Towards perfect water line intensities Lorenzo Lodi University College London, Dept of physics & Astronomy, London, UK

Theoretical methods. Line positions. Line intensities and their uncertainties. Comparison of H 2 18 O and H 2 17 O 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, (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 H 2 18 O and H 2 17 O [ 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 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, (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, (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, (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, (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 cm/molecule for H 2 18 O and H 2 17 O. 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 H 2 16 O almost done. Preliminary results with experimental line intensities by Geoffrey Toon from JPL are very encouraging.