Time resolved and size selective infrared-spectroscopic studies on the structure and dynamics of neutral (water) clusters generated in supersonic expansions

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The isomerism and the dynamics of water clusters reflect on the microscopic scale the special properties of macroscopic water that enable the existence of life as we know it. In this project an infrared-spectroscopy approach is pursued for characterizing size-selectively neutral water clusters exploiting the nanosecond cluster dynamics after IR excitation for isomer discrimination. For larger clusters a similar isomer selectivity has been so far only achieved for ionic clusters. The method utilizes the supersonic expansion technique and relies on the special, temperature dependent interaction of sodium atoms with water clusters enabling their soft single photon ionization. The spectroscopic data accumulated in the this project will provide references for benchmarking and improving water model potentials in size ranges so far difficult to access. Central goals are the determination of the critical lower size for the emergence of a crystalline core in water clusters, the detection of (H2O)n isomers of high stability in the size range n=20-60 and the exploration of the presence of temperature range in which liquid and crystalline water clusters can coexist, a so far only theoretically predicted water cluster property.