Time-of-flight mass spectrometry of nanometer diameter metal clusters

A Time-of-Flight Mass Spectrometer (TOFMS) designed to determine the size distributions of nanometer diameter metal clusters produced in a MECS source was built and tested. This TOFMS is capable of analyzing cluster samples deposited on a substrate as well as samples obtained from a cluster beam. Aerodynamic deceleration is used to reduce cluster velocity when analyzing clusters from the beam. A two-stage detector is used to allow the detection of very large cluster ions. This detector operates by accelerating the ions through a 25kV potential and allowing them to collide with a stainless steel target. The secondary electrons emitted from the target are focussed by electric and magnetic fields onto a multi channel plate (MCP). Threshold averaging of MCP spectra is used to improve the signal-to-noise ratio. Studies of Gold clusters both by direct sampling from the beam and by desorption of samples deposited on substrates suggest the existence of a precursor cluster formed in the multiple expansion cluster source used to produce the gold clusters for this study. It is believed that this precursor cluster is a regular truncated octahedron with four atoms on an edge.