Planar laser-induced fluorescence (PLIF) is used to monitor Cu atoms and Cu2 produced by excimer laser ablation of a copper target (308 nm, ≳10 J/cm2, 1–3 GW/cm2) expanding rapidly into helium background gas at pressures ranging from 10 to 100 Torr. The Cu2 results from gas phase condensation of the copper atoms ablated from the target in the regions of highest Cu atom density as expected, but the maximum Cu2 laser-induced fluorescence (LIF) signal occurs significantly after the maximum of the Cu signal. Rotationally resolved excitation scans of Cu2 utilizing the A–X (0,0) transition indicate that the Cu2 has reached equilibrium with the 300 K background gas. An extensive search for Cu3 via LIF failed, indicating that Cu3 is present only in very low ‘‘steady state’’ number density in the plume. This data is explained qualitatively by a simple kinetic model. In addition to the kinetic information, it is clear from the PLIF images that viscous eddy formation becomes more pronounced as the backing gas pressure increases; however, we see no evidence of turbulence in the plume even at the highest backing gas pressure studied. The PLIF technique allows us to observe the onset of condensation directly as well as to obtain information about the expansion dynamics of the plume not easily obtainable by other means.
Hook spectroscopy has been implemented to measure the absolute density of atomic species in a laser-ablated plasma plume. We use the hook method to determine the density of copper atoms produced by ablation of a copper target in 25 Torr of helium at power densities of ∼1 GW/cm2. The measured copper densities when ablating into 25 Torr of helium backing gas range from 1.5×1015 cm−3 at a delay of 200 μs to 4.4×1013 cm−3 at 5 ms delay. This dramatic decrease in density is due to condensation of the metal vapor to form fine particulate.
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