Collision‐induced ion‐pair formation of the thallium halides TlF and Tl2F2

摘要

Absolute cross sections have been determined for collision‐induced ion‐pair formation (polar dissociation) of TlF and Tl2F2from collisions with Xe and Kr atoms. A crossed‐beam method was used in which the Xe or Kr atoms were aerodynamically accelerated to energies up to 17 eV (in the laboratory frame). Time‐of‐flight mass spectrometry permitted the determination of partial cross sections for each positive and negative ion channel in the collision. The cross sections for both monomer and dimer thallium fluoride were considerably smaller than those previously reported for thallium chloride, the difference most likely resulting from the poor energy transfer into the molecular target due to the light F atom. The cross section for the reaction Xe(Kr)+TlF→Xe(Kr)+Tl++F−exhibited, near threshold, a power‐law rise with increasing energy sgr; =const.× (Etot−E0)n/Erel, whereEtotis the total collision energy (kinetic plus internal),E0is the threshold energy,Erelis the relative kinetic energy andn=1.85(1.95). The cross section for rearrangement ionization Xe +TlF→TlXe++F−exhibited a step behavior. Collisions of Xe with the dimer Tl2F2resulted in both Tl2F++F−and Tl++TlF−2ion products, the cross sections for the latter ion pair being about one third those for the former. With new values of the heats and entropies of formation of Tl2Cl2, Tl2Br2, and Tl2I2the absolute cross sections for the previously reported reactions of the dimers Xe(Kr)+Tl2X2→Xe(Kr)+Tl2X++X−have been recalculated. The threshold behaviors of these reactions have also been analyzed in terms of the above power law, giving values of the powernin the range 2.1–2.7. Model calculations have been performed to determine bond energies of the (Tl–X–Tl)+and (X–Tl–X)−ions, which were used to determine theoretical thresholds for the formation of these ions. The difference between the experimental and theoretical thresholds then gives the internal excitation of the molecular ions at threshold (assuming the inert gas atom carries away negligible energy). Tl2F+, in particular, is formed at threshold with considerable internal excitation (∼1.8 eV). The low cross sections for both TlF and Tl2F2and the high internal excitation of Tl2F+at threshold are consistent with a collision model in which only very dynamically constrained collisions result in the extraction of the light F−ion from either TlF or Tl2F2.