1. Update on the Leicester lab studies (WP2.2 cavity ringdown spectroscopy) Stephen Ball & Simon Neil (Leicester University) CAVIAR science meeting, NPL, 2 nd June 2008

2. Predicted (H 2 O) 2 overtones  = 3 at 960 nm  = 4 at 755 nm  = 5 at 622 nm  = 4  Monomer Red = HITRAN 04 (0.24nm FWHM) Dimer (20 cm -1 FWHM), [dimer] = 1% of [H 2 O] Blue = Schofield & Kjaergaard 2003 Green = Schofield et al 2007 Broadband cavity enhanced absorption spectroscopy (BBCEAS) Target overtone of dimer’s hydrogen-bonded OH stretch

3. 570 wavelength (nm) 670 Absorption coefficient (cm  1 ) BBCEAS spectrum (Effective path length  10.7 km at mirror’s peak) HITRAN 04 (0.44nm FWHM gaussian) 4  10  7 cm  1 Issues: [1] relative intensity of bands  mirror reflectivity vs wavelength [2] intensity of lines within a band  saturation effects BBCEAS spectrum in region of  = 5 band in (H 2 O) 2 (90% RH in nitrogen at room temp)

4. [1] Quantitative BBCEAS: mirror reflectivity vs wavelength  ( ) = absorption coefficient (absorption over a 1 cm path); d = cavity length, I (and I 0 ) = cavity transmission with (& without) sample; R( ) = mirror reflectivity Mirrors for  = 5 dimer overtoneMirrors for  = 4 dimer overtone R( ) = 0.9999 gives a cavity enhancement of 10 4 (i.e. 10km path in a 1m cavity)

5. Compare methods for measuring R( ) (i) absorption due to a known absorber: O 2 -O 2 collision complex, NO 2 (ii) difference in Rayleigh scattering: He vs N 2 or Ar vs N 2 R( )  O 2 -O 2

6. [2] Accounting for saturation effects in water monomer lines over the path lengths accessible by BBCEAS Transmission at resolution of spectrograph (0.44nm FWHM) Time = 21.6  s => effective path length = 6.5 km (c.f. ringdown time  35  s) HITRAN 04

7. Under-resolved saturated absorptions => cross sections (at instrument resolution) depend on path => cross sections vary within a ringdown event t = 0  s, optically thin t = 21.6  s, path = 6.5 km t = 100  s, path = 30 km t = 220  s, path = 66 km 596.66nm (strong absorption) 614.66nm (weak absorption)

8. Ringdown decays 614.66 nm (higher mirror reflectivity) 596.66 nm, empty cavity (lower mirror reflectivity) 596.66 nm, cavity + sample BBCEAS signal = integrated intensity from a ringdown event (note that BBCRDS measures time-labelled spectral intensity)  effective cross sections constructed from a sum of path-length dependent cross sections weighted according to ringdown decay time  rel intensity  Effective absorption cross sections for BBCEAS (black = direct convolution of HITRAN)

9. End result: quantitative fit to BBCEAS spectrum BBCEAS spectrum Fitted spectrum Residual spectrum

10. [3] Pulsed nozzle apparatus: developments Adjustable bellows mounts for cavity mirrors Pumping system; pulsed nozzle Leak tested down to 1  10  7 Torr Align first BBCEAS cavity!