Coverage-Dependent Molecular Ejection from Ion-Bombarded C₆H₆/Ag{111}

Abstract

Time-of-flight distributions, angular distributions, and relative sputtering yields of neutral benzene (C 6 H 6 ) molecules ejected from submonolayer to multilayer coverage of C 6 H 6 on Ag{111} have been measured after 8 keV ion bombardment. Two components are present in the time-of-flight distributions obtained using Ar + ion as the projectile. For low coverage a peak corresponding to kinetic energies ranging between 0.25 and 1 eV dominates the distribution, whereas for multilayer coverage a peak corresponding to extremely low kinetic energy (0.04 eV) becomes dominant. The total yield of the ejected neutral C 6 H 6 molecules is largest for a monolayer coverage and decreases to ∼50% of the maximum for multilayer samples. For low coverage, the C 6 H 6 kinetic energy and angular distributions take on the same characteristics as that of silver particles ejecting from the substrate, indicating that collisions originating in the metal substrate lead to the ejection of C 6 H 6 molecules. The low kinetic energy emission of molecules from the multilayer films is proposed to occur due to exothermic chemical reaction of fragments formed in a molecular collision cascade initiated by the projectile ions. Finally, for the entire coverage range investigated, no C 6 H 6 signal is observed when H 2 + ion is used as the projectile, indicating that a momentum-transfer process is important in the ejection of C 6 H 6 molecules.

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