Atomic resonance fluorescence spectrometry for rate constants of rapid bimolecular reactions. Part 3.—Oxygen atom resonance O 3S1–3P2, 1, 0

Abstract

Measurements at 298 K of the intensities of atomic resonance absorption and of atomic resonance fluorescence, as functions of ground state O ³P_J oxygen atom concentrations, have been made for each of the three transitions (near λ 130 nm) of the O ³S₁–³P_2,1,0 multiplet. The light source was an oxygen atom lamp in argon carrier gas, and O ³P_J oxygen atoms were generated in a discharge–flow system, both at low pressure (< 200 N m^–2).

Simple theory shows that the resonance absorption intensity, A=I_abs/I₀, varies with optical depth in the absorber, k₀l, (k₀l∝[O ³P_J]), according to a polynomial, A=(k₀l)α–½(k₀l)²β+…, always leading to the limiting case A→(k₀l)α as k₀l→0. Several models for the line profile of the atomic resonance lamp are used to compute values for A and the coefficients α, β…, for comparison with the experimental values obtained for these parameters. The results are extended to include the case of Lorentz-broadened absorption lines, in order to relate absorption measurements in low-pressure flow systems to those in higher pressure experiments (e.g. pulsed photolysis studies).

The intensity of resonance fluorescence I_F at low pressure is considered as a function of the intensity of absorption from the source, and a fluorescence emission function. The effective absorption coefficient for O ³P_J atoms is much higher for the non-reversed fluorescent radiation emitted homogeneously in the fluorescence cell, than for the radiation originating from a typical (self-reversed) source. Consequently, the concentration of O ³P_J atoms can be low enough to be optically thin with respect to absorption of radiation from the source but high enough to be optically thick with respect to re-absorption of fluorescence emitted within the homogeneous mixture of emitters and absorbers in the fluorescence cell. These considerations are used to obtain an approximate model for the variation of I_F with [O ³P_J]—as for absorption, the model leads to a polynomial in k₀l with the same experimentally-observed limiting case I_F→C′(k₀l) as k₀l→0.

The results show that whilst the O ³S₁–³P₂(λ 130.2 nm) line is more sensitive than the O ³S₁–³P₀(λ 130.6 nm) line for the detection of O ³P_J atoms, the range of linear dependence of I_F upon total oxygen atom concentration extends to much higher concentrations for λ 130.6 than for the λ 130.2 transition.