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

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 Leicesters 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 OH b - 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: OH b stretching overtones of water dimer Predicted (H 2 O) 2 overtones = 3 at 960 nm a = 4 at 755 nm a = 5 at 622 nm b a Schofield et al b Kjaergaard 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 Cambridges 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

Current experiments: Why orange wavelengths? High reflectivity of mirrors around WD feature (R(λ)~ ) means that a very high effective path length should be achievable (~7800 passes) FWHM = 35 nm LED emission

Pulsed nozzle apparatus: developments Adjustable bellows mounts for cavity mirrors Pumping system; pulsed nozzle (continuous nozzle???) Leak tested down to 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

H 2 O in N 2 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 H 2 O in N 2 atmospheric pressure

PI Acton vs Chromex/Wright spectrometer PI Acton Chromex/Wright [NO 2 ]= ~48 ppbv [NO 2 ]= ~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 nozzleContinuous 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 availabilityMust 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

JanFebMarchAprilMayJune 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 ( nm) for = 4 at 755 nm MChem student Characterise IR mirrors ( nm) for = 3 at 960 nm Timetable for work