modelling water dimer band intensities and spectra
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Modelling Water Dimer Band Intensities and Spectra. Matt Barber Jonathan Tennyson University College London 10 th February 2011 [email protected]. Band Intensities. Calculated using the “forbidden” J=0-0 transition. - PowerPoint PPT PresentationTRANSCRIPT
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Modelling Water Dimer BandIntensities and Spectra
Matt Barber
Jonathan Tennyson
University College London
10th February 2011
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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• Need to use 1992 version of DVR– Band models subsequently superseded– Calculate monomer bands from recent line lists
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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
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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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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
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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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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
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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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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/
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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