Phase-selective laser-induced breakdown spectroscopy of metal-oxide nanoparticle aerosols with secondary resonant excitation during flame synthesis

Abstract

Low-intensity phase-selective laser induced breakdown spectroscopy (PS-LIBS) with secondary resonant excitation from the same single laser pulse is employed in the in situ study of flame synthesis of TiO₂ nanoparticles. Excitation from the third harmonic (i.e. 354.71 nm as measured) of an injection-seeded Nd:YAG laser breaks down flame-synthesized titanium-dioxide nanoparticles in situ into their elements and then subsequently resonantly excites the titanium electrons. With low-intensity laser excitation (∼30 mJ per pulse or 2.4 GW cm⁻²), only the nanoparticle phase is selectively broken down, forming nano-plasmas without any observable Bremsstrahlung radiation. The induced emission at 497.534 nm resulting from the secondary resonant Ti excitation is markedly stronger than other/primary emissions. Compared to 532 nm excitation (with no secondary resonant excitation), where atomic emission intensity saturates at a laser power of ∼20 mJ per pulse (1.6 GW cm⁻² irradiance), the emissions from 354.71 nm excitation exhibit no saturation with laser irradiance because of the secondary resonant excitation, until gas-phase breakdown occurs at a laser power of 40 mJ per pulse (3.2 GW cm⁻² irradiance). Temporal evolutions of the emissions are also studied, revealing that the 497.534 nm emission maximizes earlier, but is shorter lived. Interestingly, transient line splitting with dependence on excitation laser wavelength is observed and investigated. Additional experiments scanning the excitation laser wavelength around other resonant excitation lines show that the emission intensity can be enhanced by about 130 times at 282.347 nm excitation. Thus, using the secondary resonant excitation effect, detection thresholds could be remarkably improved for phase-selective measurements of nano-aerosols.