Laser studies in flames

Laser excited fluorescence was used to investigate minority species in premixed H₂:N₂:O₂ flames with temperatures ranging from about 1500 K to 2400 K. The species CH and NH were produced in a series of flames but detection by laser fluorescence failed. Fluorescence studies of metal atoms enabled the rate of spin-orbit relaxation Pb (7s³P₁°) → Pb (7s³P₀°) in collisions with atomic hydrogen and flame bulk-constituents to be measured. For hydrogen as the collision partner, the rate constant showed a negative temperature coefficient, varying as T⁻¹·⁷, whereas for other flame bulk constituents the rate constant showed a positive coefficient, varying as T¹·⁸. The process is described in terms of the Landau-Zener theory of non-adiabatic transitions between potential curves corresponding to excited states of the transient molecules formed during collisions. The magnitude of the rate constants and the signs of the temperature coefficients are predicted correctly by the application of the Landau-Zener theory