Molecular dynamics simulations have been performed to model 5-keV C60 and Au3 projectile bombardment of an amorphous water substrate. The goal is to obtain detailed insights into the dynamics of motion in order to develop a straightforward and less computationally demanding model of the process of ejection. The molecular dynamics results provide the basis for the mesoscale energy deposition footprint model. This model provides a method for predicting relative yields based on information from less than 1 ps of simulation time.
Simulations of the initial oxidation process of a SiC
surface exposed
to O2 and H2O molecules was studied with ReaxFF,
an atomically detailed reactive molecular dynamics method that naturally
models the breaking and forming of bonds. In this work, the ReaxFF
forcefield was first expanded by training it with new quantum mechanics
data of the binding energy, equation of state, and heat of formation
of the SiC crystal, along with data from earlier studies that describes
Si – Si, Si – O, and Si – H interactions. This
expanded ReaxFF forcefield is capable of simultaneously describing
both Si–C–O and Si–O–H bonding interactions.
Using the forcefield, oxidation simulations were performed at various
temperatures (in the range of 500 to 5000 K), and the trajectories
were analyzed. The analyses showed that SiC gradually transforms into
the oxides of silicon with simultaneous formation of a graphite-like
layer. In presence of excess O2, the graphite-like layer
was further oxidized to CO and CO2. We also analyzed the
trajectories with two-atom and three-atom clusters to quantitatively
track the oxidation progress. This analysis clearly showed Si–O
and C–C bond formation at the expense of O–O and Si–C
bond consumption indicating SiC oxidation with simultaneous formation
for carbon-like structure. Consumption of SiC with O2 was
found to be faster than that with H2O. We have also reported
the oxidation simulation of SiC with a mixture of H2O and
O2. Oxidation proceeded effectively as a two-part sequence,
with O2 first oxidizing the SiC, followed then by H2O.
The total sputtering yields for water ice due to kiloelectronvolt cluster bombardment have been measured and compared to the predictions made by the mesoscale energy deposition footprint (MEDF) model. For C60 bombardment, the experimental yield varies almost linearly from 820 water molecule equivalents at an incident kinetic energy of 10 keV to 10 100 water molecule equivalents at a kinetic energy of 120 keV. For Au3 bombardment, the experimental yield varies almost linearly from 630 water molecule equivalents at an incident energy of 10 keV and rises to 1200 water molecule equivalents at 25 keV. The MEDF model is used to calculate relative yield trends with respect to incident energy using short-time molecular dynamics simulations. The results of these calculations indicate that the model can effectively predict the yield trends observed for these two clusters in experiments, although there is a consistent overestimate of the predicted induced C60 yield. It is hypothesized that this overestimate can be explained by the absence of reactions and ionization processes in the current simulations. Despite this omission, experimental yield trends can be accurately predicted using relatively small amounts of computer time. The success of the model in predicting the yield of water from ice films using a variety of energies and projectiles suggests this approach may greatly aid in the optimization of experimental configurations.
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