Polarization of laser-induced fluorescence in OH in an atmospheric pressure flame

Measurements have been made of the polarization of the laser-induced fluorescence in OH in the burnt gases of a CH4/O2 flame diluted with N2, Ar, or He. A significant degree of polarization is observed for the lines emitted by the pumped level. Although elastic depolarizing collisions occur, they must compete with rapid quenching, resulting in a freezing of the coherence generated by the polarized laser. The variation of apparent line strength ratios with pump line and J is in agreement with theoretical expectations. The coherence is preserved to a small degree following collisional transfer to nearby rotational levels. The implications concerning relative collision rates and for flame diagnostic measurements are discussed.