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