Collision induced dissociation of CsI and Cs2I2 to ion pairs by Kr, Xe, and SF6

Abstract

Absolute cross sections as functions of collision energy have been determined for collision induced dissociation of cesium iodide monomer and dimer to ion pairs. In these studies a beam of accelerated Xe, Kr, or SF6 projectiles was crossed with a thermal beam of cesium iodide. The partial cross sections for each product-ion channel were determined by time-of-flight mass spectrometry. For the rare gas-monomer collisions, the dependence of each partial cross section on the internal temperature of the CsI was also obtained. Collisions of Xe with CsI produced three-body dissociation as well as the formation of the molecular ions CsXe+ and IXe−. The formation of both the positive and negative molecular ions is primarily a reflection of the similar masses of Cs+ and I−, and was not observed in previously studied systems. For the same reason, Cs2I+ and CsI−2 resulting from collisions of Xe with Cs2I2 were formed with comparable intensities. At energies well above threshold, the total dissociation cross section for the rare gases colliding with CsI or Cs2I2 is large (≳10 Å2). Those for SF6 are approximately a factor of 5 smaller for the monomer, but only slightly smaller for the dimer. No ions containing SF6 were observed. The cross sections for three-body dissociation as well as molecular ion formation are relatively small in the region of the thermodynamic threshold (decreasing in the series Xe, Kr, and Ar). Analysis of the experimental results indicates that dissociation in this region only occurs for CsI molecules having considerable internal excitation, an effect related almost entirely to the projectile-target relative masses. A model which takes into account the coupling of internal motion with relative translational motion is shown to give an excellent description of the dissociation in the threshold region. Collinear trajectory calculations of the rare gases colliding with CsI were also performed in order to determine the threshold for dissociation as a function of the vibrational state of CsI.