Search

A highly accurate, global dipole moment surface (DMS) is calculated for the water molecule using ab initio quantum chemistry methods. The new surface is named LTP2011 and is based on all-electron, internally contracted multireference configuration interaction, including size-extensivity corrections in the aug-cc-pCV6Z basis set. Dipoles are computed as energy derivatives and small corrections due to relativistic effects included. The LTP2011 DMS uses an appropriate functional form that guarantees qualitatively correct behaviour even for most high energies configuration (up to about 60,000 cm(-1)), including, in particular, along the water dissociation channel. Comparisons with high precision experimental data show agreement within 1% for medium-strength lines. The new DMS and all the ab initio data are made available in the supplementary material.

A new spectroscopically determined potential energy surface (PES) for HD(16)O is presented. This surface is constructed by adjusting the high accuracy ab initio PES of Polyansky et al. [Science 299, 539 (2003)] by fitting to both published experimental data and our still unpublished data. This refinement used experimentally derived term values up to 25,000 cm(-1) and with J< or =8: a data set of 3478 energy levels once some levels with ambiguous assignment is excluded. To improve the extrapolation properties of the empirical PES, the restraint that the resulting PESs remain close to the ab initio surface was imposed. The new HDO&#95;07 PES reproduces the experimental data, including high J levels not included in the fit, with a root mean square error of 0.035 cm(-1). Predictions for rotation-vibration term values up to J=12 are made.

A valence-only (V) dipole moment surface (DMS) has been computed for water at the internally contracted multireference configuration interaction level using the extended atom-centered correlation-consistent Gaussian basis set aug-cc-pV6Z. Small corrections to these dipole values, resulting from core correlation (C) and relativistic (R) effects, have also been computed and added to the V surface. The resulting DMS surface is hence called CVR. Interestingly, the C and R corrections cancel out each other almost completely over the whole grid of points investigated. The ground-state CVR dipole of H(2) (16)O is 1.8676 D. This value compares well with the best ab initio one determined in this study, 1.8539+/-0.0013 D, which in turn agrees well with the measured ground-state dipole moment of water, 1.8546(6) D. Line intensities computed with the help of the CVR DMS shows that the present DMS is highly similar to though slightly more accurate than the best previous DMS of water determined by Schwenke and Partridge [J. Chem. Phys. 113, 16 (2000)]. The influence of the precision of the rovibrational wave functions computed using different potential energy surfaces (PESs) has been investigated and proved to be small, due mostly to the small discrepancies between the best ab initio and empirical PESs of water. Several different measures to test the DMS of water are advanced. The seemingly most sensitive measure is the comparison between the ab initio line intensities and those measured by ultralong pathlength methods which are sensitive to very weak transitions.

Water is predicted to be among the most abundant (if not the most abundant) molecular species after hydrogen in the atmospheres of close-in extrasolar giant planets ('hot Jupiters'). Several attempts have been made to detect water on such planets, but have either failed to find compelling evidence for it or led to claims that should be taken with caution. Here we report an analysis of recent observations of the hot Jupiter HD 189733b (ref. 6) taken during the transit, when the planet passed in front of its parent star. We find that absorption by water vapour is the most likely cause of the wavelength-dependent variations in the effective radius of the planet at the infrared wavelengths 3.6 mum, 5.8 mum (both ref. 7) and 8 mum (ref. 8). The larger effective radius observed at visible wavelengths may arise from either stellar variability or the presence of clouds/hazes. We explain the report of a non-detection of water on HD 189733b (ref. 4) as being a consequence of the nearly isothermal vertical profile of the planet's atmosphere.

The infrared spectrum of hot water observed in a sunspot has been assigned. The high temperature of the sunspot (3200 K) gave rise to a highly congested pure rotational spectrum in the 10-micrometer region that involved energy levels at least halfway to dissociation. Traditional spectroscopy, based on perturbation theory, is inadequate for this problem. Instead, accurate variational solutions of the vibration-rotation Schrödinger equation were used to make assignments, revealing unexpected features, including rotational difference bands and fewer degeneracies than anticipated. These results indicate that a shift away from perturbation theory to first principles calculations is necessary in order to assign spectra of hot polyatomic molecules such as water.

Quantum beat spectroscopy is combined with triple-resonance vibrational overtone excitation to measure the Stark coefficients (SCs) of the water molecule for 28 rovibrational levels lying from 27 600 to 41 000 cm-1. These data provide a stringent test for assessing the accuracy of the available potential energy surfaces (PESs) and dipole moment surfaces (DMSs) of this benchmark molecule in this energy region, which is inaccessible by direct absorption. SCs, calculated using the combination of a high accuracy, spectroscopically determined PES and a recent ab initio DMS, are within the 1% accuracy of available experimental data for levels below 25 000 cm-1, and within 4.5% for coefficients associated with levels up to 35 000 cm-1. However, the error in the computed coefficients is over 60% for the very high rovibrational states lying just below the lowest dissociation threshold, due, it seems, to lack of a high accuracy PES in this region. The comparative analysis suggests further steps, which may bring the theoretical predictions closer to the experimental accuracy. [ABSTRACT FROM AUTHOR]

