Secondary ion emission from keV energy atomic and polyatomic projectile impacts on sodium iodate

Abstract

Sodium iodate and sodium iodide are inorganic solids in which iodine exists in different chemical environments. In sodium iodate, the iodine atom is bonded to oxygen to make the trigonal pyramidal IO 3 anion, which in turn is incorporated with sodium into an ionic crystal. In sodium iodide, however, the iodide anion and sodium cation are ionically bound in a crystal lattice. Nearly half of the negative secondary ion yield generated from keV energy polyatomic ion impacts on a sodium iodate surface is characteristic of ion emission expected from sodium iodide (i.e. (NaI) n I −), yet x-ray photoelectron spectroscopy data indicate that the solid material resulting from aliquots of aqueous sodium iodate dried on stainless steel contains no more than 2% sodium iodide. To determine how the number of atoms in the primary ion influences the amount of iodide type ion formation from sodium iodate, secondary ion yield measurements were performed using Cs, (CsI)Cs, and (CsI) 2Cs projectiles incident at energies ranging from 10 to 25 keV. The experiments were run on an event-by-event basis at the level of single ion impacts. The yields of iodate (composed of Na, I, and O) and iodide type secondary ions increase with the energy of the projectile and the number of constituent atoms. When compared on a per-incident atom basis, however, we found that the yields of secondary ions characteristic of iodate saturate at three total projectile atoms, but continue to increase nonlinearly for iodide species (i.e. the yield per impacting atom increases).