31 No. 5
A Database of Water Transitions from Experiment and Theory
The water molecule is arguably the single most important species from a spectroscopic point of view. It is certainly molecule number 1 in the standard atmospheric database HITRAN since it is both the largest absorber of sunlight in the earth’s atmosphere and the major greenhouse gas. This means, for example, that its isotopologue H218O is the fifth biggest absorber in the earth’s atmosphere. Water spectra are equally important astronomically: absorption by hot water dominates the atmospheres of M-dwarfs, the most common stars in our galaxy, and has recently been identified in the extra-solar planet HD189733b.
There have been many years of work and hundreds of papers on the laboratory spectroscopy of water but obtaining reliable spectroscopic parameters for water remains an unresolved problem. The Task Group was created to synthesize the known data on water spectra, whether experimental or theoretical, and carefully validate them.
As a first major task, the project has established a protocol for inverting transition frequencies taken from high-resolution laboratory spectra to give energy levels. The procedure, called MARVEL (Measured Active Rotational-Vibrational Energy Levels)1 is based on the use of the so-called X-matrix method, which has been in use for some time, for producing energy levels from a single spectrum. Adapting the X-matrix to large datasets of heterogeneous data raises a number of issues to do with both consistency of the underlying data and that of the published uncertainties. These are addressed in MARVEL itself before and during the inversion process, and post hoc by comparing the labelled MARVEL energy levels with those obtained from high-accuracy variational nuclear motion calculations and with combination differences.
The Task Group recently published the first part of a series of articles reporting critically evaluated rotational-vibrational line positions, transition intensities, pressure dependence, and energy levels, with associated critically reviewed assignments and uncertainties, for all the main isotopologues of water. The article,2 which also sets out the strategy the Task Group wishes to follow, contains energy levels and data for line positions of the singly substituted isotopologues H217O and H218O. The procedure and code MARVEL is used extensively in all stages of determining the validated levels and lines and their self-consistent uncertainties. The spectral regions covered for both H217O and H218O are 0–17125 cm-1. The energy levels are checked against ones determined from accurate variational calculations. The number of critically evaluated and recommended levels and lines are, respectively, 2687 and 8614 for H217O and 4839 and 29364 for H218O. Among other things, the list of validated energy levels can be used to give very precise predictions of yet-to-be measured line frequencies. The active nature of this activity is illustrated by the publication of new spectra subsequent to the completion of this paper by some task group members. Similar studies are now underway for H216O, H217O, and H218O, which will be reported shortly, and for the main isotopologue, H216O, for which the dataset collected contains more than 100 000 transitions.
- T. Furtenbacher, A. G. Császár, J. Tennyson, MARVEL: Measured Active Rotational-Vibrational Energy Levels, J. Mol. Spectry. 245, 115–125 (2007).
- J. Tennyson, P.F. Bernath, L.R. Brown, A. Campargue, M.R. Carleer, A.G. Csaszar, R.R. Gamache, J.T. Hodges, A. Jenouvrier, O.V. Naumenko, O.L. Polyansky, L.S. Rothmam, R.A. Toth, A.C. Vandaele, N.F. Zobov, L. Daumont, A.Z. Fazliev, T. Furtenbacher, I.E. Gordon, S. N. Mikhailenko and S. V. Shirin, IUPAC Critical Evaluation of the Rotational-Vibrational Spectra of Water Vapor. Part I. Energy Levels and Transition Wavenumbers for H217O and H218O, J. Quant. Spectrosc. Rad. Transf., 110, 573–596 (2009). DOI: 10.1016/j.jqsrt.2009.02.014
For more information and comments, contact Task Group Chair Jonathan Tennyson <[email protected]>.
last modified 14 September 2009.
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