Assignment of H2O-HF rotational lines via ab initio calculations.

Significant progress in the assignment of rotational transitions in the H2O-HF dimer using ab initio calculations is described. For the first time, rovibrational calculations are performed up to J = 6 using a full-dimensional Hamiltonian, including all intra- and inter-molecular interactions. Previous work had assigned lines of H2O-HF belonging to the ground state and one excited vibrational state νβ(o) with Ka ≤ 4. Two existing spectra covering the J ranges 12 to 24 and 10 to 26 are re-analyzed using the new ab initio calculations, yielding assignments and corresponding rotational constants for 25 more vibrational state Ka combinations. In particular, five new vibrational states, νβ(i), νσ, 2νβ(o), νσ + νβ(o), and νβ(o) + νβ(i), are assigned for Ka = 0 and several excited Ka values. Differences between the predicted, ab initio rotational B constants and the experimentally determined ones are systematic and about 40 MHz, which serves as confirmation of the correctness of these assignments.