Clusters formed in laser-induced ablation of Si, SiC, Pt, UO2 and evaporation of UO2 observed by laser ionization time-of-flight mass spectrometry and scanning tunneling microscopy

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Abstract
Cluster formation is traditionally observed by mass spectrometry, which has the disadvantage that the detection sensitivity often decreases with increasing mass. Alternatively, one may collect the clusters onto an atomically flat substrate and identify them by scanning tunneling microscopy (STM). Both techniques were used here. For the first technique, a Nd:YAG laser (frequency quadrupled to 266 nm, 5 ns pulse width) focused onto spots of 4-100-mu-m diameter was used to ablate refractory materials, and a reflectron time-of-flight tube served to mass-analyze the plumes. The observed mass spectra for Si, Pt, SiC, and UO2 varied in the distribution of ablation products among atoms, molecules, and clusters, depending on laser power density and target material. For the second technique, cleaved surfaces of highly oriented pyrolytic graphite were positioned either 10 cm away from materials ablated at 10(-5) Torr by 1-3 excimer laser (308 nm) pulses of 20 ns duration, or 1 m away from materials vaporized at 10(-8) Torr by 10 Nd:Glass laser pulses of 1 ms duration. The deposited material was then analyzed by STM in air. Clusters up to 30 angstrom in diameter were observed for all materials, making the use of STM at room temperature for mass analysis questionable.

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