Scaling law of angular emission distributions of laser ablated particle pulses from monoatomic and compound targets

In the present experiment we have investigated angular emission distributions of freely expanding laser produced plasma pulses quantitatively and over a wide mass range (27-184) both for monoatomic and binary species. The following major results have been obtained: (1) All emission shapes can be closely fitted by Gauß-functions. (2) The emission cones of all species steepen continuously as a function of the (average) atomic mass A. Thereby the widths can be well described by a hyperbolic behavior k/A + c, whereby k and c systematically depend on the size of the ablation area. (3) The measured emission widths of the compound systems agree with averaged data of its single pure components if conservation of stoichiometry is assumed in the calculation. As an essential consequence, e.g. for practical applications, the results allow predictions of emission distributions of monoatomic species hardly to measure (e.g. gases) and of compound systems of arbitrary compositions. Specimen targets have been monoatomic Al, Ti, Cu, Mo, W and their compounds TiAl, MoCu and WCu with stoichiometric mass ratios of 25/75, 34/66, 50/50 and 75/25 for TiAl, 70/30 for MoCu and 60/40, 80/20 and 90/10 for WCu, respectively. Particles were ablated by obliquely incident Q-switched laser pulses (TEM00, EL = 125 mJ, τ = 5 ns and λ = 1.06 μm) focused to a focal area of 0.3 mm2 corresponding to a focal intensity of about 1010 W/cm2.