Communication: Mapping water collisions for interstellar space conditions

Abstract

We report a joint experimental and theoretical study that directly tests the quality of the potential energy surfaces used to calculate energy changing cross sections of water in collision with helium and molecular hydrogen, at conditions relevant for astrophysics. Fully state-to-state differential cross sections are measured for H2O–He and H2O–H2 collisions at 429 and 575 cm−1 collision energy, respectively. We compare these differential cross sections with theoretical ones for H2O+H2 derived from state-of-the-art potential energy surfaces [P. Valiron et al., J. Chem. Phys. 129, 134306 (2008)] and quantum scattering calculations. This detailed comparison forms a stringent test of the validity of astrophysics calculations for energy changing rates in water. The agreement between theory and experiment is striking for most of the state-to-state differential cross sections measured.

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

“…As a result, in our opinion, the water-hydrogen V08 PES is once more successfully tested, with a special emphasis on the mid-to long-range region of the PES and the isotropic part. Thus, here we complement various tests performed recently on the water-hydrogen system, such as differential measurements [4] and molecular beam scattering experiments [5], which so far have all confirmed the high accuracy of the V08 PES. Another series of experiments, now under way, would aim at the spectroscopy of the bound H 2 O-H 2 van der Waals molecule.…”

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.…”

“…2,[22][23][24][25][26] In high temperature H 2 /O 2 combustion, for example, collisions of the highly energized product H 2 O molecules with H 2 reactant provide an important mechanism for heat flow as well as establishing local thermodynamic equilibrium in the flames. 3 As a result, there is a broad set of energy dependent cross sections and temperature dependent kinetic rates for such processes at the stateto-state level that prove critical in achieving the desired optimal combustion efficiency.…”

State-to-state differential and relative integral cross sections for rotationally inelastic scattering of H2O by hydrogen

“…1 We assume that both water and hydrogen molecules are rigid bodies, at their average geometries in their respective ground vibrational states. This approximation is valid for the collision energies studied here (< 600 cm --1 ), since the first vibrational transition, the H2O bending mode is at 1595 cm --1 , resulting in a clear separation between the rotational and vibrational energies.…”