Kinetic study of Sr(53PJ) by time-resolved emission, 53P1→ 51S0+hν(λ= 689.3 nm), following dye-laser excitation

Abstract

Electronically excited strontium atoms in the low-lying 5s5p(³P_J) state, 1.8 eV above the 5 s²(¹S₀) ground state, have been studied in the time-resolved mode by recording the forbidden emission at λ= 689.3 nm [Sr(5³P₁)→ Sr(5¹S₀)+hν]. Sr(5³P₁) was generated by the repetitive pulsed dye-laser excitation of ground-state strontium atoms, in equilibrium with the solid at elevated temperatures, in a slow-flow system kinetically equivalent to a static system. The resulting resonance fluorescence at λ= 689.3 nm, following rapid Boltzmann equilibration within the 5³P_J spin-orbit manifold, was then monitored by means of boxcar integration. Particular consideration was given to the development of a pre-trigger photomultiplier gating system which permitted decay measurements necessarily to be made on time scales considerably shorter than achieved hitherto with this type of investigation. Measurement of the fluorescence decays in the noble gases and across a range of atomic strontium concentration revealed significant effects due to radiation trapping. Extrapolation of the decay data to [Sr(5¹S₀)]= 0 yielded the mean radiative lifetime τ_e= 20.1 ± 0.4 µs for the λ= 689.3 nm transition. This is compared with the range of values reported previously. The following upper limits are reported for the collisional-quenching constants of Sr(5³P_J) by the noble gases (k_Q/cm³ atom^–1 s^–1, T= 950 K): He, <5.5 × 10^–15; Ne, <3.5 × 10^–15; Ar, <3.9 × 10^–15; Kr, <5.5 × 10^–15 and Xe, <3.1 × 10^–15, which are compared with data derived from the phase-shift technique. A limit is also reported of the rate constant for quenching of Sr(5³P_J) by Sr(5¹S₀) of <2 × 10^–13 cm³ atom^–1 s^–1(T= 850–1000 K). The quenching of Sr(5³P_J) by H₂ and D₂ is studied in detail, yielding the following results: k_H2= 3.7 ± 0.4 × 10^–12 cm³ molecule^–1 s^–1(950 K, 1 σ), k_D2= 3.0 ± 0.2 × 10^–12 cm³ molecule^–1 s^–1(950 K, 1 σ). The results for these two isotopes are compared with analogous data reported from other measurements for Ca(4³P_J) and Mg(3³P_J).