Identification of hydrocarbon molecules using emission spectra of femtosecond laser-induced plasma

Hydrocarbon molecular species are identified, and their concentrations are measured using femtosecond laser-induced breakdown spectroscopy (fs-LIBS). For comparison, conventional nanosecond laser-induced breakdown spectroscopy (ns-LIBS), which is sensitive only to atom compositions and not to molecular species, is conducted simultaneously. Hydrocarbon-containing gas mixtures are prepared by mixing methane (CH₄), ethylene (C₂H₄), propane (C₃H₈), carbon dioxide (CO₂), oxygen (O₂) and nitrogen (N₂). While the molecular compositions of the mixtures vary, the atomic mole fractions of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) are kept constant within each set of prepared mixtures to evaluate the molecule detection capabilities of fs- and ns-LIBS techniques. Laser pulses from a Ti:Sapphire femtosecond laser (pulse duration 35fs) and a nanosecond-pulsed Nd:YAG laser (pulse duration 9 ns) are focused on the gas mixtures to induce laser-induced plasmas (LIP). In the fs-LIP spectra, CN violet bands (B²Σ-X²Σ,Δυ=0,-1,+1; 370-395 nm, 350-365 nm, 410-425 nm, respectively), C₂ swan bands (d³Π_g-a³Π_u,Δυ=0,-1,+1; 500-520 nm, 450-480 nm, 540-570 nm, respectively), and a CH radical peak (A²Δ-X²Π; 430 nm) are observed, which are sensitive to the molecular composition. Conversely, all the ns-LIP spectra, dominated by atomic emission lines, are nearly identical when the atomic fractions are fixed while varying the molecular composition. This is likely due to the ns-LIP completely dissociating the molecules in it with the inverse-Bremsstrahlung (IB) photon absorption by densely populated electrons. For reduced-order analyses of the spectra, proper orthogonal decomposition (POD) is used, while support vector regression (SVR) and support vector classification (SVC) models are employed for the calibration of the atomic concentrations and the identification of molecular species, respectively.