1. Current CAVIAR activities at Cambridge A.J.L. Shillings 1, S.M. Ball 2 and R.L. Jones 1 1 University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge, CB1 2EW 2 University of Leicester, Department of Chemistry, Leicester, LE1 7RH

2. Outline 1.BBCRDS Measurements Outstanding issues (and how we plan to address them) 2.CRDS Measurements Monochromatic studies to thoroughly test monomer databases 3.IR-BCEAS Measurements Difficulties encountered Planned Experiments

3. Summary of 750nm region (1) HITRAN 04UCL 08

4. Summary of 750nm region (2) Inferred maximum dimer signal averaged over measurement interval (736 – 760 nm) UCL_08 data Expected peak absorption using linestrength of 2.8×10 -22 cm molecule -1 – the smallest estimate. Inferred maximum dimer signal averaged over measurement interval (736 – 760 nm) HITRAN 04 data Using HITRAN monomer data instead of UCL_08 data results in significantly larger inferred upper limit for water dimer absorption – how to reference this database?

5. As in the 750 nm region, HITRAN, contains a poor representation of the water monomer absorption – UCL_08 is much better but discrepancies still remain. Summary of 607 to 622 nm region (1)

6. Summary of 608 to 622 nm region (2) Inferred maximum dimer signal averaged over measurement interval (608 – 622 nm) UCL_08 data Expected peak absorption using linestrength of 4.0×10 -23 cm molecule -1 – the smallest estimate. Inferred maximum dimer signal averaged over measurement interval (608 – 622 nm) HITRAN 04 data Using HITRAN monomer data instead of UCL_08 data results in significantly larger inferred upper limit for water dimer absorption.

7. BBCRDS Summary Inferred upper limits for water dimer absorption in spectral regions where water monomer is sufficiently well treated. Imply widths of dimer transitions in regions investigated are much larger than in the mid IR – Could linestrength be incorrect? Consistent with theory? Can these widths be rationalised? How big could they be? In other regions (towards the centre of the bands) we are currently limited by water monomer knowledge See next section BBCRDS system will move to Leicester in mid-October See Steve’s talk

8. Dealing with BBCRDS water monomer absorption

9. Investigated methods to use a low [H 2 O] spectrum to fit a high [H 2 O] spectrum – should remove monomer component form measured absorption. Results interesting but not conclusive (still need broadening parameters to be quantitative) Need a different approach Dealing with BBCRDS water monomer absorption

10. Monochromatic CRDS (1) CRDS Setup essentially the same as BBCRDS but….. Replace broadband dye laser with a monochromatic dye laser (i.e. with a grating) (Borrowed from Corey Evans at Leicester) Replace CCD detector with PMT directly behind cavity output mirror Use same (well tested and understood) absorption cell Have experienced difficulties with hairline fracture in cell and T controlled mirror mounts –cavity stability has been poor

11. Orange region first – (595-630 nm) – where current agreement is poorest. Everything in Cambridge essentially ready to go – plan to start experiments in the next couple of weeks. Monochromatic CRDS (2) High resolution CRDS Similar sensitivity to BBCRDS but much higher resolution (~0.1 cm -1 (0.004 nm) cf 0.15 nm) Attempt to resolve individual (broadened) monomer lines To (thoroughly) test the monomer database performance Remove non B-L effects, remove any possible convolution effects

12. IR BBCEAS (1) Everything assembled – works (surprisingly) well in Red (660 nm) but having difficulty with mid IR Light sources Had hoped to use LEDs – cheap, convenient, stable, group has plenty of experience with them but those available for this region are not bright enough. Very newly developed ‘Cooled Red’ light source (essentially a 1800K black body), looks ideal for our application, very bright, fibre coupled and very stable (claimed) We have one on order (from USA) that is due to arrive next week – to be compared to ocean optics (LS-1 and HL2000 (Tungsten Halogen)) light sources.

13. IR BBCEAS Calibration CEAS not inherently calibrated (unlike CRDS) – need a method to determine mirror reflectivity Introduce known absorber CO 2 very convenient Absorbs throughout the near and mid IR Very stable, non toxic and readily available at a range of concentrations c.f.

14. IR BBCEAS (2) Typical mirror high reflectivity bandwidth ~ 200 nm. Bandwidth of detector (Grating Dependent), Resolution (Slit Dependent) First experiment in this region to make use of excellent bandwidth of instrument to particularly target the weakest continuum away from band centres – high sensitivity and broadband approach – novel set of measurements of high [H 2 O] ‘continuum’ over a range of T’s. Grating Bandwidth / nm Resolution FWHM / nm 1~300~1 2~100~0.3 3~20~0.1

15. Initial IR BBCEAS Experiments 550 – 2500nm water bands Any requests about the sequence of experiments? Specific areas to investigate? (BB)CRDS Planned First Measurements (1250 to 1450 nm)

16. BBCRDS Fitting Methods If Beer-Lambert law obeyed, these plots would be straight lines