Progress in water vapour spectroscopy at RAL Dr Robert McPheat CAVIAR Progress Meeting University of Leicester, 24/09/2009

Previous MSF Measurements Measurements in the mid- and near-IR. These measurements were intended to target the weak water vapour continuum. –Long path lengths. Measurements carried out at moderate (0.01cm -1 ) & high resolution (0.002cm -1 ) using the Bruker IFS125 HR. Initial measurements revealed a need for better line data too.

Water line parameter measurements Although CAVIAR is targeted at improving the water vapour continuum, errors in line parameters contribute to the measured continuum First measurements with upgraded Bruker IFS120 HR –Upgraded electronics / moving mirror to same specification as IFS125 High resolution measurements cm -1 –Long integration times (~ 12 hours) in mid-IR

5 mm Cell 5 mm path length Electropolished stainless steel for low surface adsorption Spherical reservoir volume to increase volume to surface ratio All metal vacuum system Parylene coated KBr windows attached with low vapour pressure epoxy resin

5 mm Cell

Water line parameter measurements 486 measurements covering mid & near IR Up to 13 hPa pure water Room temperature Spectra delivered to Reading Ongoing analysis of pressure measurements to be completed early next week –Leak rate –Outgassing

Mid-IR Spectra Wavenumber /cm -1

Mid-IR Spectra Wavenumber /cm -1

Near-IR Spectra Wavenumber /cm -1

Near-IR Spectra Wavenumber /cm -1

SPAC upgrade OpticsInnerCooling Jacket

High temperature pressure gauge & filling line Temperature & pressure logging Temperature control Outer vacuum jacket Transfer optics vacuum tank Detector box Electrical feedthroughs

SPAC upgrade modifications Additional base heaters added Inner modified to avoid problems with thermal expansion

Example Spectra Wavenumber /cm – 450 K Up to 1 bar water vapour Up to 5 bar total pressure

SPAC upgrade problems Problems with water condensation on bottom mirror Largely solved with shield & higher temperatures Degradation of mirror surfaces Move feedthrough to bottom of cell Will also solve wiring problems Kapton wires More PRTs Remove humidity sensors Replace O ring Simplify connections

SPAC upgrade modifications Electrical feedthroughs

LPAC Measurements Previous Measurement conditions –Room temperature, ~ 290 K –Water pressures up to 0.75 SVP ( < 20 hPa) –Resolution up to 0.002cm -1 but mainly at 0.01cm -1 –Water/air mixtures at total pressures of 150, 500 & 1000 hPa –Pathlengths of 32, 128 & 512m Made measurements in 3 spectral ranges covering mid/near- IR Planned measurements in 8 – 10 cm -1 region for other work. –Will extend to higher wavenumber if window materials allow

Schedule Complete analysis of 5 mm cell measurements –Next week Complete analysis of existing SPAC measurements & deliver data –Early next month Finish modifications to SPAC & start new measurements –November