Vibrational predissociation spectra and dynamics of small molecular clusters of H₂O and HF

Experimental results are presented for the vibrational predissociation spectra in the frequency range 3000-4000 cm^-1 for the species (HF)_n and (H²O)_n, n = 2-6, using molecular-beam techniques and a tunable infrared laser. The observed spectra show a dramatic change between the dimer and larger clusters which is thought to be a result of the cyclic structure of the trimer and larger clusters. The spectra are compared with calculated harmonic force constants of available intermolecular potentials to understand how these small, gas-phase clusters relate to the liquid and solid phases of HF and H₂O. Additionally, the angular distributions of the predissociation products show that little energy appears as translational motion of the fragment molecules. This conclusion is consistent with recent theoretical models of the predissociation process. An upper limit of ca. 2 μs is observed for the lifetime of the vibrationally excited clusters.