Three-photon-excited laser-induced fluorescence detection of atomic hydrogen in flames

In many recent studies, ultrashort femtosecond (fs) two-photon (2p) laser-induced fluorescence (LIF) of atomic hydrogen (H) has been demonstrated using a 205 nm excitation. However, 205-nm- deep ultraviolet (UV) pulses can be problematic in practical devices containing thick transmissive optics and can also be susceptible to photolytic production at high laser energies. In this Letter, we investigate the three-photon (3p) excitation scheme of H by using red-shifted 307.7 nm fs laser pulses. Efficient 3p excitation resulting from fs laser pulses enable the 3pLIF detection of H, which was previously unattainable in most flame conditions using ns or ps pulses. Measurements are reported in CH₄/O₂/N₂ Bunsen jet flames and premixed CH₄/air flames and compared to similar 2pLIF schemes with fs pulses. Saturation effects, photolytic interferences, and stimulated emissions effects are studied, as well as the benefits of 3pLIF in diagnostic hardware with thick optical windows.