Water ortho–para conversion by microwave background radiation in space

Abstract

ABSTRACT A theoretical model of water ortho–para conversion induced by blackbody radiation in space is developed. The model is based on two main ingredients: the mixing of water ortho and para states by a hyperfine spin-rotation interaction in the molecule and the interruption of this mixing by surrounding blackbody radiation. The model predicts the lifetime of water spin isomers τ = 2.7 Myr for radiation with a temperature of 100 K and τ = 1.3 Gyr for microwave background radiation. The time dependence of the ortho-to-para ratio (OPR) of water molecules interacting with microwave background radiation is determined. The temperature dependence of the stationary OPR predicted by the model is found to be different from the usually accepted OPR for water spin isomers.