Signal detection efficiency in multiphoton ionization flame measurements

Multiphoton ionization is often the most sensitive method available for detecting radical species in flame environments. To make accurate relative concentration measurements, however, the electron (or ion) detection efficiency as a function of flame position must be known. Two methods are presented for determining this quantity in a laminar CH(4)/air diffusion flame burning at atmospheric pressure: (1) simultaneous detection of ionization and fluorescence in CO, following two-photon excitation of the B(1)Sigma(+) state at 230.0 nm; (2) comparison of 3 + 1 multiphoton ionization of the 4' state of argon at 314.4 nm with mass spectrometric measurements. The results show significant variation of the electron detection efficiency in the lean, stoichiometric, and rich flame regions, with the greatest detection sensitivity observed in the high temperature, primary reaction zones (i.e., near stoichiometric conditions). Corrections to multiphoton ionization data obtained for H atoms are discussed in terms of determining relative concentration profiles across the methane/air diffusion flame.