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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 30.04.2010 UCL, London.

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water vapour self continuum recent interpretation
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 30.04.2010 UCL, London

slide2

Water continuum and water dimers(1600-8000 cm-1)

The "problem of the third peak"…

slide3

Water continuum and water dimers(1600-8000 cm-1)

Metastable dimers are expected to produce similar to "smoothed" H2O spectral features

slide4

Water continuum and uncertainty in H2O line parameters

In 3600 cm-1 band:

100 to 300% systematic error in strongest H2O lines' parameters would be “required” to explain deviation from MTCKD

There can’t 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)

In 1600 cm-1 band:

50 to 100% systematic error in H2O line para-meters is “required” to explain the deviation from MTCKD.

slide5

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-collisioninduced (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 (O2-O2, CO2-CO2, N2-N2,H2O-H2O) on the basis of statistical partitioning of the molecular pairs in the phase space (Vigasin, Kluwer-2003).

slide6

a)

r/re

Etr

b)

L

Statistical partitioning of molecular pairs

in the phase space

A. Vigasin (Kluwer, 2003):

The family of the effective intermolecular potentials Ueff 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 quasiboundstates in 3D

space of energy variables of the complex:H = U(R,) + Etr + EL + Er .

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)

Free-pairs

Quasi-

bound

Bound

H2O-H2O:

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.

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.

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

slide7

( Inspired by the paper Vigasin, Mol. Phys., 2010, in print )

Vigasin & Pavlyuchko:

(Prague -2008)

Cs()– cross-section of the experimental continuum [cm2molec-1atm-1]

Keqbound– equilibrium constant for true bound dimers formation [atm-1]

SboundandSmetastable – intensities of the bound and metastable dimer bands [cm/molec]

QboundandQmetastable– partition functions for true bound and metastable dimers

Sboundare 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)

Smetast.are assumed 2Smonomerfor near-IR spectral region (HITRAN-2008).

Partitioning of H2O-H2O pairs using CAVIAR experiments

(1)

(2)

slide8

Partitioning of H2O-H2O pairs using CAVIAR experiments

Keqbound (T) is taken fromCurtiss et al.(1979)

slide9

Partitioning of H2O-H2O pairs using CAVIAR experiments

Keqbound (T)which brings togetherQbound/Qtotal for all bands

slide10

Partitioning of H2O-H2O pairs using CAVIAR experiments

Keqbound (T) is taken fromCurtiss et al.(1979)

slide11

Partitioning of H2O-H2O pairs using CAVIAR experiments

Qmetast./Qbound  2 <=

Keqbound (T)which brings togetherQbound/Qtotal for all bands

slide12

0.03 atm-1 2 2.SmonomerHWHM= 30cm-1

Decomposition of the in-band continuum (1600-8000 cm-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 250-300 cm-1 wide.

slide14

Water vapour self-continuum in band wings:

“Hot” measurements at MSF RAL

slide15

Water vapour self-continuum in band wings:

Can the far-wing model "increase" out-of-band continuum by factor 10 without bringing its parameters beyond their physically justified region?

slide17

+

2

2+

3

Water vapour continuum & dimers (2002-2004)

V. Vaida et al.(QJRMS, 2001): Dimers Ptashnik et al.(QJRMS, 2004): Dimers & MTCKD

Simulated spectra of H2O lines and water dimers in 10 mbar pure water vapour, 296K

Dimer band intensities:Kjaergaard et al., Salmi et al. (JPC-2008)calculations, andKyuanov etal. (JCP-2010) and Bouteiller & Perchard (CP-2004) experiments in matrix and He droplets.

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.

continuum measurements at msf ral 2002 2004 5000 5600 cm 1
Continuum measurements at MSF RAL (2002-2004):5000-5600 cm-1

I. Ptashnik, K. Smith, K. Shine, D. Newnham,Q. J. R. Meteorol. Soc., v. 130, 2391-2408 (2004)

Measurements:

10 m, 98 mbar H2O, 342 K (heated cell)

128 m, 20 mbar H2O, 299 K (long path)

Continuum retrieval:

Experiment – H2O_lines_with_M&Tcont.

?

slide20

22(PD)

1(PD)3(PD),

3(PA)

Recent CAVIAR measurements at MSF RAL(3700 cm-1 band)

Retrieval of the water vapour self-continuum :

< RAL_measurement – Calculated_H2O_lines_contribution >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).