At the Institut des Sciences Moléculaires d’Orsay, (ISMO), within the framework of the ANR HyTRAJ project, the study of the sticking of an hydrogen atom on a graphenic surface will be studied taking into account the coupling with the vibrational modes of the substrate (phonons). The aim is to study the dynamics of collage using the quantum trajectory method developed in the ANR project.

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 post-doctoral position 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

[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.
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graphene : I. System-bath modeling, J. Chem. Phys., 143:114705, 2015.
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[9] B. Lepetit, D. Lemoine, Z. Medina, and B. Jackson. J. Chem. Phys., 134:114705, 2011.