A pulsed valve connected to a closed-cycle cryostat was optimized for producing helium droplets. The pulsed droplet beam appeared with a bimodal size distribution. The leading part of the pulse consists of droplets suitable for doping with molecules. The average size of this part can be varied between 10(4) and 10(6) helium atoms, and the width of the distribution is smaller as compared to a continuous-flow droplet source. The system has been tested in a single pulse mode and at repetition rates of up to 500 Hz with almost constant intensity. The droplet density was found to be increased by more than an order of magnitude as compared to a continuous-flow droplet source.

Computer simulations and experiments have been performed to quantify the effects of nozzle shape and skimmer placement on high-density supersonic jets. It is shown that the on axis beam intensity achieved is much higher than intensity achieved using standard sonic nozzles. Changes in skimmer design and positioning are required to allow this intense jet to propagate in a typical supersonic beam setup.

We present a new excitation source for pulsed supersonic beams. The excitation is based on dielectric barrier discharge in the beam. It produces cold beams of metastable atoms, dissociated neutral atoms from molecular precursors, and both positive and negative ions with high efficiency and reliability.

Experimental results for the hypersonic impact of singly charged, mass selected, water clusters at a hard surface demonstrate cooling of the heated cluster by sequential loss of a water molecule. The trends in the experiment are most simply summarized as an evaporation. This behavior is in contrast to the shattering phenomena observed for rare gas clusters. The same information theoretic approach that predicted the shattering transition is fully consistent with the results for the water clusters. It shows that the relatively high binding energy per water monomer is the key reason for the anomalous fragmentation. The tetra coordinated oxygen atom in a cluster does however imply that the fragmentation is not a pure evaporation but has residual elements of a shattering transition. For water clusters the transition is broadened to span the finite range in energy between the onset of boiling and the energy when the solvation of the hydronium ion is over and it is bare.