Global spectroscopy of the water monomer

Abstract

Given the large energy required for its electronic excitation, the most important properties of the water molecule are governed by its ground potential energy surface (PES). Novel experiments are now able to probe this surface over a very extended energy range, requiring new theoretical procedures for their interpretation. As part of this study, a new, accurate, global spectroscopic-quality PES and a new, accurate, global dipole moment surface are developed. They are used for the computation of the high-resolution spectrum of water up to the first dissociation limit and beyond as well as for the determination of Stark coefficients for high-lying states. The water PES has been determined by combined ab initio and semi-empirical studies. As a first step, a very accurate, global, ab initio PES was determined using the all-electron, internally contracted multi-reference configuration interaction technique together with a large Gaussian basis set. Scalar relativistic energy corrections are also determined in order to move the energy determinations close to the relativistic complete basis set full configuration interaction limit. The electronic energies were computed for a set of about 2500 geometries, covering carefully selected configurations from equilibrium up to dissociation. Nuclear motion computations using this PES reproduce the observed energy levels up to 39 000 cm −1 with an accuracy of better than 10 cm −1 . Line positions and widths of resonant states above dissociation show an agreement with experiment of about 50 cm −1 . An improved semi-empirical PES is produced by fitting the ab initio PES to accurate experimental data, resulting in greatly improved accuracy, with a maximum deviation of about 1 cm −1 for all vibrational band origins. Theoretical results based on this semi-empirical surface are compared with experimental data for energies starting at 27 000 cm −1 , going all the way up to dissociation at about 41 000 cm −1 and a few hundred wavenumbers beyond it.

On the relative absorption strengths of water vapour in the blue wavelength range

“…More recently, water vapour absorption was also observed (Maksyutenko et al, 2012) and modelled (Polyansky et al, 2012) in the UV range below 370 nm. The HITEMP absorption line database for H 2 O (Rothman et al, 2010) relies to a large extent on the ab initio line list BT2 calculated by Barber et al (2006).…”

Water in the gas phase

“…Polyansky et al [13] discuss how theory is rising to these challenges and present encouraging results on how their calculations reproduce the latest dipole measurements. Calculated transition dipoles and the associated transition intensities and Einstein-A coefficients are playing an increasingly important role in practical applications of spectroscopy such as those found in astrophysics.…”

Molecular opacities for exoplanets

“…For H 2 O, for example, the experimental line intensities (e.g. in HITRAN) typically have errors of a few percent, while it is becoming clear that the line intensities calculated ab initio are now substantially better than this [28,41].…”

“…This success led to more experimental observations in other spectral regions and at higher temperatures (3000 K in an oxyacetylene torch) and to increasingly accurate theoretical predictions. The experimental [27] and theoretical work [28] continues, but Barber et al [6] have computed about half a billion water lines using an ab initio potential energy surface adjusted slightly to reproduce experimental observations and an ab initio dipole surface. The Barber et al BT2 line list is generally suitable for exoplanet molecular opacities.…”