Schematic representation of the instrument used in the current study. Helium droplets are formed by expanding ultrahigh purity helium from a 5 μm pinhole nozzle into vacuum. The expansion is then skimmed to form a droplet beam. The droplets are doped with water molecules by directing the droplet beam through a pickup cell filled with water vapor. The doped droplets continue downstream to a time-of-flight mass spectrometer where the droplets are ionized by incident electrons from an electron gun. The resulting ions are focused into the flight tube and detected by a multichannel plate (MCP) detector.
Mass spectra of the helium droplet beam doped using several different pressures in the water pickup cell. Pickup cell pressure increases from bottom to top. Spectra were recorded at a mean droplet size of ∼25 000 atoms/droplet. Inset shows the dependence of the (H2O)H+ ion signal on the helium nozzle temperature. The pickup cell pressure was optimized at each nozzle temperature.
Droplet size thresholds observed for the detection of (H2O)+, (H2O)H+, (H2O)2H+, and (H2O)3H+ ions in the mass spectrometer. In each trace, the pickup cell pressure was re-optimized at each droplet size. Least-squares linear fits to the data points in the vicinity of each threshold are shown.
Observed droplet size thresholds for the (H2O)+ signal obtained with the water pickup cell located in two different port upstream of the mass spectrometer, along with linear extrapolation to zero pickup-ionizer distance.
Listing of the water multimers investigated, the ion signals monitored, and the observed droplet size thresholds for each. The differences between the thresholds of each multimer of order n and n-1 are listed. Subtracting N capture from the difference in threshold values yields the number of helium atoms evaporated to dissipate the binding energies of the clusters. N capture was taken to be 635 atoms (Ref. 9). Incremental and total binding energies calculated from these values are shown, along with theoretical values obtained from Refs. 16 and 17. Error bars are in parentheses. We note that the D0 values in the rightmost column were tabulated by adding the zero point energies calculated in Ref. 17 to the values of De listed in Ref. 16.
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