ELECTRON DIFFRACTION STUDIES OF CLUSTERS FORMED IN SEEDED NOZZLE BEAMS

A multistage molecular beam has been constructed to provide a continuous supply of clustered species into a 10-8 torr scattering chamber where the beam is crossed with a 40 keV electron beam. Diffraction patterns from the molecular beam are magnetically scanned over a scintillator-photomultiplier detector. The molecular beam is mechanically chopped and patterns taken in either an analog mode using a lock-in amplifier, or in a digital pulse counting mode. The pulse counting scheme provides equal time windows when the molecular beam is on and off eliminating the shutter opening and closing parts of the cycle. Since one scattered high energy electron can produce many photons, single photon noise events can be discriminated against using a pulse height analyzer. Experiments are carried out in two modes. One is the conventional mode of expanding an unsaturated gas mixture through the nozzle where the nucleation occurs in the free jet expansion. The other mode, for metals, consist of mixing with a cold carrier gas just prior to the nozzle expansion causing nucleation before the free jet formation. The mean cluster size is varied by changing the temperature, pressure and condensable species mole fraction. Diffraction patterns, for H2O and for CO2 in the former mode, and Ag plus Ar in the latter mode, will be presented and the resultant cluster size and structure discussed