The temperature dependence of ’’single collision’’ bimolecular beam–gas chemiluminescent reactions. II. Experimental studies

Abstract

We consider ’’single collision’’ bimolecular beam–gas chemiluminescent reactions in which a metal beam formed effusively intersects a tenuous atmosphere of oxidant gas (10−4–10−6 torr) resulting in the emission of visible radiation from excited electronic states of the reaction products. From recently derived relationships which allow the determination of the temperature dependence for the observed chemiluminescence we deduce upper bounds for ΔH vaporization (ΔHsub). In addition, for systems in which ΔH is accurately known through independent studies Eexp may be evaluated. Derived relationships are demonstrated experimentally through studies of ten bimolecular reactions. We consider the reaction of strontium atoms and fluorine molecules. Here, reaction occurs with one internal state of the metal atom (ground 1S0 states). ΔHsub(Sr), Eexp(SrF, C2Π), and Eexp(SrF, D2Σ+) are determined by monitoring the chemiluminescence from the SrF C2Π and D2Σ+ excited electronic product states. By combining the determined activation energies with reasonable estimates of relative frequency factors for the C2Π and D2Σ+ states we deduce that the D2Σ+ state of SrF is long lived (metastable). Results are also presented for the reactions of Sc, Y, and La with O2, NO2, and N2O. Here the metal beams are characterized by a substantial internal state population. The effect of the internal state population characterizing the metal beams is analyzed. We deduce an upper bound for ΔHvap (Sc, Y, La) from the nine reactions studied. Eexp for formation in the v′=0 level of the ScO and YO A2Π states, and the v′=0 level of the LaO C2Π state, is deduced for the three possible oxidations with O2, NO2, and N2O. The resultant experimentally determined activation energies are compared with those expected on the basis of mechanistic models used to describe the oxidation reactions of Group IIIB atoms.