update on the leicester lab studies (wp2.2: crds measurements) matthew dover & stephen ball...
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Update on the Leicester lab studies(WP2.2: CRDS Measurements)
Matthew Dover & Stephen Ball
(University of Leicester)
CAVIAR science meeting, Imperial College, 16th December 2008
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• Appointment – 22 September 2008• My background – PhD high resolution LIF spectroscopy
of transient silicon containing species• Used the same vacuum system as CAVIAR pulsed
nozzle experiments• Training since appointment – have carried out my first
BBCEAS experiments using the field instrument• Last few weeks first BBCEAS experiments using vacuum
chamber
Leicester’s CAVIAR postdoc appointed!
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• Positive identification of WD absorption features
• In regions away from strong WM absorptions BBCEAS study, of the third, fourth and fifth water dimer OHb-stretching overtone transitions
• Supersonic expansion:– Non-equilibrium concentrations of WM and WD– Collapse WM structure
• Initial experiments are under way with an aim to examining the = 5 at 622 nm (orange/red region)
Target: OHb stretching overtones of water dimer
Predicted (H2O)2 overtones
= 3 at 960 nm a
= 4 at 755 nm a
= 5 at 622 nm b
aSchofield et al. 2007bKjaergaard 2003
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• Visible light makes cavity alignment easier than infrared
• Cavity mirrors already well characterised, and have good reflectivity (next slide)
• Bright LED, peak emission at 617 nm (nearly gaussian emission spectrum)
• The = 5 water dimer overtone feature is predicted to be at ~622 nm – between WM lines (see above)
• Consistent with Cambridge’s BBCRDS search for 615 nm (and 760 nm) dimer bands
Current experiments: Why orange wavelengths?
Kjaergaard predicts WD feature
Spectrum recorded by Simon Neil using field instrument
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560 580 600 620 640 660 6800.9994
0.9996
0.9998
Mir_samj2
Mir_samj2
regress_mirrorj2
Mir4
Xwavelengthj2 Xwavelengthj2 Xwavelengthj2 Mir0
Current experiments: Why orange wavelengths?
• High reflectivity of mirrors around WD feature (R(λ)~0.99987) means that a very high effective path length should be achievable (~7800 passes)
FWHM = 35 nm
LED emission
d
R
I
I )(11)( 0
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Pulsed nozzle apparatus: developments
• Adjustable bellows mounts for cavity mirrors
• Pumping system; pulsed nozzle (continuous nozzle???)
• Leak tested down to 1107 Torr
• Aligned first BBCEAS cavity and taken some preliminary measurements
LED
Nozzle
Spectrograph/CCD camera
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New Spectrometer: PI Acton SpectraPro 2500i
• Very sensitive instrument as a cooled ICCD camera is used for light collection
• Particularly attractive for pulsed nozzle experiments because of fast gating electronics supplied
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Fibre coupler for new spectrometer
• Manufacturer supplied fibre f-matcher not ideal for BBCEAS. Therefore built our own
• It was essential to design and engineer a suitable fibre coupler for the system
• The fibre coupler was designed so as to give maximum throughput of light into the spectrometer by using a fast achromat to focus the light into the monochromator slit
• Fibre is mounted on an x,y,z translator to allow optimal focus and positioning of fibre relative to monochromator entrance slit
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• H2O in N2 through pulsed nozzle
• Gated detection on ICCD camera
• This is a VERY preliminary result with much scope to improve when compared to the previous result obtained from the field instrument…
First vacuum experiments
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Vacuum instrument vs field instrument
•PI Acton•Pulsed nozzle
•Chromex/Wright•H2O in N2 atmospheric pressure
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PI Acton vs Chromex/Wright spectrometer
•PI Acton
•Chromex/Wright
[NO2]= ~48 ppbv
[NO2]= ~57 ppbv
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• Although the PI Acton spectrometer allows gating type experiments, the noise levels and signal strengths do not look very promising:
– Broader lineshape– Narrower bandwidth– Noisier!
• Revert back to Chromex/Wright spectrometer – issue of gating experiment suitably to record spectra using a pulsed setup
• Investigate possibility of a continuous source for the nozzle…
Conclusion from first vacuum experiment
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• Probably the most important part of the overall system design• Good arguments for pulsed system and continuous system
Future developments: nozzle design
Pulsed nozzle Continuous nozzle
Larger orifice - Higher concentrations of absorbing species in each pulse
Smaller orifice – Lower concentrations of absorbing species, but a continuous flow
Better cooling effects in supersonic expansion
Good cooling may be achieved by using the correct orifice size
Out-of-the-box availability Must be engineered to exacting specifications
Only potential issue is getting the timing of experiments right
Potential frosting issues
Ideally requires a detector capable of gating experiments
Continuous source requires no gating of detector
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Jan Feb March April May June
Continue to take measurements in the = 5 region
Locate dimer feature!
Optimise vacuum conditions
Setup Chromex/Wright spectrometer
Continuous vs pulsed nozzle experiments
Continuation of nozzle design
Characterise NIR mirrors (730-780 nm) for = 4 at 755 nm
MChem student
Characterise IR mirrors (910-1000 nm) for = 3 at 960 nm
Timetable for work