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.