Molecular pairs in the atmosphere, the carriers of continuum-like absorption

1. 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

2. Water vapor continuum absorption n, cm-1

3. Water vapor continuum absorption(after Baranov, 2008) Terrestrial radiation peaks here Solar radiation peaks there 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].

4. 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 To avoid continuous controversy it looks rational to start AB OVO

5. 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

6. The real gas virial equation of state

7. Interaction induced correction to unimolecular absorption

8. Interaction induced correction to unimolecular absorption Linear term vanishes in case of dipole forbidden transitions (highly symmetrical molecules such as N2, O2, CO2 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.

9. Bound, quasibound, and free pair states Ueff = U(r) +

10. Phase space subdivision forstructureless monomers Translational kinetic energy Free pairs True bound Metastable End-over-end rotational energy

11. The use of kinetic energy termsto subdivide the phase space No matter what intermolecular pair EL ER Ei Ei EL ERELEi

12. Partitioning in the phase spaceof polyatomic dimers

13. The Ar-CO2 system Potential Energy Surface Ab initio PES from …….J. Hutson et al., J. Chem.Phys.,105,9130 (1996).

14. Typical trajectories showing the difference among free andquasibound pair states Free pair state Quasicomplex formation TIME

15. Classical 3D trajectory analysis of the Ar-CO2 pair statesProjections of two slices across Ei onto (ER, EL) planeat selected separation R S. Lokshtanov, S. Ivanov, and A. Vigasin J. Molecular Structure, 742, 31 (2005).

16. Collision-induced absorption in the region of the CO2 Fermi dyad A. Vigasin,Y. Baranov, and G. Chlenova J. Molecular Spectroscopy, 213, 51 (2002).

17. Normalized collision-inducedintensity in the Fermi dyadand triad ranges

18. Collision-induced absorption in theN2 fundamental True dimer absorption is buried here and only here T = 343 K S-branch O-branch Ripple structure is OUT-OF-PHASE with N2 rotational lines! Q-branch Y. Baranov, W. Lafferty, and G. Fraser, J. Molecular Spectroscopy, 233, 160 (2005).

19. Normalized contributions to CIA intensity in the N2 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

20. Temperature variations of the CIA intensity in the N2 fundamental

21. Temperature variations of the CIA intensity in the N2 fundamental

22. Temperature variations of the CIA intensity in the N2 fundamental 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).

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

24. Drastic effect of close collisionson induced dipole Low temperature limit: Bab ~ |m(Re)|2 x exp(-De/kT) m(Re) Re De

25. Drastic effect of close collisionson induced dipole High temperature limit: Bab ~ kT

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

27. Temperature dependence of continuum at 944 cm-1, CRDS (from Cormier et al., JCP, 2005) Dimer model Vigasin, JQSRT, 2000 -1/2 De = -15.7 kJ/mole VRT(ASP-W)III Goldman e.a., JPC, 2004 Far wing H2O-H2O

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

29. The fraction of true bound orquasibound pair statesin the water vapor

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

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

32. Salient features of variational anharmonic calculations for the water dimer Vibrational problem: B3LYP/6-311G(3df,3pd) method is used to find trial harmonic or Morse potential parameters. These parameters are adjusted to meet the water dimer intermolecular fundamentals measured in the Ne matrix. Variational solution is found then with 18564 functions in the basis set. Rovibrational problem: Variational basis set consists of the direct product of vibrational and Wigner functions. Maximum basis set at J = 10 consists of 40887 functions. Intensities: Dipole function (as well as its first and second derivatives) is found with the use of ab initio calculations at MP2/6-311G(3d,2p) level. Average density of rovibrational lines reaches 32000 per one wavenumber. A. Pavlyuchko and A. Vigasin, 2008, to be published

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

34. Temperature dependence of the continuum binary absorption coefficient

35. Spectral variations of the metastable (monomer) and stable dimerabsorption cross-sections Metastable dimers (monomers) True dimers

36. Gibbs free energyof dimerization

37. Monomer far wings or water dimers? • The involvement of different types of pair states in the water vapor continuum absorption is essentially subject to temperature. • In the vicinity of room temperature these are true bound and metastable dimer states which are likely to dominate the continuum. • 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.

38. Conclusions • 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

39. Acknowledgments Yuri I. Baranov, Obninsk Beatrice Bussery-Honvault, Besançon Sergey V. Ivanov, Troitsk Sergey E. Lokshtanov, Moscow Anatoly I. Pavlyuchko, Volgograd RFBR Grant 08-05-00140

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