Abstract

We have recently observed that focused UV lasers can ignite chemically reactive gas flows at atmospheric pressure with unusually small values of incident laser energy (ILE). For example, ignition has been achieved with ILE values of 0.3 mJ for H₂/O₂ and 0.03 mJ for C₂H₂/O₂. Furthermore, for reactive flows which involve O₂ or N₂O as oxidizers, we have observed a distinct wavelength dependence in the ignition process which is related to O-atom two-photon resonance excitation. Apparently, microplasma formation is responsible for this efficient ignition and the microplasmas are formed by first multiphoton photolysis of either the fuel or oxidizer molecules into atomic components followed by resonance-enhanced ionization of the atomic fragments to liberate free electrons in the focal volume. These, then, are the seed electrons for the UV laser-produced microplasmas. It is clear that this type of laser-produced spark is qualitatively different from the typical laser-produced spark, particularly in terms of efficiency and threshold effects. To understand the UV-produced microplasma process better, we have performed spectral and time-resolved studies on them. We have found, for example, that UV laser-produced microplasmas from N₂O are much brighter than from O₂ for a fixed value of ILE, but the O-atom emission at 777.5 nm is much weaker. This appears to be due to the differences in the photochemical formation of ground and excitedstate fragments from these molecules. Results are presented for UV laser-produced microplasmas in H₂, O₂, N₂O, and C₂H₂ flows and a comparison of these plasmas with those from a green (532-nm) laser is given.

© 1986 Optical Society of America