Collisional ionization between fast alkali atoms and selected hexafluoride molecules

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Abstract
Negative ion products resulting from collisions between orthogonal, crossed beams of alkali metal atoms (Na, K, Cs), and the octahedral hexafluorides MF6 (M=S, Se, Te, Mo, W, Re, Ir, and Pt) have been examined in the energy range from ∼0 to 40 eV (lab). Studies of the dependence of the reaction thresholds upon the temperature of the target molecules SF6, SeF6, and TeF6 have provided electron affinities for these molecules; E.A.(SF6) =0.46±0.2, E.A.(SeF6) =2.9±0.2, and E.A.(TeF6) =3.3±0.2 eV. Energy loss measurements of the alkali, A, in the reaction A+MF6→A++MF−6 at small scattering angles are consistent with these values. Measurements for SF4 together with temperature dependent thresholds for the formation of SF−5 from SF6 and SF−3 from SF4 combined with known bond dissociation energies for D (SF5–F) and D (SF3–F) yield electron affinity values for the SFn series; E.A.(SF6) =0.46±0.2, E.A.(SF5) =2.71±0.2, E.A.(SF4) =0.78±0.2, and E.A.(SF3) =3.07±0.2 eV. Lower limits of ∼5 eV for the electron affinities of MoF6, WF6, ReF6, IrF6, and PtF6 are implied by the detection of MF−6 ions when only the thermal (T∼100 to 300 °C) alkali beam (atoms plus dimers) is incident on MF6. The cross sections for charge transfer A+MF6→A++MF−6 are large at low energies which raises a question as to the mechanism of charge transfer since the zeroth order ground state covalent and ionic curves can never cross.