Hyperfine coupling and large amplitude motions interaction in the water dimer

Experimental and theoretical analyses of the hyperfine structure of the non-rigid water dimer (H2O)2 were carried out. Measurements were performed with an FT-MW spectrometer allowing us to resolve most of the hyperfine components. Eight hyperfine patterns were recorded involving rotational-tunneling sublevels with J ⩽ 2. Hyperfine patterns were analyzed accounting for the magnetic spin-rotation and spin-spin hyperfine couplings. Symmetry adapted nuclear spin wavefunctions were built to account for the interaction of the hyperfine coupling with the large amplitude motions displayed by the water dimer and to build total rotational-tunneling-hyperfine wavefunctions obeying the Pauli exclusion principle. This accounts for the strong dependence of the hyperfine patterns on the symmetry species of the rotational-tunneling sublevels. These theoretical results were used to perform individual and global analyses of the hyperfine patterns. The latter analysis yields values for some of the components of the spin-rotation coupling tensors.