Collection of ions produced by continuous wave laser-enhanced ionization in a hydrogen/air flame

Abstract

Laser-enhanced ionization (LEI) - or the optogalvanic effect in flames - is known to result when a tunable laser is used to significantly populate an excited state of an atomic species in an atmosphere pressure flame. The perturbed ionization rate may be sensed with external electrodes, providing the opportunity for applications to trace metal analysis and combustion research. The present study correlates experimental studies of the spatial and temporal characteristics of the LEI signal with theoretical expectations. The experiments are performed with a c-w dye laser exciting the 3S ..-->.. 3P/sub 3/2/ transition of sodium introduced into a H/sub 2//air flame. Saturation currents with and without laser excitation are found to be consistent with expected ionization rate constants for the Na group and excited states. Vertical spatial profiles - using a unique imaging method - show the physical size of the excess ion region generated by the laser, and the influence of external voltage, flame velocity, diffusion, and Coulombic expansion on the excess ion region. Rise and fall times - measured at different voltages with the laser switched by an acousto-optic modulator - show the dependence on ion mobility, electric field, and excited-state ionization rate constant. The relationships derived andmore » illustrated should be of value for the improvement of precision and accuracy in analytical and diagnostic LEI.« less