Reaction Dynamics Following keV Cluster Bombardment

Abstract

The nature of the regions that are favorable for chemical reactions has been investigated for a pure amorphous water ice substrate following energetic bombardment by C60 and Au3 cluster projectiles using molecular dynamics (MD) simulations. The simulations show that both projectiles, especially C60, produce regions where a plethora of reactions occur at elevated densities indicating that multiple atoms or molecules are involved simultaneously in the reactions initiated by cluster bombardment. The total number of reacted water molecules is significantly less than the total sputtering yield, which confirms that both cluster projectiles are useful for molecular depth profiling experiments.

Molecular ions in cluster bombardment: what clues do the molecular dynamics simulations provide?

“…The logical conclusion is that intact molecular ions must rise from the part of the sample that also gives rise to the ejection of intact neutral molecules. Moreover, the simulations show that hundreds of free H atoms [5] and even a few H + ions [4] incorporate in the crater's edge. These incorporated ions are positioned to create (M+H) + ions within the solid.…”

“…[4] The energy density in this cylindrical region is sufficiently high that numerous reactions occur. Too many ions eject compared to the experiment, therefore a direct ionization mechanism is not likely.…”

RIMS analysis of ion induced fragmentation of molecules sputtered from an enriched U3O8 matrix

“…Cluster ion beams such as Au 3 + , Bi 3 + and C 60 + provide ~3 orders of magnitude increase in ion yield as compared to monatomic ion sources [17,18]. This is because, unlike the collision cascade generate by monatomic ions, cluster ions deposit a majority of their energy within the first few nanometers of the surface, resulting in significant increases in sputter yield [19]. The observed cluster ion sputter yields increase non-linearly with cluster nuclearity; indicating a significant deviation from the collision cascade model.…”

Cluster SIMS of Organic Materials: Theoretical Insights

“…69,70 Second, the ionization produced in water ice by Au 3 and C 60 was investigated using a water potential with an activation energy of ∼17 eV for the dissociation into H + and OH − . 71,72 These simulations show that the energized nanovolume is a cauldron where ground-state atoms coexist with many excited state atoms and ions in a compressed phase for a very short time, and where virtually any reaction is energetically possible. Beyond the specific results, these simulations emphasize the difficulty of properly describing the complex chemistry at work in the first picosecond following cluster impact and, thereby, make us reflect on the limits of the existing models.…”