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1 Water vapour self-continuum: Recent update from Reading/RAL Semi-annual CAVIAR meeting 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

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2 Recent CAVIAR measurements at MSF RAL: cm Kevin Smith, Robert McPheat, David Paynter IFS 120HR, IFS 125HR Short-path cell (up to 20m) Long-path cell (from 32 to 512m), Pressures: mbar Temperatures: K Spectral resolution: 0.3 – 0.001cm -1

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3 Water continuum and water dimers ( cm -1 ) The "problem of the third peak"…

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4 Water continuum and water dimers ( cm -1 )

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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 % 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)

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6 Water continuum and water dimers ( cm -1 ) Metastable dimers are expected to produce similar to "smoothed" H 2 O spectral features. What is the fraction of metastable dimers?

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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)× s metastable pair free-pair collision metastable pair

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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).

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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.

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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)

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11 K eq bound (T) is taken from Curtiss et al. (1979) Partitioning of H 2 O-H 2 O pairs using CAVIAR experiments

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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

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13 K eq bound (T) is taken from Curtiss et al. (1979) Partitioning of H 2 O-H 2 O pairs using CAVIAR experiments

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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 <=

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15 Decomposition of the in-band continuum ( cm -1 ) 0.03 atm S monomer HWHM = 30cm -1 Collisionally and predissociatively broadened lines of stable and metastable water dimers overlap, producing broad ~60cm -1 wide continuum sub-bands. Contribution from metastable dimers replicates smoothed spectrum of water monomers. All together they form in-band water vapour continuum cm -1 wide.

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16 Temperature dependence of the in-band self-continuum

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17 Water vapour self-continuum in band wings: Hot measurements at MSF RAL (350K - 470K) Robert McPheat, Kevin Smith, Gary Williams

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18 Plan for MSF RAL measurements in mid-infrared

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20 Statistical partitioning of molecular pairs in the phase space A. Vigasin (Kluwer, 2003): The family of the effective intermolecular potentials U eff for different angular momentums L of the molecular pair as a function of the intermolecular distance. The auxiliary function G outlines domain of quasibound states (light grey area). Above and below lie respectively free pair and bound states' areas. Domains of bound and quasibound states in 3D space of energy variables of the complex: H = U(R, ) + E tr + E L + E r. 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 Having the phase space subdivided, the truncated partition functions for true-bound and quasibound states can be obtained by integration of Boltzmann factor over respected domain in the phase space The idea of subdivision in the phase space lies in reducing the Hamiltonian to such variables which would make obvious the definition of true bound, quasibound and free pair states. It was shown by Andrey Vigasin that the combination of spatial coordinates and particular kinetic and potential energies is rather convenient choice for such variables. Free-pairs Quasi- bound Bound E tr L b) 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.

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