We are interested in probing the spectroscopy and structure of molecules
such as: Short lived species like C3O, NCN, vinylamine etc. of astrophysical interest,
planetary interest and structural interest. CFC's, HFC's, HCFC's and other molecules of atmospheric
interest. Small species used as models for biological systems such as formamide. Larger H-bonded
species of biological and chemical interest. We have developed a number of methods for generation
of transients, typically flow pyrolysis, photolysis and IR laser powered pyrolysis (IRLPP) and these
have been coupled with our Bruker HR120 FTIR system. Vibration-rotation spectra of molecules with
lifetimes ranging from NCN (t ½ » 10-3 sec) through C3O (t½ » 1
sec), propadienone (t½ » 10 sec) and vinylamine(t½ » 10
min) to difluoroacetylene (t½ » 30 min) and chlorophosphaethyne (t½ » 30
min) have been recorded.
a typical flow through experimental system based on a White Cell
Spectra from a flow experiment generating C3O. The bottom spectra shows the vibration-rotation
assignment for C3O.
The experimental setup to generate NCN. For asymmetric molecules in particular, assignment of a spectral
band is often complicated because just a single band can contain thousands of resolved vibration-rotation
lines. Programs have been written for computer assisted assignment of the spectra of species ranging
from simple linear molecules, through near symmetric tops to highly asymmetric tops. In addition
to using computer aided assignment to deal with the plethora of lines an experimental alternative
has been used to reduce the number of lines. We have built a system based on a supersonic jet expansion
coupled to our Bruker HR120 spectrometer and have used this to cool species down to a rotational
temperature of typically 20 - 60K. Schematics of our jet nozzle cooling system. Recently we have
developed an enclosive flow cooling system that allows much more flexibility for varying the temperature
of the gas under study |
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The experimental setup to generate NCN
For asymmetric molecules in particular, assignment of a spectral band is often complicated because
just a single band can contain thousands of resolved vibration-rotation lines. Programs have been
written for computer assisted assignment of the spectra of species ranging from simple linear molecules,
through near symmetric tops to highly asymmetric tops. In addition to using computer aided assignment
to deal with the plethora of lines an experimental alternative has been used to reduce the number
of lines. We have built a system based on a supersonic jet expansion coupled to our Bruker HR120
spectrometer and have used this to cool species down to a rotational temperature of typically 20
- 60K. Schematics of our jet nozzle cooling system. Recently we have developed an enclosive flow
cooling system that allows much more flexibility for varying the temperature of the gas under study.
Schematic of the Enclosive Flow Cell (EFC)
The advantages that collisional cooling has over jet-cooling for FTIR spectroscopy include:
- Thermal equilibrium: in the collisionally cooled sample translational, rotational,
and vibrational degrees of freedom are equally cooled. In the jet, samples remain vibrationally
(and ‘conformationally’) warm.
- Temperature control: the temperature is readily increased above the boiling point
of the cooling gas, but may also be lowered by pumping on the coolant or by using a different
gas.
- Reduced linewidths: jet expansions for FTIR applications are not skimmed, and the
range of transverse velocities probed by the beam leads to no reduction of the Doppler linewidth.
The linewidths of collisionally cooled samples in thermal equilibrium are reduced.
- Greater sensitivity: the absorption path of the gas is ca. two to three orders of
magnitude greater than in a supersonic jet expansion.
- Lower sample quantities: the absorption efficiency relative to the quantity of sample
gas is 5-6 orders of magnitude greater.
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