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1 Water vapour self-continuum: Recent update from Reading/RAL Semi-annual CAVIAR meeting 13.05.2010 UCL, London Igor Ptashnik, Keith Shine, Andrey Vigasin Robert McPheat, Kevin Smith, David Paynter Department of Meteorology, University of Reading (UK) MSF, Rutherford Appleton Laboratory (UK) Zuev Institute of Atmospheric Optics, RAS, Russia Obukhov Institute of Atmospheric Physics, RAS, Russia Slide 2 2 Recent CAVIAR measurements at MSF RAL: 1200 - 8000 cm -1 + 2 Kevin Smith, Robert McPheat, David Paynter IFS 120HR, IFS 125HR Short-path cell (up to 20m) Long-path cell (from 32 to 512m), Pressures: 20 - 280 mbar Temperatures: 293 - 350K Spectral resolution: 0.3 0.001cm -1 Slide 3 3 Water continuum and water dimers (1600-8000 cm -1 ) The "problem of the third peak" Slide 4 4 Water continuum and water dimers (1600-8000 cm -1 ) Slide 5 5 Water continuum and uncertainty in H 2 O line parameters In 1600 cm -1 band: 50 to 100% systematic error in H 2 O line para- meters is required to explain the deviation from MTCKD. In 3600 cm -1 band: 100 to 300% systematic error in strongest H 2 O lines' parameters would be required to explain deviation from MTCKD There cant be up to 100-200% deviation from Lorentzian profile in WM lines within just 1-3 cm -1 from the line centres at these pressures (It is not far wings) Slide 6 6 Water continuum and water dimers (1600-8000 cm -1 ) Metastable dimers are expected to produce similar to "smoothed" H 2 O spectral features. What is the fraction of metastable dimers? Slide 7 7 Classical 3D trajectory analysis for CO 2 -Ar pair: Formation of metastable dimer Ivanov (Kluwer, 2003); Lokshtanov, Ivanov, Vigasin (J.Mol.Struc., 2005): Typical trajectory resulting in formation of a collisional quasicomplex (upper panel). Decrease in the angular momentum L of the colliding pair of molecules (middle panel) at the cost of transforming part of the translational energy of the molecules to the rotational energy of CO 2 (lower panel). Approximate conservation of L and E i =J 2 /(m o r 2 ) + V(R, ) for the major part of the trajectory. (1-5)10 -12 s metastable pair free-pair collision metastable pair Slide 8 8 Bimolecular absorption. Partitioning of pairs in the phase space Bimolecular absorption can be formally split in three parts: Free-pair collisions (or CIA), caused by single-collision induced (or changed) dipole moment; True bound (stable) dimers; and Quasibound (metastable) dimers. The "water continuum question" then is: Which parts of BA contribute most to the continuum? The answer depends on intermolecular potential and temperature, and has been demonstrated for a few molecular pairs ( O 2 -O 2, CO 2 -CO 2, N 2 -N 2, H 2 O-H 2 O ) on the basis of statistical partitioning of the molecular pairs in the phase space (Vigasin, Kluwer-2003). Slide 9 9 Statistical partitioning of molecular pairs in the phase space Vigasin (Chem. Phys. Lett., 1985) Vigasin (Infrared Phys., 1991) Epifanov & Vigasin (Molec. Phys.,1997) Vigasin et al. (JMS, 2002) Lokshtanov et al. (J. Mol. Struc., 2005) Vigasin (Mol. Phys., 2010, in print) a) r/r e Free-pairs Quasi- bound Bound H 2 O-H 2 O: 1) The role of free-pair states is almost negligible at near room temperatures as compared to metastable and true bound states. 2) The fraction of true bound and metastable dimers must be comparable at room temperatures. Slide 10 10 S bound are taken for every band from VPT2 calculation by Kjaergaard et al. (J.Phys.Chem., 2008) or low-T experiment by Kuyanov et al. (J.Chem.Phys., 2010) S metast. are assumed 2 S monomer for near-IR spectral region (HITRAN-2008). Partitioning of H 2 O-H 2 O pairs using CAVIAR experiments C s ( ) cross-section of the experimental continuum [cm 2 molec -1 atm -1 ] K eq bound equilibrium constant for true bound dimers formation [atm -1 ] S bound and S metastable intensities of the bound and metastable dimer bands [cm/molec] Q bound and Q metastable partition functions for true bound and metastable dimers Vigasin & Pavlyuchko: (Prague -2008) ( Inspired by the paper Vigasin, Mol. Phys., 2010, in print ) (1) (2) Slide 11 11 K eq bound (T) is taken from Curtiss et al. (1979) Partitioning of H 2 O-H 2 O pairs using CAVIAR experiments Slide 12 12 Partitioning of H 2 O-H 2 O pairs using CAVIAR experiments K eq bound (T) which brings together Q bound /Q total for all bands Slide 13 13 K eq bound (T) is taken from Curtiss et al. (1979) Partitioning of H 2 O-H 2 O pairs using CAVIAR experiments Slide 14 14 K eq bound (T) which brings together Q bound /Q total for all bands Partitioning of H 2 O-H 2 O pairs using CAVIAR experiments Q metast. /Q bound 2