Dissipative quantum dynamics of Eley-Rideal H2 recombination on graphite

Pasquini1, M. Bonfanti1 and R. Martinazzo1,2

1.Dipartimento di Chimica, Università degli Studi di Milano, v. Golgi 19, 20133 Milano, Italy

2.Istituto di Scienze e Tecnologie Molecolari, Centro Nazionale Ricerche, v. Golgi 19, 20133 Milano, Italy

The molecular hydrogen recombination on graphitic surface has been widely studied in the last decades, mainly because of its importance for the chemistry of the InterStellar Medium (ISM). In fact, H2 is the most abundant molecule in the universe, but its formation mechanism is not yet fully understood. It is now generally accepted that the reaction occurs

on the graphitic surface of interstellar dust grains [1]. Among the the typical mechanism of gas-surface chemistry, we focused on the Eley-Rideal reaction. This mechanism occurs when one H atom ( the so called “targon”) is previously adsorbed on the substrate and in thermal equilibrium with it; a second hydrogen atom (called the “incidon”), coming from the gas phase, collides and forms the molecule, which leaves the surface.

Due to the collision between the reactants and the products formation, a large amount of energy is released. Part of the available energy goes in the H2 excitation, while the remaining part is transferred to the substrate and dissipated through phonons and surface heating. In order to fully understand the process dynamics, it is crucial to exploit a quantum approach and to correctly describe the energy transferred to the substrate. Thus, we developed a new approach in which a system-bath description is adopted and used to investigate the zero temperature quantum dynamics with high dimensional wavepacket calculations [2].

In our simulations the system is composed of three degrees of freedom (DOFs), which are the distances from the graphitic surface of the C atom bonded to the targon (zc) and of the two involved hydrogen atoms (zt and zi); only the collinear geometry as been considered. The system can exchange energy with a bath of independent harmonic oscillators.

The oscillators are chosen and characterized so that they can reproduce the fluctuating-dissipative properties of the environment and its coupling with the system.

We performed quantum dynamical simulations with the Multi Layer Multi Configuration Time Dependent Hartree (ML-MCTDH) method, as well as classical and quasiclassical calculations, in order to study the reaction probability as functions of the incidon collision energy.

[1] R. J. Gould and EE Salpeter, Astrophys. J., 138:393, 1963; D. A. Williams and E. Herbst, Surf. Sci., 500(1):823–837, 2002.

[2] M. Bonfanti, B. Jackson, K. H. Hughes, I. Burghardt and R. Martinazzo, J. Chem. Phys., 143, 124703, 2015; M. Bonfanti, B. Jackson, K.H. Hughes, I. Burghardt and R. Martinazzo, J. Chem. Phys., 143, 124704, 2015.