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Numerical investigation of the elastic scattering of hydrogen (isotopes) and helium at graphite (0001) surfaces at beam energies of 1 to 4 eV using a split-step Fourier method.

Huber SE, Hell T, Probst M, Ostermann A - Theor Chem Acc (2013)

Bottom Line: The hydrogen- and helium-graphite potentials are derived from density functional theory calculations using a cluster model for the graphite surface.We observe that the elastic interaction of tritium and helium with graphite differs fundamentally.Our investigations imply that wave packet studies, complementary to classical atomistic molecular dynamics simulations open another angle to the microscopic view on the physics underlying the sputtering of graphite exposed to hot plasma.

View Article: PubMed Central - PubMed

Affiliation: Institute for Ion Physics and Applied Physics, Innsbruck University, Technikerstrasse 25, 6020 Innsbruck, Austria.

ABSTRACT

We report simulations of the elastic scattering of atomic hydrogen isotopes and helium beams from graphite (0001) surfaces in an energy range of 1-4 eV. To this aim, we numerically solve a time-dependent Schrödinger equation using a split-step Fourier method. The hydrogen- and helium-graphite potentials are derived from density functional theory calculations using a cluster model for the graphite surface. We observe that the elastic interaction of tritium and helium with graphite differs fundamentally. Whereas the wave packets in the helium beam are directed to the centers of the aromatic cycles constituting the hexagonal graphite lattice, they are directed toward the rings in case of the hydrogen beams. These observations emphasize the importance of swift chemical sputtering for the chemical erosion of graphite and provide a fundamental justification of the graphite peeling mechanism observed in molecular dynamics studies. Our investigations imply that wave packet studies, complementary to classical atomistic molecular dynamics simulations open another angle to the microscopic view on the physics underlying the sputtering of graphite exposed to hot plasma.

No MeSH data available.


Related in: MedlinePlus

Dwell times in the case of H, D or T scattering at graphite (0001) surfaces for different impact energies
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Fig6: Dwell times in the case of H, D or T scattering at graphite (0001) surfaces for different impact energies

Mentions: The reason for the difference between He and H/D/T are the various local minima in the H/D/T-graphite potential. This is clearly seen in the plots of the dwell time for hydrogen, deuterium and tritium in Fig. 6. For an energy of 1 eV, the dwell time exhibits a long tail which is a consequence of partial reflection when the wave packet passes through the minima in front of the steep repulsive increase of the potential. Thus, the wave packet is first reflected by the potential wall and then by the edges of the minima, parts of it several times. Hence, the dwell time decreases significantly slower than in the case of He in agreement with an earlier wave packet study focusing on impact energies in a range of 0.1–0.9 eV [31]. At higher energies, the dwell time becomes more Gaussian-shaped as the influence of these features of the H/D/T-graphite potential become less significant in agreement with physical intuition, because as energies become higher the importance of quantum dynamic effects diminishes. Nevertheless, the results at higher energies are in agreement also with a recent investigation of the sticking coefficients of H/D/T on graphite which have been shown to decrease with increasing impact energy [47]. These features as well as the scaling with the square root of the impinging particle’s mass are well reproduced in Fig. 6. The dwell time at 2 eV approaches zero still slower as in the case of He reflection, but the difference is much smaller than at 1 eV. The dwell time curve for 3 eV is already Gaussian-shaped. The curve for the impact energy of 4 eV again exhibits a small tail. This could be due to the fact that the classical permeation energy at bridge site is 4.5 eV and thus is only slightly higher than the impact energy. This subsequently increases the possibility of encountering small potential walls when the wave packet is reflected into regions of higher energy.Fig. 6


Numerical investigation of the elastic scattering of hydrogen (isotopes) and helium at graphite (0001) surfaces at beam energies of 1 to 4 eV using a split-step Fourier method.

Huber SE, Hell T, Probst M, Ostermann A - Theor Chem Acc (2013)

Dwell times in the case of H, D or T scattering at graphite (0001) surfaces for different impact energies
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4551279&req=5

Fig6: Dwell times in the case of H, D or T scattering at graphite (0001) surfaces for different impact energies
Mentions: The reason for the difference between He and H/D/T are the various local minima in the H/D/T-graphite potential. This is clearly seen in the plots of the dwell time for hydrogen, deuterium and tritium in Fig. 6. For an energy of 1 eV, the dwell time exhibits a long tail which is a consequence of partial reflection when the wave packet passes through the minima in front of the steep repulsive increase of the potential. Thus, the wave packet is first reflected by the potential wall and then by the edges of the minima, parts of it several times. Hence, the dwell time decreases significantly slower than in the case of He in agreement with an earlier wave packet study focusing on impact energies in a range of 0.1–0.9 eV [31]. At higher energies, the dwell time becomes more Gaussian-shaped as the influence of these features of the H/D/T-graphite potential become less significant in agreement with physical intuition, because as energies become higher the importance of quantum dynamic effects diminishes. Nevertheless, the results at higher energies are in agreement also with a recent investigation of the sticking coefficients of H/D/T on graphite which have been shown to decrease with increasing impact energy [47]. These features as well as the scaling with the square root of the impinging particle’s mass are well reproduced in Fig. 6. The dwell time at 2 eV approaches zero still slower as in the case of He reflection, but the difference is much smaller than at 1 eV. The dwell time curve for 3 eV is already Gaussian-shaped. The curve for the impact energy of 4 eV again exhibits a small tail. This could be due to the fact that the classical permeation energy at bridge site is 4.5 eV and thus is only slightly higher than the impact energy. This subsequently increases the possibility of encountering small potential walls when the wave packet is reflected into regions of higher energy.Fig. 6

Bottom Line: The hydrogen- and helium-graphite potentials are derived from density functional theory calculations using a cluster model for the graphite surface.We observe that the elastic interaction of tritium and helium with graphite differs fundamentally.Our investigations imply that wave packet studies, complementary to classical atomistic molecular dynamics simulations open another angle to the microscopic view on the physics underlying the sputtering of graphite exposed to hot plasma.

View Article: PubMed Central - PubMed

Affiliation: Institute for Ion Physics and Applied Physics, Innsbruck University, Technikerstrasse 25, 6020 Innsbruck, Austria.

ABSTRACT

We report simulations of the elastic scattering of atomic hydrogen isotopes and helium beams from graphite (0001) surfaces in an energy range of 1-4 eV. To this aim, we numerically solve a time-dependent Schrödinger equation using a split-step Fourier method. The hydrogen- and helium-graphite potentials are derived from density functional theory calculations using a cluster model for the graphite surface. We observe that the elastic interaction of tritium and helium with graphite differs fundamentally. Whereas the wave packets in the helium beam are directed to the centers of the aromatic cycles constituting the hexagonal graphite lattice, they are directed toward the rings in case of the hydrogen beams. These observations emphasize the importance of swift chemical sputtering for the chemical erosion of graphite and provide a fundamental justification of the graphite peeling mechanism observed in molecular dynamics studies. Our investigations imply that wave packet studies, complementary to classical atomistic molecular dynamics simulations open another angle to the microscopic view on the physics underlying the sputtering of graphite exposed to hot plasma.

No MeSH data available.


Related in: MedlinePlus