ISMO

Institut des Sciences Moléculaires d'Orsay


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CNRS UPS




dimanche 6 décembre


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Accueil > Équipes scientifiques > Systèmes Moléculaires, Astrophysique et Environnement (SYSTEMAE) > Offres de stages, thèses et post-docs > Sticking of a H atom and a graphenic surface including relaxation of the substrate of the substrate : effect of the anharmonicity on phonon modes

Stage théorique de niveau M2

Sticking of a H atom and a graphenic surface including relaxation of the substrate of the substrate : effect of the anharmonicity on phonon modes

Niveau M2

Molecular hydrogen H2, one of the most abundant molecules in InterStellar Medium (ISM), can be formed by association of H atoms on interstellar dust grains. In our study, these grains will be modelled as graphenic surfaces. Two distinct mechanisms are usually assumed for these reactive processes : (i) two H atoms, in weak interaction with the grain (physisorption), may collide to form an H2 molecule, which is subsequently released in the gas phase ; (ii) a H atom in strong interaction with the surface (chemisorption) can react with an impinging H atom from the gas phase to form H2. In both cases, a precise determination of the probability of the first adsorption of a H atom is crucial for subsequent simulations of the H2 formation. This information, in turn, is essential in other more applied areas such as hydrogen storage [1] or graphene technology [2].
Both of the mechanisms of formation of H2 involve energy transfer between the sticking particles and the surface, thus leading to energy redistribution among the phonon bath of the substrate. Consequently, in addition to describing the motion of the reacting atoms, it is very important to accurately take into account the dissipation processes (phonon modes) induced by the nuclear degrees of freedom of the surface. The H atom interaction with a graphene surface may involve either weak dispersion interactions at large distances (physisorption) or stronger interactions at shorter distances (chemisorption). More specifically, an H atom chemisorbed on a surface C atom induces a local deformation (”puckering”) of the surface. This, in turn, translates into an activation barrier (of about 0.2 eV) for the chemisorption process. Conversely, the physisorption process is barrierless. In this respect, the sticking of an H atom on graphenic surfaces has been object of many dynamical studies (quantum, classical, and mixed quantum-classical), either in a chemisorbed state [3-7] or in a physisorbed state [8,9]. However, the simultaneous study of both chemisorbed and physisorbed states in a same calculation has not been tackled yet.
The objective of this internship is to treat both the chemisorption and physisorption processes in a single reaction dynamics calculation, using a novel quantum-classical trajectory method. For this, we propose the calculation of the vibrational modes and couplings between the surface and the motion of the individual H atoms. The coupling among the vibrational modes will be obtained from the eigenvectors of the dynamical matrix of the solid. In the harmonic approximation, the vibrational modes will be calculated by means of the Quantum Espresso computer code suite. Moreover, we will investigate the effect of the anharmonicity on the phonon modes. To tackle this study, notions in solid state physics would be useful.
These calculations will constitute the first step in the ANR HYTRAJ project (HYbrid quantum TRAJectory approach for low temperature reactive processes in condensed phase) in which novel quantum-classical trajectory methods are being developed. This ANR project is a collaboration between three laboratories (ISMO and ICP at Orsay and LUPM at Montpellier).

This internship will be supervised by Sabine Morisset, Nathalie Rougeau, and Daniel Pelaez-Ruiz.

References :
[1] V. Tozzini, V. Pellegrini, Prospects for hydrogen storage in graphene, Phys. Chem. Chem. Phys., 15:80-89, 2013.
[2] M. Bonfanti, S. Achilli, R. Martinazzo, Sticking of atomic hydrogen on graphene, J. Phys. : Condens. Matter., 30:283002, 2018.
[3] S. Morisset and A. Allouche. J. Chem. Phys., 129:024509, 2008.
[4] M Bonfanti, B. Jackson, K.H. Hugues, I. Burghardt, and R. Martinazzo, Quantum dynamics of hydrogen atoms on
graphene : I. System-bath modeling, J. Chem. Phys., 143:114705, 2015.
[5] M Bonfanti, B. Jackson, K.H. Hugues, I. Burghardt, and R. Martinazzo, Quantum dynamics of hydrogen atoms on
graphene. II. Sticking, J. Chem. Phys., 143:124704, 2015.
[6] F. Karlicky, B. Lepetit, and D. Lemoine. J. Chem. Phys., 140:124702, 2014.
[7] S. Morisset, Y. Ferro, A. Allouche, J. Chem. Phys., 133:044508, 2010.
[8] B. Lepetit and B. Jackson. Phys. rev. Lett., 107:236102, 2011.
[9] B. Lepetit, D. Lemoine, Z. Medina, and B. Jackson. J. Chem. Phys., 134:114705, 2011.

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