Detection of molecular oxygen using nanosecond-laser-induced plasma

Molecular oxygen (O₂) concentration is measured by employing nanosecond laser-induced plasmas (ns-LIP) over a broad temperature spectrum ranging from 300 K to 1000 K, in the presence of an additional oxygen-containing molecule, CO₂. Typically, emission spectra emanating from ns-LIP are devoid of molecular information, as the ns-LIP causes the dissociation of molecular species within the plasma. However, atomic oxygen absorption lines that momentarily appear at 777 nm in the broadband emission from the early-stage plasma are determined to be highly sensitive to the O₂ mole fraction but negligibly affected by the CO₂ mole fraction. The atomic O absorbing the plasma emission originates from the O₂ adjacent to the plasma: robust UV radiation from the early-stage plasma selectively dissociates adjacent O₂, exhibiting a relatively low photodissociation threshold, thus generating the specific meta-stable oxygen capable of absorbing photons at 777 nm. A theoretical model is introduced, explicating the formation of the meta-stable O atom from adjacent O₂. To sustain the UV radiation from the plasma under high-temperature and low-density ambient conditions, a preceding breakdown is triggered by a split laser pulse (532 nm). This breakdown acts as a precursor, seeding electrons to intensify the inverse-Bremsstrahlung photon absorption of the subsequent laser pulse (1064 nm). Techniques such as proper orthogonal decomposition (POD) and support vector regression (SVR) are employed to precisely evaluate the O₂ mole fraction (<1% uncertainty), by analyzing the short-lived (<10 ns) O₂-indicator depicted in the early-stage plasma.