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Modelling Water Dimer Band Intensities and Spectra

Modelling Water Dimer Band Intensities and Spectra. Matt Barber Jonathan Tennyson University College London 29 th September 2010 matt@theory.phys.ucl.ac.uk. Monomer line list published. “Short” list Intensity cutoff at 10 -30 cm mol -1 202246 lines Quantum numbers for all lines

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Modelling Water Dimer Band Intensities and Spectra

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  1. Modelling Water Dimer BandIntensities and Spectra • Matt Barber • Jonathan Tennyson • University College London • 29th September 2010 • matt@theory.phys.ucl.ac.uk

  2. Monomer line list published • “Short” list • Intensity cutoff at 10-30 cm mol-1 • 202246 lines • Quantum numbers for all lines • “Long” list • Intensities as low as 10-35 cm mol-1 • 1464204 lines • Energy levels for all lines An upper limit for water dimer absorption in the 750nm spectral region and a revised water line list, AJL Shillings et al., ACP, Sep 25 2010

  3. 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 • Need to use 1992 version of DVR • Band models subsequently superseded • Calculate monomer bands from recent line lists

  4. 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

  5. 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

  6. 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

  7. Partition function and equlibrium constant 800 vibrational energy levels J 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

  8. 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

  9. Simulate spectra at “16 K” • Assume higher equilibrium constant • Rotational band profile 0.3-1 cm-1 • Damped by experiment • Predictions give absolute intensities • Vibrational substructure valid for low T • Most dimers will be in the ground state • Comparison against Helium droplet experiment • Unfortunately, band where our model is weakest

  10. 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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