Laser-induced fluorescence from collisionally excited Si atoms in laser ablation plume

Abstract

Laser-induced fluorescence (LIF) from neutral Si atoms in a laser ablation plume is investigated using a probe laser beam at 251.6 nm. Fluorescence at 288 nm from the 4s(1P1) state is observed, aside from the deexcited fluorescence at 251.6 nm from the 4s(3P2) state. The coincidence of the 288 nm fluorescence and the 251 nm fluorescence strongly indicates that the Si atoms in the 4s(3P2) state are responsible for the 288 nm fluorescence. The 288 nm LIF signal is detectable only when the probe laser beam passes near the Si surface, and has maximum intensity for a time delay of 20 ns. The 288 nm LIF could be emitted when the Si atoms in the 4s(3P2) state, pumped by the probe laser, collide with other Si atoms in the gas phase, since a high-density gas phase of ejected particles exists near the surface. The LIF intensities from the ablated Si atoms decrease for large time delays of the probe laser (0.2 μs<td<100 μs), and the 288 nm fluorescence originating from the droplets (probably microparticles) is observed instead. Since droplets moving at ∼100 m/s are fragmented by the probe beam, the collisional excitation among these fragmented atoms can generate Si atoms in the 4s(1P1) state.