Low-Energy Water–Hydrogen Inelastic Collisions

Abstract

New molecular beam scattering experiments are reported for the water−hydrogen system. Integral cross sections of the first rotational excitations of para-and ortho-H 2 O by inelastic collisions with normal-H 2 were determined by crossing a beam of H 2 O seeded in He with a beam of H 2 . H 2 O and H 2 were cooled in the supersonic expansion down to their lowest rotational levels. Crossed-beam scattering experiments were performed at collision energies from 15 cm −1 (below the threshold for the excitation to the lowest excited rotational state of H 2 O: 18.6 cm −1 ) up to 105 cm −1 by varying the beam crossing angle. The measured state-to-state crosssections were compared to the theoretical cross-sections (close-coupling quantum scattering calculations): the good agreement found further validates both the employed potential energy surface describing the H 2 O−H 2 van der Waals interaction and the state-to-state rate coefficients calculated with this potential in the very low temperature range needed for the modeling of interstellar media.

New H2O–H2O collisional rate coefficients for cometary applications

“…Para-water initial level: Ortho-water initial level: atom-molecule collisions (Dubernet et al 2013) used in astrophysical applications lack of experimental comparisons. Nevertheless recently was reported a new molecular beam scattering experiments for the water-hydrogen system (Bergeat et al 2020): the good agreement found with theoretical calculations validated both the employed potential energy surface (Faure et al 2005) describing the H 2 O-H 2 van der Waals interaction and the state-to-state rate coefficients (Dubernet et al 2006;Daniel et al 2011) calculated with this potential in the very low-temperature range needed for the modelling of interstellar media. This group has an excellent record (Bergeat et al 2015(Bergeat et al , 2018(Bergeat et al , 2019Chefdeville et al 2015;Stoecklin et al 2017;Klos et al 2018;) of producing useful comparisons to theoretical inelastic collisions that are of relevance to astrophysical applications, and hopefully they will be able to handle other collisional systems such as other moleculewater systems.…”

Three new barium dwarfs with white dwarf companions: BD+68°1027, RE J0702+129 and BD+80°670

“…Classical Ba stars are GK-type giant stars that show enhancement in carbon and heavy elements produced by the s-process nucleosynthesis, firstly discovered by Bidelman & Keenan (1951). The production of neutron capture elements in a star is expected to happen on the thermally pulsating phase of asymptotic giant branch (TP-AGB), but the luminosity estimations for Ba stars are below the threshold for the onset of TP-AGB to experience selfenrichment of s-process elements (Bergeat & Knapik 1997;Escorza et al 2017). McClure et al (1980) proposed a masstransfer scenario within a binary system with a possible companion WD (AGB progenitor) that could be responsible for s-process overabundances in these stars.…”

Probing Low-Energy Resonances in Water-Hydrogen Inelastic Collisions

“…Water is observed ubiquitously in the Solar System, in the Milky Way, and in the far Universe. Because of its large abundance in our own atmosphere, the main isotopologue, 1 H 2 16 O, is difficult to detect from the ground, except for some strong maser lines [1]. As a result, the rich rotational and rovibrational spectrum of water has been essentially observed from space, thanks to the (far) infrared space missions ISO, SWAS, Odin, Spitzer, and Herschel [2].…”

“…More recently, Bergeat et al [16] have measured relative integral cross sections (ICSs) for H 2 O colliding with normal-H 2 at much lower collisional energies, in the range 15-105 cm −1 . The agreement with theory was found to be very good.…”

“…Experiment.-We performed experiments in a crossed molecular beam apparatus [16,17], with variable collisional angle, under single collision conditions. A pulse of D 2 O seeded in helium was generated with an Even-Lavie valve maintained at 320 K. As the best collisional energy resolution is obtained when the two beams have almost the same velocity, for the para H 2 beam, a cryogenically cooled valve was used (see the Supplemental Material for details).…”