Collisional ionization between fast alkali atoms and selected hexafluoride molecules

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 SF_n 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.