Update on the Leicester lab studies (WP2.2: CRDS Measurements)

1. 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

2. Leicester’s CAVIAR postdoc appointed! • 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

3. Target: OHb stretching overtones of water dimer • 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 Dn = 5 at 622 nm (orange/red region) Dn Predicted (H2O)2 overtones Dn = 3 at 960 nm a Dn = 4 at 755 nm a Dn = 5 at 622 nm b aSchofield et al. 2007 bKjaergaard 2003

4. Current experiments: Why orange wavelengths? Spectrum recorded by Simon Neil using field instrument • 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 Dn = 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 Kjaergaard predicts WD feature

5. 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) LED emission FWHM = 35 nm

6. Pulsed nozzle apparatus: developments Nozzle Spectrograph/CCD camera LED • Adjustable bellows mounts for cavity mirrors • Pumping system; pulsed nozzle (continuous nozzle???) • Leak tested down to 1107 Torr • Aligned first BBCEAS cavity and taken some preliminary measurements

7. 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

8. 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

9. First vacuum experiments • 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…

10. Vacuum instrument vs field instrument • PI Acton • Pulsed nozzle • Chromex/Wright • H2O in N2 atmospheric pressure

11. PI Acton vs Chromex/Wright spectrometer • PI Acton [NO2]= ~48 ppbv • Chromex/Wright [NO2]= ~57 ppbv

12. Conclusion from first vacuum experiment • 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…

13. Future developments: nozzle design • Probably the most important part of the overall system design • Good arguments for pulsed system and continuous system

14. Timetable for work