Effect of ambient pressure on microwave-enhanced laser-induced plasma: Influence on signal stability and intensity at different laser energies.

The stability and intensity of microwave-enhanced laser-induced breakdown spectroscopy (ME-LIBS) are significantly influenced by laser energy. The ME-LIBS signal exhibits high enhancement at low laser energies, but with poor repeatability. For higher laser energies, the enhancement decreases, and the ME-LIBS signal shows a nonlinear dependence on increasing laser energy. In this study, various laser energies (5-40 mJ) were used to evaluate the effects of different pressures (1-100 kPa) on ME-LIBS emission intensity and signal repeatability of the titanium sample. At low laser energy, the results reveal that the signal relative standard deviation (RSD) was noticeably reduced from ∼65% at 100 kPa to ∼ 13% at 20 kPa. Whereas, at high laser energy, significant improvement has been observed in the ME-LIBS signal intensity at 5 kPa. The optimal pressure shifts towards lower values with the increase in laser energy, i.e., ∼20 kPa suitable for stable reheating at 5 and 10 mJ, 10 kPa for 15 mJ, and 25 - 40 mJ favors ∼5 kPa for efficient microwave coupling. Temporal plasma imaging further showed that reduced pressure stabilized the plasma morphology and enhanced microwave-plasma coupling for low and high laser energies. At normal pressure, the ME-LIBS signal suffers from strong signal fluctuations, particularly for low laser energies, due to the intrinsic instability of weak laser-induced plasmas (LIP). Furthermore, the ME-LIBS signal also showed lower intensity at high laser energies because of the excessive electron density of LIP that reflect microwave radiation. This work demonstrates that low ambient pressure simultaneously enhances plasma stability and microwave absorption efficiency, effectively resolving the high-RSD issue at low laser energy and low-enhancement limitation at high laser energy in ME-LIBS.