Time-of-flight high-mass spectrometer observation of large size Nb clusters toward assembling of size controlled clusters

Using a plasma-gas-condensation-type cluster source, we could obtain nano-s cale clusters with the average diameter between 2 and 13 nm. In order to de tect such large free clusters containing 1-10(5) atoms, we have constructed a time-of-flight high-mass spectrometer (TOF-HMS). The size distribution o f the Nb-n clusters observed by the TOF-HMS is in good agreement with the r esult of the transmission electron microscope observation. The mass spectra have been measured with varying the operational parameters of the cluster source. They are shifted to the smaller size direction and the mass distrib ution becomes narrower with increasing the He gas flow rate, V-He, and keep ing the Ar gas flow rate, V-Ar, constant. These features are attributable t o the decrease in the residence times of gaseous species in the cluster sou rce. The cluster size increases with increasing the Ar gas pressure and low ering the temperature of the cluster source. (C) 1999 American Vacuum socie ty. [S0734-211X(99)04005-6].