Kinetic study of Mg(3 3PJ), Mg(3 1P1) and Mg(4 3S1), including energy pooling, following pulsed dye-laser excitation at λ= 457.1 nm [Mg(3 3P1)â†� Mg(3 1S0)]

Abstract

The kinetic study of Mg(3 ¹P₁) and Mg(4 ³S₁), 4.346 and 5.108 eV above the 3 ¹S₀ ground state, has been investigated as a function of temperature and pressure of the gases He and Kr following pulsed dye-laser excitation of magnesium vapour at λ= 457.1 nm [Mg(3 ³P₁)â†� Mg(3 ¹S₀)]. Mg(3 ³P_J), Mg(3 ¹P₁), and Mg(4 ³S₁) were monitored in the time-domain by measurements of the atomic emission at λ= 457.1 nm, λ= 285.3 nm [Mg(3 ¹P₁)→ Mg(3 ¹S₀)+hν], and λ(average)= 517.6 nm [Mg(4 ³S₁)→ Mg(3 ³P_J)+hν]. Kinetic measurements on the decay of Mg(3 ³P_J) in He and Kr in the range 600–1100 K yielded the temperature dependence of the diffusion coefficients, when expressed in the form D[Mg(3 ³P_J)–He] and D[Mg(3 ³P_J)–Kr]∝Tⁿ, of n= 1.64 ± 0.09 and 1.65 ± 0.09, respectively. The collisional quenching of Mg(3 ³P_J) by Kr has also been investigated in this temperature range, yielding the following alternative forms for the temperature-dependent rate constant: k_Kr=(2.9 ± 0.7)× 10^–15 exp (–3.1 ± 0.5 kJ mol^–1/RT) cm³ atom^–1 s^–1 or k_Kr=(6.0 ± 0.5)× 10^–17T^1/2+(1.1 ± 0.8)× 10^–16 cm³ atom^–1 s^–1, the temperature-independent term lying within the error of the measurements. The mean radiative lifetime (τ_e) of Mg(3 ³P₁) has been redetermined from all the decay measurements in the range 600–1100 K, yielding the improved result τ_e= 2.3 ± 0.3 ms. An approximate estimate of k_Mg= 2 × 10^–15 cm³ atom^–1 s^–1 is reported for the collisional removal of Mg(3 ³P_J) by Mg(3 ¹S₀). Energy pooling to yield Mg(3 ¹P₁) has been studied in the range 800–1100 K, the first-order decay profiles for Mg(3 ¹P₁) and Mg(3 ³P₁) being consistent with Mg(3 ³P_J)+ Mg(3 ³P_J) annihilation at all temperatures and pressures. Estimates of the ratio of the initial yields of [Mg(3 ¹P₁)]_{t= 0}/[Mg(3 ³P₁)]_{t= 0} determined at different temperatures are found to be sensibly consistent with thermodynamic data for the vapour pressure of atomic Mg. Energy pooling to yield Mg(4 ³S₁) has been investigated from detailed measurements of the decay profiles for this atomic state using the first-order kinetic approximation at ‘long decay times’ in the range 750–1100 K. This has been interpreted quantitatively in terms of a mechanism involving the low-lying states Mg₂(³Π_g, ³Σ⁺_u), 2.05 and 2.19 eV above the Mg₂(X¹Σ⁺_g) ground state, and produced from the three-body recombination of Mg(3 ³P_J)+ Mg(3 ¹S₀)+ He, Kr which undergoes subsequent reaction with Mg(3 ³P_J) to yield Mg(4 ³S₁), in accord with the earlier proposal of Breckenridge and coworkers. The mechanism is established from the dependence of the decay profiles at λ(average)= 517.6 nm on the pressure of helium and krypton at all temperatures, and by comparison with those at λ= 457.1 nm. This indicates that Mg₂(³Π_g, ³Σ⁺_u) can only be placed in stationary state in this type of system when T > 1000 K. The temperature dependence of the initial yields of [Mg(4 ³S₁)]_max/[Mg(3 ³P₁)]_{t= 0} is also found to be in sensible accord with expectation based on vapour pressure data for Mg(3 ¹S₀).