Modelling Water Dimer Band Intensities and Spectra Matt Barber Jonathan Tennyson University College London 29 th September 2010

Monomer line list published “Short” list –Intensity cutoff at cm mol -1 – lines –Quantum numbers for all lines “Long” list –Intensities as low as cm mol -1 – 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

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 Need to use 1992 version of DVR –Band models subsequently superseded –Calculate monomer bands from recent line lists

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, (2008) Issues: PES used to generate monomer wavefunctions Cut through 12 D DMS used

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

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

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

Simulate spectra at “296 K” Assume 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

Simulate spectra at “16 K” Assume higher equilibrium constant Rotational band profile 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

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