Preferential adsorption of para and ortho water molecules on charged nanoparticles in planetary ice clouds

In the Earth mesopause, nanometer-size singly charged particles form by condensation of evaporated meteorite material. Tey exhibit an enhanced water adsorption cross section due to the strong charge-dipole-interaction. In this work, we study how the nuclear spin state of water molecules afects this enhancement and whether there are conditions that could lead to the formation of spin-polarized ice. Due to symmetry constraints on the total molecular wave function, ortho (proton spins parallel) and para (spins antiparallel) water occupy
diferent rotational states, resulting in a diferent average dipole orientation in
electric felds. Terefore, we expect ortho and para water to exhibit distinct ad-
sorption enhancement factors onto charged nanoparticles. Based on Stark-shifs
of individual rotational states of water, average dipole orientations of a molecu-
lar ensemble and the resulting collision cross section was calculated for various
temperatures and particle sizes. We found that in the mesosphere of the Earth
(T~150K) the adsorption enhancement of ortho- and para- water is approxi-
mately equal while at lower temperatures prevailing around ice giant planets and
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their moons, signifcant spin polarizations up to 15% occur.