Molecular Emissions from Stretched Excitation Pulse in Nanosecond Phase-Selective Laser-Induced Breakdown Spectroscopy of TiO₂ Nanoaerosols

In phase-selective laser-induced breakdown spectroscopy (PS-LIBS), gas-borne nanoparticles are irradiated with laser pulses (∼2.4 GW/cm²) resulting in breakdown of the nanoparticle phase but not the surrounding gas phase. In this work, the effect of excitation laser-pulse duration and energy on the intensity and duration of TiO₂–nanoparticle PS-LIBS emission signal is investigated. Laser pulses from a frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (532 nm) are stretched from 8 ns (full width at half maximum, FWHM) up to ∼30 ns at fixed pulse energy using combinations of two optical cavities. The intensity of the titanium atomic emissions at around 500 nm wavelength increases by ∼60%, with the stretched pulse and emissions at around 482 nm, attributed to TiO, enhanced over 10 times. While the atomic emissions rise with the stretched laser pulse and decay around 20 ns after the end of the laser pulse, the TiO emissions reach their peak intensity at about 20 ns later and last longer. At low laser energy (i.e., 1 mJ/pulse, or 80 MW/cm²), the TiO emissions dominate, but their increase with laser energy is lower compared to the atomic emissions. The origin of the 482 nm emission is explored by examining several different aerosol setups, including Ti–O, Ti–N, and Ti–O–N from a spark particle generator and Ti–O–N–C–H aerosol from flame synthesis. The 482 nm emissions are attributed to electronically excited TiO, likely resulting from the reaction of excited titanium atoms with surrounding oxidizing (carbonaceous and/or radical) species. The effects of pulse length are attributed to the shift of absorption from the initial interaction with the particle to the prolonged interaction with the plasma through inverse bremsstrahlung.