A first step for the understanding of disequilibrated ortho to para ratios found in space : studies of nuclear spin conversion of H_{2}O in rare gas matrices

Details A first step for the understanding of disequilibrated ortho to para ratios found in space : studies of nuclear spin conversion of H_{2}O in rare gas matrices A first step for the understanding of disequilibrated ortho to para ratios found in space : studies of nuclear spin conversion of H_{2}O in rare gas matrices

Nov 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sf2a.conf..337p&link_type=abstract

"SF2A-2008: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics Eds.: C. Charbonnel, F. Combes

Astronomy and Astrophysics

Astrophysics

Scientific paper

%Water and other molecules of astrophysical interest like H_{2}, H_{2}O, or H_{2}CO, exist in two nuclear spin species. They are called ortho and para depending if the spins of the protons are parallel (total nuclear spin I=1) or anti-parallel (total nuclear spin I=0). In gas phase, each rotational state is associated with only one of the nuclear magnetic species and in the high temperature limit (>50 K), it is known that 1/4 of the molecules are para while 3/4 are ortho. Below 50 K, the ortho-to-para ratio at equilibrium becomes strongly temperature dependent. From the ortho-to-para abundance ratios of molecules measured in cometary comae [1, 2] or in dark clouds [3], it is expected to determine the formation conditions of molecules in space, and especially the formation temperature.
As a first step before studying ices of astrophysical interest, we have investigated the parameters involved in the nuclear spin conversion of water isolated in rare gas matrices at low temperatures (4.2 K). In these environments, the water molecule rotates almost freely and is able to perform translational oscillations within the cage made of rare gas atoms. We present here a study, in the mid-infrared, of H_2O in neon, argon, krypton and xenon matrices. In all the matrices, we observed an acceleration of the nuclear spin conversion as the concentration of water in the sample increased. Calculations performed by our group show clearly that intermolecular magnetic interactions are responsible for this concentration dependence. The intermolecular process is found to slow down from neon to xenon because the lattice parameter continuously increases from neon to xenon. For diluted samples we measured times between 100 and 700 minutes depending upon the rare gas atom. It is then surprising that these times observed in cryogenic matrices are much shorter than months estimated in ice by Tikhonov & Volkov (2002) at 77 K.

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