Kinetic study of the collisional behaviour of Mg(3 3PJ) with alkanes at elevated temperatures following pulsed dye-laser excitation

Abstract

The collisional behaviour of electronically excited magnesium atoms in the Mg(3s3p(³P_J)) state, 2.712 eV above the 3s²(¹S₀) ground state, is investigated by time-resolved atomic fluorescence at λ= 457.1 nm [Mg(3 ³P₁)→ Mg(3 ¹S₀)+hν] following pulsed dye-laser excitation of magnesium vapour at the resonance wavelength. Decay profiles for Mg(3 ³P₁), which demonstrate clear exponential behaviour, are recorded on time-scales within which a Boltzmann equilibrium has been established in the ³P_J spin–orbit manifold using signal averaging and computerised analysis. The kinetic behaviour of Mg(3 ³P_J) is investigated as a function of temperature in the range 680–905 K in He and Xe alone and in the presence of CH₄, C₂H₆, C₃H₈ and C₄H₁₀ using He as the excess buffer gas both in a static system and in a flow system, kinetically equivalent to a static system. The mean radiative lifetime, τ_e[Mg(3 ³P₁)] has been re-investigated with particular care to eliminate impurity quenching and employing low densities of Mg(3 ¹S₀) and low laser pulse energies to minimise effects arising from both collisional quenching by the ground state and by energy pooling. This yields τ_e= 3.9 ± 0.2 ms, in accord with more recent theoretical calculations. The diffusion of Mg(3 ³P_J) is also investigated in some detail as a function of temperature and considered quantitatively in terms of the first-approximation Chapman–Enskog equation. Absolute second-order rate constants for the collisional removal of Mg(3 ³P_J) by the alkanes (RH) are measured as a function of temperature, yielding the following Arrhenius forms [k_R=A exp(–E/RT)]: [graphic omitted]. These data are considered in terms of the bond-energy, bond-order (BEBO) approach, yielding estimates of activation energies consistent with the experimental observations and with H-atom abstraction to yield MgH. The results are also compared with analogous kinetic data for the removal of Mg(3 ³P_J) by H₂ and D₂ investigated hitherto by a similar method.