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

Abstract

Laser-induced fluorescence ͑LIF͒ 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( 1 P 1 ) state is observed, aside from the deexcited fluorescence at 251.6 nm from the 4s( 3 P 2 ) state. The coincidence of the 288 nm fluorescence and the 251 nm fluorescence strongly indicates that the Si atoms in the 4s( 3 P 2 ) 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( 3 P 2 ) 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 sϽt d Ͻ100 s͒, and the 288 nm fluorescence originating from the droplets ͑probably microparticles͒ is observed instead. Since droplets moving at ϳ100 m/s are fragmented by the probe beam, the collisional excitation among these fragmented atoms can generate Si atoms in the 4s( 1 P 1 ) state.

Internal state populations and velocity distributions of monatomic species ejected after the 1064 nm laser irradiation of barium

“…Similar values have been reported previously for the propagation velocities of macroparticles ejected from various target materials and using very different laser wavelengths. [36][37][38] Consistent with other recent studies, 39 it is suggested here that explosive boiling is the most plausible formation mechanism for such apparently delayed macroparticles following nanosecond laser irradiation of barium. Substantial interactions of the nascent plume with the ablation laser radiation can also be anticipated 18 at the present range of incident laser fluences, which is within the saturation region of the ablated monatomic Ba mass ͑see Fig.…”

Comment on “Evidence for phase-explosion and generation of large particles during high power nanosecond laser ablation of silicon” [Appl. Phys. Lett. 76, 783 (2000)]

“…It is interesting to note that the ejection of droplets was detected during laser ablation of Si at 1.06 m for an irradiance as low as 0.11 GW/cm 2 . 5 A very large quantity of droplets, which are described as ''debris round and smooth, and appears as if it has resolidified from liquid droplets'' was observed after laser ablation of Si by an ArF excimer laser at an irradiance of 0.28 GW/cm 2 , only four times higher than the ablation threshold at that wavelength ͑see One should regard the results of computer simulation with caution because the physics of the ablation process is still not fully understood and the optical and thermophysical properties of liquid materials high above their melting points and close to the critical temperature ͑or spinodal temperature ϳ0.8-0.9 T c ͒ are poorly known. Even a much simpler problem, that of predicting the laser heating and melting of Si with nanosecond-long laser pulses is still a matter of debate.…”

“…To explain the droplet emission reported at much lower irradiance values [3][4][5][6] it seems necessary to accept that the phase-explosion mechanism could appear at much lower irradiance levels. For example, Song and Xu 8 suggested that for Ni this should occur at irradiance levels around ϳ0.2 GW/cm 2 .…”

High-enthalpy test environments, flow modeling and in situ diagnostics for characterizing ultra-high temperature ceramics

“…Both B atoms 71 and BO molecules 72,73 have been detected by LIF schemes, as have Si atoms 74,75 and SiO molecules. 74, 76 Feigl and Auweter-Kurtz 77 have used LIF to monitor SiO concentrations in front of SiC materials oxidized in argon-oxygen-nitrogen plasma flows, by exciting the SiO (A l Π ← X l Σ + (0, 0)) transition near 234 nm and collecting fluorescence near 260 nm from the (A l Π → X l Σ + (0, 3)) band.…”