Collision-induced ion pair formation of thallium halides: Absolute cross sections

Abstract

Collision-induced dissociative ion pair formation (polar dissociation) of thallium chloride, bromide, and iodide by aerodynamically accelerated Xe and Kr atoms has been studied by the crossed-beam method. Absolute cross sections for total positive ion formation from both the monomer and the dimer were determined from the threshold to several electron volts relative energy above threshold. The cross sections for the monomer varied over three orders of magnitude, ranging from 0–1.0 × 10−16 cm2 for TlCl, to 0–8 × 10−18 cm2 for TlBr, to 0–1.6 × 10−19 cm2 for TlI when Xe was the collidant. Similar ranges were measured for Kr on TlCl and TlI, but the cross sections were approximately a factor of 10 smaller for TlBr. The observed cross sections for positive ion production from the dimer molecules were in the range of 1 to 20 × 10−16 cm2. Time-of-flight mass spectrometry permitted the determination of partial cross sections for each positive and negative ion channel in the collision. The excitation functions for the reaction A+BC → A+B++C− (direct polar dissociation) exhibited a concave upward curvature in the postthreshold region, while those for A+BC → AB++C− (complex formation) showed a step behavior for TlCl and a concave downward curvature for TlBr. The yield of Tl2X+ was used in calculating the cross section for polar dissociation of the dimer. Only monatomic halogen negative ions were observed. The experimental results are discussed within the framework of a surface crossing model. The model predicts the essential features of the experimental results for both the thallium halide monomers and dimers.