Collisional quenching of Mg(33PJ) studied by time-resolved emission, 33P1→ 31S0+hν(λ= 457.1 nm), following dye-laser excitation

Abstract

The collisional quenching of electronically excited magnesium atoms, Mg[3s3p(³P_J)], 2.71 eV above the Mg[3s²(¹S₀)] ground state, has been studied for a number of added gases following repetitive dye-laser pulsing. Subsequent to the dye-laser excitation at λ= 457.1 nm of magnesium vapour to the 3s3p(³P₁) state and Boltzmann equilibration within the ³P_J manifold through collisions, the resulting slow, forbidden spontaneous emission, Mg(3³P₁)→ Mg(3¹S₀)+hv, was then used to minitor the decay of Mg(3³P_J) in the presence of various collision partners, using boxcar integration. The following absolute second-order quenching rate constants at T= 800 K are reported (k_Q cm³ molecule^–1 s^–1; errors 1σ): D₂, (1.9 ± 0.1)× 10^–13; CO₂, (1.0 ± 0.1)× 10^–12; N₂O, (4.9 ± 0.1)× 10^–13; CF₄, (6.5 ± 0.2)× 10^–14; C₂H₂, (3.8 ± 0.2)× 10^–11; C₂H₄, (1.4 ± 0.1)× 10^–10; and C₆H₆, (3.5 ± 0.1)× 10^–10. These data, together with previous results using this signal-averaging technique on a flow system, kinetically equivalent to a static system, are compared, where possible, with absolute rate data derived either from a slow flow–discharge system or dye-laser excitation coupled with either atomic resonance absorption or fluorescence in the ‘single-shot mode’. The significant isotope effect for the collisional removal of Mg(3³P_J) by H₂ and D₂ is considered in some detail.