A new band system between 6200 and 7500 Å has been observed in the ’’single collision’’ beam–gas chemiluminescent spectrum resulting from the reactions of Sc and Y atoms with O2. A strong, well-developed system is observed in the YO spectrum resulting from the Y–O2 reaction. A vibrational analysis of two distinct subsystems, each consisting of double-headed bands (ΔΛ?1), yields Te=14 531.2 cm −1, ωe′=794.0 cm−1, ωexe′=3.23 cm−1, ωe″=861.9 cm−1, ωexe″=2.95 cm−1 for the lower component, and Te=14 870.4 cm−1, ωe′=794.9 cm−1, ωe′=3.3 cm−1, ωe″=862.1 cm−1, ωexe″=3.025 cm−1 for the upper component. Semiempirical calculations predict Te=15 330.4 cm−1 for the A′ 2Δr state of YO. Perturbation calculations predict that mixing of the A 2Π state with the A′ 2Δ state of YO may be as large as 3.5%. On the basis of these calculations, the new band system is attributed to the A′ 2Δ–X 2Σ+ transition of YO. The new band system in the Sc+O2 spectrum is weaker and not as well resolved as the YO system. However, analysis of the system, by analogy with that of YO, yields T0=14 965.9 cm−1, ΔG′1/2 =834.0 cm−1 for the ScO A′ 2Δ3/2 state, and T0=15 072.0 cm−1, ΔG′1/2=837.0 cm−1 for the ScO A′ 2Δ5/2 state.

No abstract

The chemiluminescent reactions of the group 3 metals Sc and Y with F 2 , Cl 2 , Br 2 , ClF, ICl (Sc), IBr (Y), and SF 6 and La with F 2 , SF 6 , Cl 2 , and ClF have been studied at low pressures (6 × 10 −6 to 4 × 10 −4 Torr) using a beam-gas arrangement and extended to the 10 −3 Torr multiple collision pressure range. Contrary to previous reports, the observed chemiluminescent spectra are primarily attributed to emission from the metal monohalides. Extensive pressure and temperature dependence studies and high-level correlated molecular orbital theory calculations of the bond dissociation energies support this conclusion and the attribution of the chemiluminescence. Evidence for the "selective" production of a monohalide excited electronic state is obtained for several of the Sc and Y reactions. All reactions producing the metal monofluorides are first order with respect to the oxidant, while reactions producing the monochlorides and monobromides are found to be "faster than first order" with respect to the oxidant. This difference is associated with the metal halide bond dissociation energies and the metal halide product internal density of states. Analysis of the temperature dependence for six representative reactions indicates that the "selective" excited-state formation of the metal monohalides proceeds via a direct mechanism with negligible activation energy. We compare and contrast the present results with previous experiments and interpretations which have assigned the selective emission from these systems to the group 3 dihalides produced in a two-step reaction sequence analogous to an electron jump process. The current results suggest a distinctly different interpretation of the observed processes in these systems. The observed selectivity observed in these studies is remarkable given the significant number of known and potential excited states in the scandium and yttrium halides as well as their different electronic configurations.

Using a beam–gas arrangement, we have studied the chemiluminescent emission which results when a thermal beam of La atoms (1750–2400 K) intersects a tenuous atmosphere (10−6 to 10−4 torr) of one of the thermal oxidants O2, NO2, N2O, or O3 (300 K). The La–O2 reaction is characterized by visible chemiluminescence from the B 2Σ+ and C 2Π states of LaO. The La–NO2, La–N2O, and La–O3 reactions are characterized by emission from the B 2Σ+, C 2Π, and D 2Σ+ states of LaO. In addition all four systems are characterized by extensive emission from the A 2Π state of LaO. From the chemiluminescent spectrum for the La–O2 system, a minimum value of 8.19±0.04 eV is deduced for the dissociation energy of lanthanum monoxide. This lower bound is in good agreement with previous mass spectrometric determinations (8.28±0.11 and 8.38±0.2 eV). We discuss the temperature dependence of the four chemiluminescent reactions deducing an upper bound for the heat of vaporization of lanthanum metal. We combine the results of our temperature dependent studies with reactant–product correlations and demonstrate the manner in which this blend can be used to elucidate the significance and dynamics of reaction of low lying atomic states in the oxidation reactions of high temperature metallic beams.

The kinetics of a beam–gas chemiluminescent experiment are investigated. The first observations of collision induced emission from within an excited molecular electronic state are presented. The importance of a ’’loss mechanism’’ whereby metastable molecules are deactivated in wall collision is discussed. Comparisons with beam–beam chemiluminescent experiments are presented and the role of the excited state radiative lifetime is demonstrated.