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Modelling Water Dimer Band Intensities and Spectra Matt Barber Jonathan Tennyson University College London 10 th February 2011 matt@theory.phys.ucl.ac.uk

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Band Intensities Calculated using the forbidden J=0-0 transition. Water dimer is too complicated for full ro- vibrational modelling. However, we can model vibrations of monomers within dimer and simulate additional rotational structure. Need to use 1992 version of DVR –Band models subsequently superseded –Calculate monomer bands from recent line lists

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Dimer band intensities Calculate from (perturbed) monomer vibrational wavefunctions Requires Eckart embedding of axis frame Use HBB 12 D dipole moment surface (DMS) corrected with accurate monomer DMS CVR: L. Lodi et al, J Chem Phys., 128, 044304 (2008) Issues: PES used to generate monomer wavefunctions Cut through 12 D DMS used

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Perturbing the dimer configuration Many possible configurations Transition intensities vary considerably from small changes in geometry Equilibrium may not be best choice Pick to strengthen donor bound stretch

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Estimating transition frequencies Band centre from monomer DVR3D calculation Blue/red shift from calculation on perturbed PES Vibrational fine structure from dimer dimer transitions Rotational structure simulated by overlaid Lorentzian

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Partition function and equlibrium constant 800 vibrational energy levels J up to 5 calculated, extrapolated up to 50 Dissociation energy? Equilibrium constant at room temperature: –Around 0.03 to 0.05 for bound states –Possibly up to 0.08 for metastable

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Simulate spectra at 296 K Assume 0.045 equilibrium constant for typical atmospheric conditions Rotational band profile 30 cm -1 HWHM Vibrational fine structure mostly hidden beneath rotational structure But: Vibrational substructure still only for low T (8 J=0 states per symmetry) Possible contribution from metastable dimers

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Further Work Preliminary spectra for up to 10,000 cm -1 produced. –Band profile comparisons show some encouraging signs. –Effects of the sampling of the potential being investigated. Need all states up to dissociation for RT spectra –Only 8 states per symmetry here –It is a challenge for a much higher number of states Improved band origins

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