Harpoon reaction

Abstract: Excited state reactions of metals produce electronically excited products efficiently, as revealed by studies performed both in the gas phase and in free Van der Waals complexes. The reaction mechanism is assigned to an excited state charge transfer from the metal to the molecular reactant (i.e. a harpoon mechanism). The present work uses the well established cluster isolated chemical reaction (CICR) technique and addresses these processes when the metal···molecule Van der Waals pair is deposited at the surface of a large argon cluster. Such work is aimed at investigating the effect of the cluster substrate on the preparation and dynamics of the reaction. We have revisited the pluridimensional character of the harpoon reaction in these systems. More specifically, we studied the reaction of excited calcium with HBr near the calcium resonance line at 422.7 nm, forming CaBr in the A and B states. As in previous Van der Waals experiments, we could explore the dynamics of the reaction by recording action spectra. These spectra exhibit noticeable differences from those observed for unsupported Ca···HBr complexes. In particular the bending movement of the Ca···HBr complex which gives access to the transition state of the reaction is partly hindered by the presence of the argon cluster.

Abstract: A harpoon reaction model employing multiple crossings based on the formalism of Bauer et al. [J. Chem. Phys. 51, 4173 (1969)] and Gislason and Sachs [J. Chem. Phys. 62, 2678 (1975)] is developed to explain the large cross sections (σ∼330–975 A2) measured for the reaction Xe*(5p5np,np’, n=6,7)+Cl2. The model calculates the Landau–Zener transition probability for each intermediate ionic crossing with the covalent surface. The transition matrix elements are represented as a product of the electronic interaction (modeled by the empirical result of Olson et al. [Appl. Opt. 10, (1971)]) and a Franck–Condon factor for the Cl2→Cl−2 transition. The model predicts near unit probability for a transition to the ionic surface for impact parameters less than 20 Bohr. Once transfer occurs, the pair is captured by dissociation of Cl−2 to form XeCl*. The large temperature dependence observed qualitatively in the experiments is explained by the increased cross section for vibrationally excited Cl2. A simple model for orbit...

Abstract: Reactive quenching of the excited states of the rare gases has been studied for more than a decade. Early, Gundel et al [J. Chem. Phys. 64,4390(1976)] proposed that reactions of the metastable states of argon (3p 5 4s[3/2] 2 and 3p 5 4s′[1/2] 0 ) were analogous to reactions of alkali metal atoms with halogens and proceeded by a harpooning mechanism, which assumes the reaction proceeds through an ion intermediary, Ar * + Cl 2 → Ar + + Cl 2 − → ArCl * + Cl. Product branching fractions, vibrational energy disposal, velocity dependence, and vibrational excitation of reactants and propensity for ion-core conservation (N. Sadeghi, in previous measurements similar to studies with copper presented in an adjacent section) have now been studied for the reactions of metastable states of all the rare gases with many chloride compounds. This paper will present recent studies of the effect of excited state energy on the harpoon reaction. We present measurements of total binary and tertiary quench rates and branching fractions for the reaction, Xe (5p 5 np,np′, n=6,7)+ Cl 2 . Xenon atoms are excited by state-selective, two-photon absorption with a u.v. laser, and the time dependent fluorescence from the excited atom in the I.R., visible, and from XeCl * (B) products near 308 nm are then measured with subnanosecond time resolution. The energy dependence of the measured reaction rates are consistent with a multichannel harpoon model to be presented which is based on the formalism of Gislason and Sachs [J. Chem. Phys. 62, 2678 (1975)].