Non-resonant photochemical ignition of lean methane/air mixtures by femtosecond laser filamentation

Laser ignition (LI) is promising for green combustion of lean-fuel mixtures with controllable ignition timing and location. It was recently discovered that despite the inferior energy deposition and low thermal temperature in femtosecond (fs) laser-induced plasma, fs laser pulses can achieve a robust ignition of lean-fuel mixture through forming a "line" kernel by filamentation. Here, to clarify fs-LI mechanism, we investigated a dual-color (DC: 800 nm at 1.5 mJ and 400 nm at 0.43 mJ, similar to 50 fs) fs-LI of a lean methane/air mixture with an equivalence ratio of phi = 0.87. An optical emission spectroscopy study was conducted to probe the N-2(+) and OH emissions and characterize the ignition success rate. It was demonstrated that fs-LI can be achieved at a lower minimum ignition energy (MIE) (<0.46 mJ) by the DC scheme than that (>0.7 mJ) by a single-color (SC: 800 nm at 2.0 and 2.4 mJ, similar to 50 fs) scheme, indicating a strong wavelength effect on the successful ignition. A pump-probe measurement was carried out to reveal the effect of the ionization enhancement on the successful ignition. It was found that only when the two-color fs pulses are temporally overlapped, the OH yield is strongly enhanced and the MIE is decreased. By comparing the variation trend of the fluorescence intensity of OH with that of the direct ionization product N-2(+), we ascribed fs-LI to a non-resonant photochemical ignition mechanism, in which the enhancement in the multiphoton/tunnel ionization of the lean-fuel mixture by the high-energy 400-nm photon can increase the yields of the reactive radicals through various dissociation and chain reaction pathways, and thus result in the successful ignition at the micro-joule level. This work unravels the essential role of the non-resonant photochemical ignition mechanism in fs-LI, and provides a promising route for the ignition of lean-fuel engines by compact ultrashort-pulsed lasers in the filamentation regime.