modelling water dimer band intensities and spectra matt barber jonathan tennyson university college...
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Modelling Water Dimer BandIntensities and Spectra
Matt Barber
Jonathan Tennyson
University College London
10th February 2011
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• Need to use 1992 version of DVR– Band models subsequently superseded– Calculate monomer bands from recent line lists
Band Intensities
1.00E-025
1.00E-024
1.00E-023
1.00E-022
1.00E-021
1.00E-020
1.00E-019
1.00E-018
1.00E-017
1.00E-016
0 2000 4000 6000 8000 10000
Frequency
Inte
nsity HITRAN
BT2
DIPJ0
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
1500 2500 3500 4500 5500 6500 7500 8500 9500 10500 11500
1.00E-028
1.00E-026
1.00E-024
1.00E-022
1.00E-020
1.00E-018
1.00E-016
Donor (equilibr ium)
A c c eptor(equilibr ium)
Monomer
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
1500 2500 3500 4500 5500 6500 7500 8500 9500 10500 11500
1.00E-028
1.00E-026
1.00E-024
1.00E-022
1.00E-020
1.00E-018
1.00E-016
Donor (per turbed)
A c c eptor (per turbed)
Monomer
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 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
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
1300 1400 1500 1600 1700 1800 19000.00E+000
1.00E-021
2.00E-021
3.00E-021
4.00E-021
5.00E-021
6.00E-021
7.00E-021
8.00E-021
9.00E-021
UCL
Salmi
MTCKD-1.1, 293K
WD(S&K-2003)
RAL-2007/295K/
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