Kinetic studies of reactions involving ground state Bi(64S) atoms by time-resolved resonance fluorescence

Abstract

We describe a kinetic study of reactions involving Bi(6⁴S) which is generated photochemically from BiMe₃ by pulsed irradiation. The ground state atoms were monitored by time-resolved resonance fluorescence at λ= 306.77 nm [Bi(7⁴P_½→ 6⁴S)] in the “single-shot” mode following optical excitation to the 7⁴P_½ state, which was effected by means of a high intensity microwave-powered atomic flow spectroscopic source. The fluorescence intensity was calibrated experimentally against the density of Bi(6⁴S) atoms and absolute rate constants k₃, defined according to the equation –d[Bi]/dt=k₃[Bi(6⁴S)]. [C₂H₂, C₂H₄][He], were then determined for the removal of the atom in the presence of the gases C₂H₂ and C₂H₄. The resulting values for k₃, namely, k₃^C₂H₂= 3.3 ± 0.3 × 10^–33 and k₃^C₂H₄= 8.7 ± 0.4 × 10^–33 cm⁶ molecule^–2 s^–1(300 K) are compared with the analogous data (k^′₃) derived previously using time-resolved absorption of atomic resonance radiation, and discussed in terms of the modified Beer–Lambert law, I_tr=I₀ exp [–ε(cl)^γ]. Finally, we calculate radiation trapping effects in our system assuming the atomic line profiles to be the summation of Voigt profiles over the nuclear hyperfine interaction components. Solutions of the diffusion equation for radiation trapping using the boundary conditions for infinite slab geometry, cylindrical geometry or spherical geometry indicate that, whilst apparent radiative lifetimes for the transition are considerably enhanced, sensible linearity between fluorescence intensity and particle density may be expected over wide concentration ranges. The implications of this result to both time-resolved resonance fluorescence measurements and steady fluorescence measurements of the Stern–Volmer type are discussed.