board
which is capable of a 500 ps time resolution for acquisition times as long as
68s. The advantage of the computer-based TOF board is its ability to record
high data throughout, enabling a higher repetition rate than with the
oscilloscope-based version.
Based on the overall specifications of the whole apparatus, rate coefficients
as low as 1x10-14 cm3/s for radical-molecule reactions,
and 1x10-12 cm3/s
for radical-radical
reactions can be observed over the temperature range 240 - 900 K and the
pressure range 1 - 5 Torr. The advantage of using a TOFMS over other means is
its capability of:
- Monitoring all components of the sampled gas, i.e., reactants as well as
products
- Detecting molecules in low concentrations, e.g., (highly reactive)
radicals
- Tracking relatively fast reactions, i.e., reactive half-lives
of 0.4 to 15 ms for our instrument.
A schematic diagram of the apparatus identifying its major components can be
found here. N.B., the use of brand names in this
figure does not constitute an endorsement of any product(s), but merely
identifies the components of our instrument.
1 James A.R. Samson, Techniques of Vacuum Ultraviolet
Spectroscopy, Pied
Publications, Lincoln, NE, 1967.
This kinetics experiment is designed for observing various rapid
radical-radical reactions in the gas phase. The reactor consists of a quartz
flow tube (length: 40cm, ID: 1cm) through which a mixture of reactants,
precursors, and buffer gas flows. Mass flow controllers keep pressure and flow
velocity at a defined value which allows the calculation of the concentration of
each component in the reactor. The flow tube can be either heated (300K - 900K)
or cooled (240K - 300K). Typical experimental conditions for pressure and
velocity lie in the ranges of 1 to 5 Torr and 5 to 20 m/s.