A chemical dynamics simulation was performed to study low energy collisions between N 2 and a graphite surface. The simulations were performed as a function of collision energy (6.34 and 14.41 kcal/mol), incident polar angle (20−70°) and random azimuthal angle. The following properties were determined and analyzed for the N 2 + graphite collisions:(1) translational and rotational energy distributions of the scattered N 2 ; (2) distribution of the final polar angle for the scattered N 2 ; (3) number of bounces of N 2 on the surface before scattering. Direct scattering with only a single bounce is dominant for all incident angles. Scattering with multiple collisions with the surface becomes important for incident angles far from the surface normal. For trajectories that desorb, the parallel component of the N 2 incident energy is conserved due to the extremely short residence times of N 2 on the surface. For scattering with an incident energy of 6.34 kcal/mol, incident polar angle of 40°, and final polar angle of 50°the percentage incident energy loss is 29% from the simulations, while the value is 27% for a hard cube model used to interpret experiment (J. Phys.: Condes. Matter 2012, 24, 354001). The incident energy is primarily transferred to surface vibrational modes, with a very small fraction transferred to N 2 rotation. An angular dependence is observed for the energy transfer, with energy transfer more efficient for incident angles close to surface normal.
Energy
transfer in collisions of O2 with a graphite
surface was studied by chemical dynamics simulations. The simulations
were performed for three collision energies E
i of 2.1, 7.4, and 15 kcal/mol, with the initial incident angle
fixed at θi = 45°. Simulations were performed
for each E
i at a surface temperature T
surf = 300 K. For the higher surface temperature
of 1177 K, a simulation was only performed for E
i = 15 kcal/mol. The following properties were determined and
analyzed for the O2 + graphite collisions: (1) translational
energy distributions of the scattered O2; (2) distribution
of the final polar and azimuthal angle for the scattered O2; and (3) number of bounces of O2 on the surface before
scattering. The average energy transferred to the graphite surface
and that remaining in O2 translation, i.e., ⟨ΔE
surf⟩ and ⟨E
f⟩, exhibit a linear dependence with the initial translational
energy. For the O2 + graphite scattering, the physisorption/desorption
residence time distribution decays exponentially, with an increase
in residence time with a decrease in E
i. The rate at which the distribution decreases shows a near-linear
dependence with an increase in E
i. For
higher collisional energies of 7.4 and 15 kcal/mol, O2 scattering
from the surface follows a nearly quasi-trapping desorption process.
However, for the lowest collision energy, it mostly follows conventional
physisorption/desorption. For all of the scattering conditions considered
experimentally, the relationship between the average final translational
energy and average scattering angle for the O2 molecules
found from the simulations is in excellent agreement with the experimental
results. This experimental validation of precise simulation outcomes
is important as it indicates that collisional energy-transfer predictions
for this system can be reliably used in assessing interfacial energy
flow in a variety of technological applications, including high-performance
flight systems.
Intermolecular energy transfer for the vibrationally excited propylbenzene cation (CH) in a helium bath was studied with chemical dynamics simulations. The bond energy bond order relationship and electronic structure calculations were used to develop an intramolecular potential for CH. Spin component scaled MP2/6-311++G** calculations were used to develop an intermolecular potential for He + CH. The He + He intermolecular potential was determined from a previous explicitly correlated Gaussian electronic structure calculation. For the simulations, CH was prepared with a 100.1 kcal/mol excitation energy to compare with experiment. The average energy transfer from CH, ⟨ΔE⟩, decreased as CH was vibrationally relaxed and for the initial excitation energy ⟨ΔE⟩ = 0.64 kcal/mol. This result agrees well with the experimental ⟨ΔE⟩ value of 0.51 ± 0.26 kcal/mol for collisions of He with the ethylbenzene cation. The ⟨ΔE⟩ value found for He + CH collisions is compared with reported values of ⟨ΔE⟩ for He colliding with other molecules.
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