towards perfect water line intensities lorenzo lodi university college london, dept of physics &...
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Towards perfect water line Towards perfect water line intensitiesintensities
Lorenzo LodiUniversity College London, Dept of physics &
Astronomy, London, UK
• Theoretical methods.
• Line positions.
• Line intensities and their uncertainties.
• Comparison of H218O and H2
17O linelists with HITRAN.
Talk summary
General scheme of solution
• Born-Oppenheimer approximation.
• Obtain potential energy surface (PES) and dipole moment surface (DMS).
• Use PES for the motion of the nuclei.
• From DMS and nuclear-motion wavefunctions calculate line intensities.
L. Lodi and J. Tennyson, J. Phys. B: At. Mol. Opt. Phys. 43, 133001 (2010)
Energy levels from experiment
• Fully labelled lines → energy levels by standard analyses.
• Different experimental sources → different uncertainties, systematic errors, mislabelling / inconsistent labelling.
• MARVEL program developed to deal with these issues [T. Furtenbacher, A.G. Csaszar, J. Tennyson, J Mol Spectr 245, 115 (2007)].
• MARVEL takes in (labelled) line positions and uncertainties and gives out energy levels and uncertainty bars.
Energy levels from experiment
• Using MARVEL a IUPAC-sponsored task group analysed all experimental data for H2
18O and H217O [J Tennyson et al,
JQRST 110, 573 (2009)].
• Led respectively to 4839 and 2687 energy levels (and uncertainty bars).
• Many more energy levels remain unknown (~26000 energy levels with energy up to 19000 cm-1 and J < 19).
• Calculations necessary to supplement experimentally-derived data.
Energy levels from theory
• PES by Shirin et al [S.V. Shirin et al, J Chem Phys 128, 224306 (2008)] to compute energy levels.
• Comparing with experimentally-derived energy levels gives estimate of error, which is ~0.1 cm-1.
Line position - summary
• Line positions from experimentally-derived energy levels, if possible.
• Theoretical line positions otherwise.
• Appropriate uncertainty bars in all cases.
Line intensities
• Absolute line intensities difficult to measure with accuracies < 5%.
• The LTP2011 ab initio DMS [L Lodi, J Tennyson and OL Polyansky, J. Chem. Phys 135, 034113 (2011)] provides 1% accurate line intensities for most lines.
• Such 1% accuracy claim is supported, among others, by:
1. Recent Stark coefficient measurements [OL Polyansky et al, Phil Trans Royal Soc London A, 370, 2728 (2012)].
2. Very accurate line intensities by [D Lisak, DK Harvey and JT Hodges, Phys Rev A, 79, 052707 (2009)].
Comparison with Lisak, Harvey and Hodges
Line intensities: resonances
• Resonant transitions very sensitive to PES used.
• For ~10% line intensities not accurate.
• Strategy to identify such lines suggested in [L Lodi and J Tennyson, JQSRT 113, 850 (2012)].
Line intensity error bars
• Compute two sets of wave function using PES by Shirin et al and using ab initio PES by Barletta et al [P Barletta et al, J Chem Phys 125, 204307 (2006)].
• Use two high-quality DMS (LTP2011 and LTP2011S) to get four sets of line intensities.
• The scatter of line intensities gives an estimation of the error.
Line intensity statistics
• ~45% of lines have scatter less than 1% (stable lines).
• ~3% of lines have scatter greater than a factor of 2 (unstable lines).
• All sensitive lines are very weak.
Conclusions and future work
• Linelists complete down to 10-29 cm/molecule for H218O
and H217O.
• Most line positions with errors of ~0.002 cm-1.
• Most line intensities have accuracies of 1-2%.
• Quantities have sensible error bars.
• Corresponding linelist for H216O almost done.
• Preliminary results with experimental line intensities by Geoffrey Toon from JPL are very encouraging.
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