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1 Water vapour self-continuum: Recent interpretation Igor Ptashnik, Keith Shine, Andrey Vigasin University of Reading (UK) Zuev Institute of Atmospheric Optics (Russia) Institute of General Physics (Russia) Lab & Theory CAVIAR meeting UCL, London

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

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3 Water continuum and water dimers ( cm -1 ) Metastable dimers are expected to produce similar to "smoothed" H 2 O spectral features

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4 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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5 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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6 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 should dominate instead" 2) The fraction of true bound and metastable dimers must be comparable at room temperatures.

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7 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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8 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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9 K eq bound (T) which brings together Q bound /Q total for all bands

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10 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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11 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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12 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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13 Temperature dependence of the in-band self-continuum

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14 Water vapour self-continuum in band wings: Hot measurements at MSF RAL

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15 Can the far-wing model "increase" out-of-band continuum by factor 10 without bringing its parameters beyond their physically justified region? Water vapour self-continuum in band wings:

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17 Simulated spectra of H 2 O lines and water dimers in 10 mbar pure water vapour, 296K Dimer band intensities: Kjaergaard et al., Salmi et al. (JPC-2008) calculations, and Kyuanov etal. (JCP-2010) and Bouteiller & Perchard (CP-2004) experiments in matrix and He droplets. Water vapour continuum & dimers ( ) 1) There is striking resemblance between expected WD bands and CKD continuum model / Ptashnik et al. (QJRMS, 2004), Daniel et al. (GRL, 2004)/ 2) Most pronounced WD features should be detectable within near-IR water vapour bands. V. Vaida et al. (QJRMS, 2001): Dimers Ptashnik et al. (QJRMS, 2004): Dimers & MTCKD

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18 Continuum measurements at MSF RAL ( ): cm -1 I. Ptashnik, K. Smith, K. Shine, D. Newnham, Q. J. R. Meteorol. Soc., v. 130, (2004) Measurements: 10 m, 98 mbar H 2 O, 342 K (heated cell) 128 m, 20 mbar H 2 O, 299 K (long path) Continuum retrieval: Experiment – H 2 O_lines_with_M&Tcont. ?

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19 Dimers and continuum: Review of the former works

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20 Recent CAVIAR measurements at MSF RAL (3700 cm -1 band) Retrieval of the water vapour self-continuum : 1cm -1 Dimer intensities: VPT2 calculations (Kjaergaard et al., JPC-2008), experiments on water dimers trapped in He droplets (Kyuanov et al., JCP-2010), and measurements in matrices (Bouteiller & Perchard, CP-2004). 1 (PD) 3 (PD), 3 (PA) 2 2 (PD) ?

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