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Theoretical work on the water monomer and dimer Matt Barber Jonathan Tennyson University College London 13 th May 2010 matt@theory.phys.ucl.ac.uk

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Lab observations in the visible (broad band CRDS) For dimer spectroscopy Need accurate description of monomer contribution Including weak lines A.J.L. Shillings, S.M. Ball, M.J. Barber, J. Tennyson & R.L. Jones, Atmos. Chem. Phys. (to be submitted)

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Anomaly in HITRAN data from 8000 to 9500 cm -1

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

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Simulate spectra at 295 K Assume 4.5% dimer concentration Rotational band profile 30 cm -1 (too narrow?) Predictions give absolute intensities 6D averaging But: Vibrational substructure still only for low T (8 J=0 states per symmetry) Results preliminary (main calculations in progress)

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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 –Only 8 states per symmetry here –It is a challenge for a much higher number of states Improved band origins coming soon

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