Acceleration of the conversion of nuclear spin isomers of water molecules under their vibrational excitation

Water molecules exist in nature in the form of nuclear spin isomers ortho-H$_2$O and para-H$_2$O with the total spin of hydrogen nuclei $I = 1$ and $0$, respectively. The effect of the vibrational excitation of the molecule on the conversion rate of nuclear spin isomers is studied in this work. It is found that the conversion of isomers is anomalously accelerated (by a factor of about $50$) if the normal vibration $v_2$ is excited. The spin conversion in the state $2v_2$ is accelerated by a factor of $260$ compared to the ground state of the molecule. The effect is due to the rotational distortion of the water molecule and to the appearance of levels of ortho- and para-isomers of H$_2$O with close energies in vibrational states $v_2$ and $2v_2$. The predicted acceleration of conversion in the vibrationally excited states of water molecules can be used to enrich their spin isomers by laser radiation.