Resonantly enhanced two-photon photoionization of NO in an atmospheric flame

Abstract

Molecular multiphoton ionization experiments are reported for the first time in a flame environment. The resonantly enhanced two-photon photoionization spectrum of NO from 270 to 317 nm in an atmospheric pressure H2/air/N2O flame is essentially identical with respect to both line position and intensity to that which is predicted for the one-photon absorption to the intermediate A state. A model is developed here which accounts for this result by including rates for collisional repopulation of the laser depleted state. Based on this model, the rotational transfer rate constant for NO is estimated to be ⩾4×109 s−1 in the flame, corresponding to a cross section of ∼70 Å2. It is found that the photoionization spectra obtained in this work have far better signal-to-noise and resolution than those reported for NO in flames using laser-induced fluorescence methods and that the estimated detection limit for NO is 1 ppm.