Expansion of a laser-produced silver plume in light background gases

The expansion of a silver ablation plume in a helium and an argon background gas has been studied over the pressure range 10(-6) to 1 mbar. The angular distribution of silver atoms deposited on an array of quartz-crystal microbalances as well as time-of-flight signals of the plume ions in both gases were measured. There is a distinct sharpening of the angular distribution of the ablated silver atoms in the helium gas within a narrow range from 0.2 to 0.4 mbar. In contrast, the width of the angular distribution in the argon gas increases throughout the interval. The time-of-flight signals show a peak of fast ions which gradually merges into a more complex structure.