Laser induced flame chemistry of Li (2 2P1/2,3/2) and Na (3 2P1/2,3/2). Implications for other saturated mode measurements

Abstract

Saturated laser fluorescence measurements of sodium or lithium in a series of fuel rich, atmospheric pressure H₂/O₂/N₂ flames at 1700–2200 K indicate induced chemical interactions between the excited ²P_1/2,3/2 states of the metals and the H₂O or H₂ flame constituents. A steady state redistribution occurs among the metal’s elemental, hydroxide and hydride forms within the initial fraction of the μs laser pulse duration. A saturated absorption model incorporating these chemical effects illustrates the significant depletion of the free atom concentrations under these conditions and explains previous discrepancies between such measurements and conventional absorption experiments. Estimates of the rates of the reactions between the ²P_1/2,3/2 states of sodium or lithium with H₂O or H₂ indicate that they proceed predominantly via the nonadiabatic physical relaxation channel. For sodium the two chemical channels are relatively inefficient constituting only about 2% and 0.5% of the total interaction cross section with H₂O and H₂, respectively. Even so, they are still sufficient to drain off a reasonable fraction of the free atoms into these molecular metastable sinks. No evidence of contributions from laser enhanced ionization was noted. Applications of the saturated laser fluorescence technique as a general combustion species monitor must necessarily be restricted to the use of nanosecond pulse length lasers in order to circumvent these potential chemical relaxations.