Combustion of 1,5-dinitrobiuret (DNB) in the presence of nitric acid using ReaxFF molecular dynamics simulations.

In this study we have examined the combustion dynamics of 1,5-dinitrobiuret (DNB) and nitric acid using reactive molecular dynamics simulations. Simulations were performed using the ReaxFF force field with parameters that were fitted against quantum mechanical calculations on model compounds/clusters relevant for this particular chemical system. Several different compositions were investigated, at densities of 0.5 and 1.0 g/mL, to examine the reaction kinetics in a dense vapor and liquid phase of these mixtures. Our simulations show that at certain compositions of the mixture reaction kinetics result in a very sharp release of thermal energy, which we associate with spontaneous ignition or hypergolicity. Analysis of key reaction mechanisms responsible for this process is discussed.

Effect of formic acid addition on water cluster stability and structure.

Computational chemistry simulations were performed to determine the effect that the addition of a single formic acid molecule has on the structure and stability of protonated water clusters. Previous experimental studies showed that addition of formic acid to protonated pure water results in higher intensities of large-sized clusters when compared to pure water and methanol-water mixed clusters. For larger, protonated clusters, molecular dynamics simulations were performed on H(+)(H(2)O)(n), H(+)(H(2)O)(n)CH(3)OH, and H(+)(H(2)O)(n)CHOOH clusters, 19-28 molecules in size, using a reactive force field (ReaxFF). Based on these computations, formic acid-water clusters were found to have significantly higher binding energies per molecule. Addition of formic acid to a water cluster was found to alter the structure of the hydrogen-bonding network, creating selective sites within the cluster, enabling the formation of new hydrogen bonds, and increasing both the stability of the cluster and its rate of growth.

Sputtering yields for C60 and Au3 bombardment of water ice as a function of incident kinetic energy.

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.

Mesoscale energy deposition footprint model for kiloelectronvolt cluster bombardment of solids.

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.

Surface sensitivity in cluster-ion-induced sputtering.

The ion beam-induced removal of thin water ice films condensed onto Ag and bombarded by energetic Au, Au2, Au3, and C60 projectiles is examined both experimentally and with molecular dynamics computer simulations. For water overlayers of thicknesses greater than 10 A, the yields of sputtered Ag+ secondary ions decay exponentially with increasing ice thickness, revealing characteristic decay lengths of 24, 20, 18, and 7.0 A, respectively. It is shown that these values manifest the characteristic depths of projectile energy loss, rather than escape depths of the sputtered Ag atoms through the water ice overlayer. Computer simulations show that the mechanism of ejection involves the sweeping away of overlayer water molecules, allowing for an unimpeded escape of ejected Ag atoms. The relevance of these data with respect to surface sensitivity in secondary ion mass spectrometry is discussed.