Observation of the low frequency spectrum of water dimer as a sensitive test of the water dimer potential and dipole moment surfaces

The intermolecular interaction of bulk water is dominated by pairwise and non-pairwise cooperative interactions, with the later accounting for ca. 25 % of the total Energy. Whereas by now accurate descriptions of the pairwise interaction are available, which can be tested by precise low frequency spectra of water dimer over the entire range up to 550 cm -1 , the same does not hold for the three-body interactions. Here, we report the first comprehensive spectrum of the water trimer in the frequency region from 70 to 620 cm -1 , using our helium nanodroplet isolation setup at the free-electron lasers FELIX in Nijmegen. By comparison to accompanying high level quantum calcualtions, we are able to assign the experimentally observed intermolecular bands. The transition frequencies of the degenerate translation, the degenerate in-plane, the non-degenerate out-of-plane libration as well as additional bands of the out-of-plane librational mode are reported for the first time. These provide a benchmark for state-of-the art water potentials and dipole moment surfaces, especially in respect to the three-body interaction. Furthermore, we are able to deduce the size of the torsional splitting in the excited intermolecular modes, which increase upon vibrational excitation.