Vacuum ultraviolet excitation of large water clusters

Vacuum Ultraviolet (VUV) Photoionization of Small Water Clusters

Tunable vacuum ultraviolet (VUV) photoionization studies of water clusters are performed using 10-14 eV synchrotron radiation and analyzed by reflectron time-of-flight (TOF) mass spectrometry. Photoionization efficiency (PIE) curves for protonated water clusters (H2O)(n)H+ are measured with 50 meV energy resolution. The appearance energies of a series of protonated water clusters are determined from the photoionization threshold for clusters composed of up to 79 molecules. These appearance energies represent an upper limit of the adiabatic ionization energy of the corresponding parent neutral water cluster in the supersonic molecular beam. The experimental results show a sharp drop in the appearance energy for the small neutral water clusters (from 12.62 +/- 0.05 to 10.94 +/- 0.06 eV, for H2O and (H2O)4, respectively), followed by a gradual decrease for clusters up to (H2O)23 converging to a value of 10.6 eV (+/-0.2 eV). The dissociation energy to remove a water molecule from the corresponding neutral water cluster is derived through thermodynamic cycles utilizing the dissociation energies of protonated water clusters reported previously in the literature. The experimental results show a gradual decrease of the dissociation energy for removal of one water molecule for small neutral water clusters (3 <or= n <or= 9). This dissociation energy is discussed within the context of hydrogen bond breaking in a neutral water cluster.

Theoretical study of photoacidity of HCN: the effect of complexation with water

The character of the hydrogen bonding and the excited state proton transfer (ESPT) in the model system HCN...H(2)O is investigated. The PES of the two lowest excited states of the H(2)O...HCN complex was calculated using the CASPT2 method. The nonadiabatic coupling of the two states of the (pi-->pi*) and (pi-->sigma*) character is responsible for the excited state proton/hydrogen transfer. Compared to the ground state, the barrier for this process is significantly smaller. An increased number of water molecules in the complex with cyclic hydrogen-bonded network causes a large blue shift of the state of the (pi-->sigma*) character. The question of the dissociation of the complex in its excited state is also addressed.

The role of dimers in evaporation of small argon clusters

Evaporation of small Lennard-Jones argon clusters has been studied using molecular dynamic simulations. An extensive library of clusters with 4, 5, 6, 11, and 21 atoms has been obtained from an earlier study. Analysis of the evaporation properties of the clusters indicate, that the fraction of dimer evaporations of all evaporation events increases with the total energy of the cluster. The fraction of evaporated dimers from clusters with a constant lifetime is independent of the cluster size for short-lived clusters and increases with cluster size for long-lived clusters. Only a few percent of the clusters which are long lived enough to participate in vapor-liquid nucleation decay by emitting dimers. The mean cluster lifetime as a function of total energy shows the same exponentially decreasing trend for monomer and dimer evaporation channels. The fraction of trimer evaporations is found to be vanishingly small.

Luminescent photoproducts in UV-irradiated ice

This Account describes the near-UV and visible luminescences emitted from crystalline, polycrystalline, and amorphous ices as a result of excitation by UV light. Vibrationally resolved, short-lived luminescence around 340 nm arises from excited O(2) formed by the reaction of two O atoms. Long-lived luminescence around 420 nm is tentatively assigned to a spin-forbidden (4)sigma(-) --> X(2)Pi transition of OH. This Account gives a history of the research into this little-known phenomenon, places it in the context of other spectroscopic studies of gaseous and solid water, and proposes future directions for the work.