VACUUM-ULTRAVIOLET EXCITATION OF LARGE WATER CLUSTERS

The fluorescence excitation spectrum of water clusters containing up to 1500 water molecules has been studied using synchrotron radiation in the spectral region 60-250 nm. Fluorescence is only observed for excitation of the water clusters in the region 100-140 nm. Water clusters do not fluoresce for excitation wavelengths lower or higher than this range. The fluorescence lies in the spectral region 200-420 nm and is peaked at 300-350 nm. The fluorescence excitation spectrum closely resembles that for free water molecules, and we see no shifts in the spectrum relative to gaseous molecules. It is suggested that the fluorscence comes from Rydberg-excited water molecules lying at the surface of the cluster that are photodetached and predissociate into fluorescent fragments. Below 100 nm, the clusters photoionize to give protonated cluster ions rather than electronically excited monomer ions as in the gas phase. We see no evidence for emission similar to that recently reported for UV excitation of ice crystals [Matich, A, J.; et al. J. Phys. Chess. 1993, 97, 10539].