Ultraviolet Laser Ignition of Premixed Gases by Efficient and Resonant Multiphoton Photochemical Formation of Microplasmas

Ignition properties of premixed H2/O2 and H2/N2O flows at atmospheric pressure have been studied using an ultraviolet laser as the ignition source. Tuning the laser in the 225.6 nm wavelength region has yielded three minima in the amount of incident laser energy (ILE) that is required to ignite either mixture. The minima correspond exactly to the two-photon resonant excitation wavelengths for the three spin-orbit split ground electronic states of oxygen atoms. A determination of the ILE necessary to ignite both premixed flows as a function of equivalence ratio shows a minimum far into the fuel-lean region. Also, the minimum ILE value for the ignition of H2/O2 was found to be around 0.3 mJ, while ignition of the same mixture with the green beam from a frequency doubled Nd: YAG laser (532 nm) required an ILE value near 13 mJ. Additional time-resolved spectral studies were carried out on O2 and N2O flows alone. These indicated a resonant formation of a microplasma with a lifetime on the order of 100 nsec. All of these results lead to the conclusion that multiphoton photochemical ignition is a phenomenon consisting of three major components: (1) the multiphoton photochemical formation of oxygen atoms; (2) multiphoton ionization of these atoms to efficiently form free electrons in the laser focal volume; (3) the formation of a laser microplasma using the electrons formed in the previous process as seed electrons. As such, this new laser ignition source appears to be more efficient and more controllable than the well-known laser-produced spark (gas breakdown) process and it thus should be useful for further ignition studies.