Dissociation and suppressed ionization of H2O molecules embedded in He clusters: The role of the cluster as a cage.

Electronic structure and energy transfer in H2O doped HeN clusters (N≈10)4 is studied with photoexcitation in the spectral range of 140–40 nm (9–30 eV). The reaction dynamics is investigated by fluorescence of neutral OH* and H* and ionic H2O+ fragments. The rotational temperature of embedded water molecules has been estimated from the 124 nm line shape (3pa1 C1 B1←1b1 X˜1 A1 transition!. Two different temperatures (T1≤5 K and T2≈30K) have been found. We propose that the lower temperature (T1) is due to completely thermalized water molecules trapped inside helium clusters, while the warmer molecules (T2) are formed if they are first captured by helium clusters but then leave the clusters again. Predissociation of H2O with excitation below the ionization limit (λexc.100 nm) is found to be unaffected by the cluster environment. On the other hand, the ionization (λexc,100 nm) seems to be suppressed inside helium clusters in favor of the fragmentation into neutral products.