Exploring the slow intracluster vibrational relaxation kinetics with two-color, IR-IR photoexcitation of cryogenically cooled water clusters*

Because the OH stretching quantum is close to the dissociation energy, vibrational excitation of the cold (20K) Iodide-HOD anionic complex in the OH stretching region explores the dynamics of a barely bound microscopic system. Here we examine the behavior of these levels by following the vibrational spectra of the states prepared by excitation of specific transitions in the cold complex in a two-color, IR-IR double resonance experiment. These results address the extent of IVR in the levels close to dissociation as well as the character of levels based on the OH stretching vibration that lie ~3000 wavenumbers above the onset of the dissociation continuum. Vibrational transitions from the OH(v=1) levels are observed to be quite sharp and are very long (tens of milliseconds) lived. This enables a quantitative exploration of the anharmonicities in the OH stretching degree of freedom far above the dissociation limit. We then discuss the application of these methods to follow the onset of spectral diffusion in the OH stretching spectrum of larger clusters with twenty water molecules. These clusters adopt pentagonal dodecahedron cage structures, and we find that the onset of spectral diffusion occurs at 100 K with large amplitude, rare events that occur with discrete jumps of about 300 wavenumbers. Surface melting is complete by 130 K or so.