Ab initiocomputation of the broadening of water rotational lines by molecular hydrogen

Abstract

Theoretical cross sections for the pressure broadening by hydrogen of rotational transitions of water are compared to the latest available measurements in the temperature range 65-220 K. A high-accuracy interaction potential is employed in a full close-coupling calculation. A good agreement with experiment is observed above ∼80 K, while the sharp drop observed experimentally at lower temperatures is not predicted by our calculations. Possible explanations for this discrepancy include the failure of the impact approximation and the possible role of ortho-to-para conversion of H 2 .

Overtone vibrational spectroscopy in H2-H2O complexes: A combined high level theoretical ab initio, dynamical and experimental study

“…Several 5D "rigidrotor" surfaces have been obtained, either by averaging the 9D potential over vibrational wavefunctions of H 2 29 with the VGS and VAP potentials shown to provide very similar cross sections even at collision energies below 1 cm −1 . The high accuracy of these H 2 -H 2 O PES's has also been confirmed recently by a number of comparisons between theory and experiment including inelastic differential cross sections, 26 pressure broadening cross sections, 30,37 elastic integral cross sections, 18 and IR spectra of the complex. 15,17,21 In the present work, we use two different vibrationally averaged 5D PES's, with (i) both H 2 and H 2 O monomers in their ground vibrational state (as discussed in Valiron et al 1 ) and (ii) ground state H 2 (v = 0) and H 2 O in its doubly excited |02 − ) state, utilizing the wavefunction of Lodi and Tennyson.…”

“…On the other hand, many of the desired energy transfer and collisional properties can be calculated from first principles, based on knowledge of a sufficiently accurate potential energy surface in high dimensionality. 24,26,[28][29][30] The need for high level predictive understanding for such high pressure/temperature combustion phenomena is clearly important, which in turn depends pivotally on benchmark accuracy of the full 9D potential H 2 + H 2 O energy surface. 1,15,17,[20][21][22][23] Of particular interest in the interstellar medium (ISM) is the corresponding weakly bound complex, H 2 -H 2 O, due in part to predominance of atomic and molecular hydrogen in the universe.…”

Recommended isolated-line profile for representing high-resolution spectroscopic transitions (IUPAC Technical Report)

“…Atoms and molecules obey the laws of quantum mechanics and ab initio quantum mechanical treatments of water line broadening are available for low-temperature collisions [32], but more approximate treatments are required at higher temperatures [33]. This is because at atmospheric temperatures a molecule such as water has too many relaxation channels for a fully quantal treatment of the problem to be practicable.…”

Water in space

“…A prime example of this interaction are the cross sections for collisions of H 2 O with the main collision partner in clouds, H 2 , which are needed to translate the observed line intensities into abundances. Discrepancies between experiment and theory have recently been resolved thanks to intense discussions in the physics community, allowing astronomers to quantitatively analyze their data [8]. Accurate line frequencies are crucial for identification of new molecules in space.…”