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

Abstract

The collisional behaviour of electronically excited magnesium atoms in the Mg(3s3p(3PJ)) 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 3PJ spin–orbit manifold using signal averaging and computerised analysis. The kinetic behaviour of Mg(3 3PJ) 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, τe[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 τe= 3.9 ± 0.2 ms, in accord with more recent theoretical calculations. The diffusion of Mg(3 3PJ) 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 3PJ) by the alkanes (RH) are measured as a function of temperature, yielding the following Arrhenius forms [kR=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 3PJ) by H2 and D2 investigated hitherto by a similar method.