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Molecular pairs in the atmosphere, the carriers of continuum-like absorption Andrei A. Vigasin General Physics & Atmospheric Physics Institutes, Russian Academy of Sciences, Moscow CAVIAR meeting, London, 2008

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Water vapor continuum absorption

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, cm -1

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Water vapor continuum absorption (after Baranov, 2008) Near surface downwelling radiation flux (for the Standard Tropical Atmospheric profile) within the wavelength range from 2000 to 3200 cm -1 is ~1.5 times in excess of that calculated using the MTCKD-2.1 continuum model [ estimated by B. Fomin, 2008 ].

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To avoid continuous controversy it looks rational to start AB OVO Monomer far wings or water dimers? The war between the two countries is over the BigEnded and the LittleEnded controversy which is a debate over which end an egg should be cracked... after Jonathan Swift, Gulliver's Travels

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One has to define without ambiguity what must be called true dimers or colliding pairs and how to distinguish among them Partitioning of pair states as a function of temperature Classical trajectory analysis in the phase space Collision-induced intensity and its T-dependence Temperature effect on the water vapor continuum

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The real gas virial equation of state

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Interaction induced correction to unimolecular absorption

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Linear term vanishes in case of dipole forbidden transitions (highly symmetrical molecules such as N 2, O 2, CO 2 etc.). Quadratic term (binary absorption coefficient) implies the aggregate effect of the totality of pair states in a gas, no matter be it true dimer, quasibound, or free state. To reveal partial effect of a selected group of pair states (e.g. dimer or whatever) one has to learn how to partition the phase space of two interacting species.

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Bound, quasibound, and free pair states U eff = U(r) + h L(L+1) 8 2 c r

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Phase space subdivision for structureless monomers End-over-end rotational energy Translational kinetic energy Metastable Free pairs True bound

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ELEL ELEL ERER EiEi EiEi No matter what intermolecular pair E R E L E i The use of kinetic energy terms to subdivide the phase space

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Partitioning in the phase space of polyatomic dimers

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The Ar-CO 2 system Potential Energy Surface Ab initio PES from …….J. Hutson et al., J. Chem.Phys., 105, 9130 (1996).

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Typical trajectories showing the difference among free and quasibound pair states Quasicomplex formation Free pair state TIME

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Classical 3D trajectory analysis of the Ar-CO 2 pair states Projections of two slices across E i onto (E R, E L ) plane at selected separation R S. Lokshtanov, S. Ivanov, and A. Vigasin J. Molecular Structure, 742, 31 (2005).

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Collision-induced absorption in the region of the CO 2 Fermi dyad A. Vigasin,Y. Baranov, and G. Chlenova J. Molecular Spectroscopy, 213, 51 (2002).

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Normalized collision-induced intensity in the Fermi dyad and triad ranges

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Collision-induced absorption in the N 2 fundamental T = 343 K O-branch Q-branch S-branch True dimer absorption is buried here and only here Ripple structure is OUT-OF-PHASE with N 2 rotational lines! Y. Baranov, W. Lafferty, and G. Fraser, J. Molecular Spectroscopy, 233, 160 (2005).

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Normalized contributions to CIA intensity in the N 2 fundamental The result of Monte-Carlo 11-fold integration of the ab initio calculated [at CCSD(T) level of theory] induced dipole over pre- selected domains in the phase space S. Lokshtanov and A. Vigasin, 2008, to be published

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Temperature variations of the CIA intensity in the N 2 fundamental

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Integrated CIA intensity is calculated using ab initio PES (Van der Avoird e.a., 1984), CCSD(T) dipole surface and classical integration S. Lokshtanov, B. Bussery-Honvault, and A. Vigasin, Molecular Physics, 106, 1227 (2008).

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Temperature variations of the CIA intensity in oxygen See also….Ubachs e.a., JQSRT, 2006

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Drastic effect of close collisions on induced dipole Low temperature limit: B ab ~ | (R e )| 2 x exp(-D e /kT) ReRe DeDe (R e )

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Drastic effect of close collisions on induced dipole High temperature limit: B ab ~ kT

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Temperature dependence of the water vapor continuum absorption (near 944 cm -1 ) (at 1200 cm -1 )

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Temperature dependence of continuum at 944 cm -1, CRDS (from Cormier et al., JCP, 2005) H 2 O-H 2 O Dimer model Vigasin, JQSRT, 2000 Far wing D e = -15.7 kJ/mole VRT(ASP-W)III Goldman e.a., JPC, 2004 -1/2

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Temperature dependence of the continuum binary absorption coefficient (Baranov et al., 2008) Temperature, 1000/Θ, K -1

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The fraction of true bound or quasibound pair states in the water vapor

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This ratio of partition functions was calculated previously after pair states partitioning in the phase space. Here K p bound (T) stands for the true bound dimers equilibrium constant; True dimer absorption cross-section bound ( ) is evaluated from anharmonic variational calculations; Metastable dimer cross-section metastable ( ) can be taken as doubled and broadened monomer cross-section: 2 monomer ( ); Binary absorption coefficient in a mixture of true bound and metastable dimers

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Variational anharmonic calculations of the far IR water dimer spectrum CKD Water dimer End-over-end rotation Intermolecular fundamentals Intermolecular overtones and combination bands A. Pavlyuchko and A. Vigasin, 2008, to be published (K p = 0.04 atm -1 )

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Salient features of variational anharmonic calculations for the water dimer A. Pavlyuchko and A. Vigasin, 2008, to be published

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Variational anharmonic calculations of the far IR water dimer spectrum CKD Water dimer End-over-end rotation Intermolecular fundamentals Intermolecular overtones and combination bands A. Pavlyuchko and A. Vigasin, 2008, to be published (K p = 0.04 atm -1 )

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Temperature dependence of the continuum binary absorption coefficient

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Spectral variations of the metastable (monomer) and stable dimer absorption cross-sections Metastable dimers (monomers) True dimers

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Gibbs free energy of dimerization

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1.The involvement of different types of pair states in the water vapor continuum absorption is essentially subject to temperature. 2.In the vicinity of room temperature these are true bound and metastable dimer states which are likely to dominate the continuum. 3.To all appearance there is no room to speculations about so- called collision-induced nature of the atmospheric continuum implying that this phenomenon might be determined by free collisional pair states. Monomer far wings or water dimers?

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Pair states in the atmosphere realize in terms of true bound, metastable, and free states Close collisions may give rise to increase in continuum absorption at elevated temperature Involvement of the metastable states is largely responsible for the observed spectral variations in the temperature dependence of the water vapor continuum absorption Conclusions

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Acknowledgments

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My thanks go to The organizers of the current CAVIAR meeting and to The audience for your attention

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