Determination of atomic oscillator strengths using resonance absorption with a Döppler line source: transitions of Br and I (n+ 1)s–np5

Abstract

A new general method is described for the determination of atomic transition probabilities using measurements of absorption intensity as a function of ground state atom concentration. The source for absorption measurements is resonance fluorescence emitted under optically thin conditions. The emission (and absorption) line profiles approximate closely to those for a Döppler line at 300 K under the conditions used. Known concentrations of Br 4p⁵²P atoms for these (and fluorescence) studies, were produced by the two rapid reactions, O ³P_J+ Br₂→ BrO + Br, O ³P_J+ BrO → O₂+ Br.

Values for the oscillator strengths, ƒ(λ nm), of various transitions of Br 4p⁵²P are reported: ƒ(157.7)=(1.16 ± 0.08)× 10^–3; ƒ(154.1)=(4.88 ± 0.59)× 10^–2; ƒ(148.9)=(4.13 ± 0.39)× 10^–2. These data were combined with resonance fluorescence intensity ratio measurements to give ƒ values for the corresponding transitions to the 4p⁵²P_½ state of Br: ƒ(163.4)=(6.4 ± 1.2)× 10^–3; ƒ(157.5)=(1.5 ± 0.5)× 10^–2. The experimental results confirm theoretical determinations by Lawrence of the ƒ values for the 5s– 4p⁵ multiplet of Br.

For I atom resonance, the source of radiation was I atom fluorescence (6s– 5p⁵) from the photolysis of ICl (X¹7 Σ⁺, v″= 0) with H λ 121.6 nm radiation. Absorption measurements were performed with known I atom concentrations generated by the rapid reaction, Cl + ICl → I + Cl₂. ƒ values for two transitions to I 5p⁵²P were determined: ƒ(178.3)=(4.52 ± 0.42)× 10^–2; ƒ(183.0)=(1.67 ± 0.23)× 10^–3. These results are in less good agreement, than those for Br, with the work of Lawrence.

Investigations of Br atom resonance fluorescence have been made in a flow system at 300 K and near 200 N m^–2 total pressure. For kinetic studies of ground state 4p⁵²P bromine atoms using resonance fluorescence, the most useful transitions are the λ 157.7 nm 5s⁴P_5/2– 4p⁵²P and the λ 154.1 nm 5s⁴P– 4p⁵²P lines. Use of certain lines which connect with the 4p⁵²P_½ state also has potential value because of the high oscillator strength of these lines combined with the absence of self-reversal of fluorescence in a system consisting of 4p⁵²P atoms.