Molecular dynamics simulations have been performed to gain microscopic insight into the factors that lead to molecular ejection after ion bombardment of an organic overlayer on a metal surface. The specific system modeled is benzene (C 6 H 6 ) adsorbed on Ag{111}. The kinetic energy and angular distributions of C 6 H 6 molecules obtained from the simulations match well with the experimentally measured distributions. The angular distributions of C 6 H 6 molecules show both normal and off-normal components. Analysis of individual trajectories reveal that the off-normal ejection arises from single collisions between substrate Ag atoms and C 6 H 6 molecules, while multiple collisions result in low-energy ejection along the surface normal. To separate issues of rotational and vibrational excitation from translational motion, calculations are also performed on an atomic adsorbate with a mass similar to that of C 6 H 6.

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.

Time distributions of neutral molecules desorbed from a chemisorbed self-assembled monolayer of phenylethanethiol on gold have been measured subsequent to 8 keV Ar + and H 2 + ion bombardment. These distributions show that, regardless of the projectile used, most of the ejected molecules leave the surface with thermal kinetic energies (∼0.03 eV). The shapes of the distributions have a strong surface temperature dependence over the range 240-300 K. This behavior is well described by a convolution of the Maxwell-Boltzmann distribution and the rate equation for first-order desorption. The results imply that kiloelectronvolt ion bombardment initiates a process which breaks the adsorbate-surface bond, leaving the resulting physisorbed molecules to evaporate after attaining thermal equilibrium with the substrate. A mechanism for this gentle cleavage of the adsorbate-substrate bond is proposed.

Kinetic energy distributions of Ni atoms in six electronic fine structure states ejected from a single crystal Ni{001} surface due to bombardment with 5 keV Ar 1 ions have been measured. These states arise from two different electronic configurations, 3d 8 4s 2 ͕a 3 F 4,3,2 ͖ and 3d 9 4s 1 ͕a 3 D 3,2 or a 1 D 2 ͖, which form three distinct fine structure manifolds within 0.422 eV of the 3 F 4 ground state. We find that the band structure effects dominate leading to larger populations in the excited 3 D 3,2 states than found for the ground state.

Time-of-flight distributions of neutral molecules ejected from various organic surfaces have been measured subsequent to 8 keV Ar+ and H2+ ion bombardment. The distributions show that depending on the physical and chemical nature of the substrate, the neutral molecules have strikingly different desorption profiles. For C6H6/Ag¿111¿, at low coverage the C6H6 molecules eject with energies in the range 0.25-1 eV while at high coverage most of the molecules desorb with thermal kinetic energies (approximately 0.04 eV). At intermediate coverage two peaks are present in the time-of-flight distribution indicating that two different mechanisms contribute to the desorption process. For self-assembled monolayers of phenylethanethiol on Au, while a minor ejection is observed at higher kinetic energy (approximately 1 eV) most of the molecules desorb with thermal kinetic energies (approximately 0.03 eV). Pyrenebutyric acid molecules ejected from monolayer and multilayer samples have kinetic energies close to 0.2 eV. One ejection mechanism is observed in this case. For tryptophan, most molecules eject with kinetic energies close to 0.1 eV. In addition, a feature unique to this case is the continuous emission of molecules from the surface that extends beyond 200 microseconds after ion impact. For all the multilayer samples investigated, a molecular collision cascade in the solid leads to ejection of molecules with kinetic energies in the range 0.1-0.3 eV.

No abstract

No abstract

No abstract