NUCLEAR SPUTTERING OF CONDENSED DIATOMIC GASES

Fluence-dependent sputtering yield measurements have been made on the condensed diatomic gases N-2, O-2, NO, and CO under bombardment by 5 keV Ar+ ions. Initial. yields for solid N-2. NO, and CO were determined for bombardment with 1-5 keV Ne+, Ar+, Kr+, and Xe+ ions. While these four targets have critical temperatures, sublimation energies, and densities similar to those of argon, their sputtering yields were found to be smaller than that for solid argon, and their fluence dependence quite different. For bombardment with 5 keV Ar+ ions, the sputtering yield for solid Ar is 1425 atoms/ion and approaches slowly a limiting value of about 700 atoms/ion as fluence increases. This decrease has been attributed to surface morphology changes. Under these conditions, the sputtering yield for CO starts out at 817 molecules/ion, increases slowly to 1300 molecules/ion with increasing incident ion fluence, and then decreases. The yield for NO starts out at 1396 molecules/ion and rapidly decreases to a steady 1140 molecules/ion. For O-2, the yield starts at no more than 700 molecules/ion and then increases extremely rapidly to a constant 900 molecules/ion. For N-2, the yield is only 350 molecules/ion and does not show any fluence dependence. For all projectiles tested and at all bombardment energies, it remains equal to one-fourth of the sputtering yield of solid argon. These results are accounted for by collision cascade theory and the gas-flow model of frozen gas sputtering, taking into, taking into account the expected chemical transformations. Most of the anticipated reaction products have been spectroscopically observed on the bombarded matrices. Initial sputtering yield measurements on the four solid targets were also performed with 1-5 keV He+ ions and are discussed in qualitative terms with reference to the low electronic and nuclear stopping power, insufficient to produce a gas-flow contribution to the sputtering yield.