Molecular beam kinetics: Angular distributions and chemiluminescence in reactions of alkali dimers with halogen atoms and molecules

Abstract

Crossed−beam studies are presented for several reactions of diatomic alkali molecules with halogen atoms and molecules. The alkali dimers are generated by association of atoms in a supersonic nozzle expansion, the halogen atoms by thermal dissociation of molecules. For both the X + K2 and X2 + K2 reactions (X = Cl, Br), the angular distributions of reactively scattered KX peak forwards (in the center−of−mass system) with respect to the incoming alkali dimer, the final relative translational energy of products is fairly high, ∼10−20 kcal/mole, and the total reaction cross sections are large, ∼50−150 Å2. For the X + A2 reactions (X = Cl, Br, I; A2 = Na2, K2, Rb2), excited alkali atom emission is observed from nearly all energetically accessible transitions which fall within the spectral range of the experiments (2200−8000 Å). Some of these lines come from states located above the reaction exoergicity and thus indicate that the initial relative translation of the reactants is converted into electronic excitation of the product atom. The chemiluminescence spectra for X + K2 correspond to a statistical distribution of excited states, roughly characterized by a temperature of ∼5000°K. The cross sections for chemiluminescence from the lowest excited 2P alkali atom state are ∼10−100 Å2. The X + A2 results are interpreted in terms of electron jumps forming X− + A2+ at crossings of potential surfaces. Calculations using a semiempirical pseudopotential method show the orbital degeneracy of the halogen atom has an important role. This is indicated also by a study of the analogous H + A2 reaction, where orbital degeneracy is lacking and no chemiluminescence is observed. For the X2 + A2 reactions, the angular distributions indicate that the dominant reaction path forms A + AX + X. Chemiluminescence is also observed from the two paths which yield A* + AX + X and AX + AX* (for X2 = Cl2, Br2, I2, ICl, and IBr; A2 = K2, Rb2, and Cs2). For the K2 reactions, the cross sections are ∼0.3−3 Å2 for A* emission and smaller for the AX* emission; for Rb2 and Cs2 the AX* emission becomes stronger. No AX* emission is observed for the analogous X2 + Na2 reactions, although it is energetically allowed. The molecular emission in reactions of ICl and IBr with K2 comes from KI*. These observations are consistent with an electron−jump mechanism which allows facile four−center reaction via (A-) A+⋅⋅⋅X− (X-), a biradical ion−pair intermediate.