Fast nuclear spin conversion from \textit{para}-H$_{2}$O to \textit{ortho}-H$_{2}$O: A matrix isolation study in solid argon

Single water molecules have been isolated in solid Ar matrices at 4 K and studied by ro-vibrational spectroscopy using FTIR in the regions of the $\nu _{1}$, $\nu _{2}$, and $\nu _{3}$ modes. Upon nuclear spin conversion at 4 K, essentially pure \textit{para}-H$_{2}$O was prepared followed by subsequent fast annealing generating ice particles. FTIR studies of the vapor above the condensed water upon annealing to T $\ge $ 250 K indicate fast re-conversion of nuclear spin to equilibrium conditions. Our results indicate that nuclear spin conversion is fast in water dimers and larger clusters, which precludes preparation of concentrated samples of para-water, such as in ice or in vapor.