The shift coefficients for the lines of the ν1 + ν2 + ν3 and ν2 + 2ν3 bands of H2O in the region from 9403 to 9413 cm−1 are measured and calculated. The measurements are performed using an intracavity laser spectrometer based on a neodymium laser with a determination error of the line center of 0.003–0.004 cm−1. The Ar, Kr, and Xe noble gases, as well as nitrogen, oxygen, and hydrogen were used as buffer gases. The coefficients of shifts in eight H2O absorption lines induced by oxygen, nitrogen, and atmospheric air pressures fall into the region from −0.004 to −0.069 cm−1/bar. The calculations are performed by a semiempirical method using variational wave functions, which, in contrast to other studies, correctly takes into account intramolecular interactions. The calculated values agree satisfactorily with experimental data. [ABSTRACT FROM AUTHOR]

The concept of quantum monodromy is introduced to give insight into the energy levels of systems with cylindrically symmetrical potential energy barriers. The K structure of bending progressions of bent molecules and the pendular states of dipolar molecules in strong electric fields are taken as molecular examples. Results are given for a two-dimensional champagne bottle model, for six computed bending progressions for water taken from the compilation of Partridge and Schwenke and for the quantal spherical pendulum. Sharp changes in the energy level patterns around the barrier energy are related to changes in the forms of the relevant classical trajectories. Analytical insight into the mathematical origin of the monodromy are also given. [ABSTRACT FROM AUTHOR]

Abstract: Water and carbon monoxide are two major compounds in the predominantly H2 atmospheres of stars, brown dwarfs and extrasolar planets. Recent radiative transfer models suggest that there is an urgent need for data on the pressure broadening of molecular transitions by H2 at high temperatures. We present a set of theoretical H2 pressure broadening parameters for 228H2O transitions in the range of 10–20,000GHz and 30 CO transitions in the 115–3500 GHz region. These parameters are appropriate for temperatures between 200 and 3000K. The random phase approximation is employed to derive the broadening parameters from recent state-of-the-art inelastic collisional rate coefficients. This approximation is compared both to full close-coupling calculations and to available experimental data. It is shown to be valid at temperatures above 200K, as expected from theoretical considerations, with an accuracy of about 25%. [Copyright &y& Elsevier]

Abstract: A comparison of the atmospheric absorption calculated with different data banks of water vapour absorption lines is made. The HITRAN database, Barber-Tennyson line list (BT2), calculation of Partridge and Schwenke (PS) are considered. The contribution of H2O lines, absent in HITRAN, to the atmospheric transmission, calculated with 10cm−1 spectral resolution in the 10000–20000cm−1 spectral region is up to 1.5% for a vertical path and 4% for a solar zenith angle of 70deg. The highest difference is observed in the 940nm band. The incoming fluxes of solar radiation, measured by a rotating solar spectroradiometer, were modeled with BT2 and HITRAN database. The difference between measured and calculated fluxes does not exceed the instrumental uncertainties. [Copyright &y& Elsevier]

Abstract: The high resolution absorption spectrum of monodeuterated water, HDO, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) in the 12145–13160cm−1 region. The achieved sensitivity (noise equivalent absorption on the order of α min ∼10−9 cm−1) allowed detecting transitions with line strengths as weak as 10−27 cm/molecule which is about 50 times lower than the weakest line intensities previously detected in the considered region. The rovibrational assignment of the 1179 lines attributed to the HDO isotopologue was based on the results of the variational calculations of Schwenke and Partridge as well as the recent calculations based on a new HDO potential energy surface refined from the fitting to the available experimental data. The overall agreement between these new calculations and the observed spectrum is very good, the rms deviation of the differences between the calculated and observed energy values being 0.05cm−1. A set of 304 new experimental HDO energy levels was obtained. In particular, band origins for the (122), (202), and (311) vibrational states, at 12568.190, 12644.652, and 12919.938cm−1, respectively, and their rotational sublevels are derived for the first time. A detailed HDO database of 1337 transitions was constructed and is provided as . [Copyright &y& Elsevier]

Abstract: The weak absorption spectrum of dideuterated water, D2O, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) between 11400 and 11900cm−1. This spectrum is dominated by the 3ν 1 + ν 2 + ν 3 and the ν 1 + ν 2 +3ν 3 centered at 11500.25 and 11816.64cm−1, respectively. A total of 530 energy levels belonging to eight vibrational states were determined. The rovibrational assignment process of the 840 lines attributed to D2O was mostly based on the results of new variational calculations consisting in a refinement of the potential energy surface of Shirin et al. [J. Chem. Phys., 120 (2004) 206] on the basis of recent experimental observations, and a dipole moment surface from Schwenke and Partridge [J. Chem. Phys. 113 (2000) 6592]. The overall agreement between these calculations and the observed spectrum is very good both for the line positions and the line intensities. [Copyright &y& Elsevier]