Laser fluorescence study of AlO formed in the reaction Al + O2: Product state distribution, dissociation energy, and radiative lifetime

Abstract

The reaction Al + O2 → AlO + O has been investigated by the method of laser−induced fluorescence. Excitation spectra are reported for AlO products formed under single−collision conditions in a ’’beam−gas’’ arrangement. The relative vibrational populations (proportional to total cross sections for formation), as well as the relative rotational populations for the v = 0 and v = 1 vibrational levels, are derived. The rotational distributions are found to differ significantly, with v = 0 having more rotational excitation than v = 1. The vibrational distribution is non−Boltzmann, the falloff with v ≳ 1 exceeding v = 1 compared to v = 0. The observed internal state distributions are compared with those calculated from phase space theory. It is concluded tentatively that the partitioning of the Al + O2 reaction energy among the product modes is not governed solely by statistical considerations. The dynamics of the Al + O2 reaction are compared with those of the exothermic Ba + O2 and endothermic K + O2 reactions. From a knowledge of the AlO internal states populated by the Al + O2 reaction and from an estimate of the thermal reactant energies, a lower bound for the ground state dissociation energy of AlO is derived. By combining the present lower bound with the upper bound previously derived from observation of the onset of an AlO absorption continuum, the value D00(AlO) = 121.5±1 kcal/mole is recommended for aluminum monoxide. Direct measurement of the fluorescence decay of the AlO B 2Σ+ state as a function of time after excitation by the short (∼5 nsec) laser pulse has allowed the determination of the radiative lifetimes τ (v′ = 0) = 100±7 nsec, τ (v′ = 1) = 102±7 nsec, and τ (v′ = 2) = 102±4 nsec, where the error estimates represent three standard deviations.