Update on the Leicester lab studies (WP2.2: CRDS Measurements) Matthew Dover & Stephen Ball (University of Leicester) CAVIAR science meeting, Imperial College, 16 th December 2008
Mar 28, 2015
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
• 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!
• 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
• 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
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
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
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
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
• 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
Vacuum instrument vs field instrument
•PI Acton•Pulsed nozzle
•Chromex/Wright•H2O in N2 atmospheric pressure
PI Acton vs Chromex/Wright spectrometer
•PI Acton
•Chromex/Wright
[NO2]= ~48 ppbv
[NO2]= ~57 ppbv
• 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
• 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
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