A tunable laser system that operates in the ultraviolet (UV) has been utilized to ignite premixed reactive gaseous flows of H2/O2, D2/O2 and CH4/N2O in a jet burner at atmospheric pressure. Multiphoton UV photodissociation of the fuel or oxidizer molecules produced ground state radicals (H and O atoms). Resonance-enhanced multiphoton excitation and ionization of these radicals formed a laser-produced microplasma that served as an ignition source. Time-resolved absorption techniques were utilized to determine minimum ignition energies for the gaseous mixtures when the laser was tuned to resonance two-photon excitation transitions of H and O atoms near 243 nm and 225.6 nm respectively. The minimum ignition energy was found to be wavelength dependent and was the least when the laser was tuned to the resonant two-photon excitation transitions. These results suggest that ignition is not only dependent upon the absorption of a certain minimum amount of energy but is also photochemically enhanced by the production of ground and excited-state radicals and ions which participate in exothermic chemical chain-branching reactions in the early stages of ignition. Therefore, a new laser ignition mechanism is proposed.
Brad E. Forch,
"Resonant laser ignition of reactive gases", Proc. SPIE 2122, Laser Applications in Combustion and Combustion Diagnostics II, (18 March 1994); doi: 10.1117/12.171284; https://doi.org/10.1117/12.171284