Selectivity and Stereodynamics Effects in the Scattering of Nitrogen Molecules from a Graphite Surface

Abstract

The scattering of N2 molecules from a graphite surface was studied using molecular dynamics simulations based on a state-to-state semiclassical collisional method in conjunction with a new reactive potential energy surface that includes the long-range part describing properly, for all accessible molecular configurations with respect to the surface, the noncovalent interactions promoting the physisorption. Molecules in well-defined initial internal vibrational and rotational states (v i , j i ) impinge on the surface with collisional energy (E coll) covering the range from thermal (0.01 eV) up to hyperthermal values (2.0 eV). In addition to the ground state of ortho and para symmetry, nitrogen molecules excited initially in low-medium v i and j i levels have been also considered. The main features of scattering dynamics, already reported in the literature, are confirmed on a larger range of both initial rotational levels and collision energies. However, new selectivity and stereodynamics effects are highlighted by studying in detail the microscopic mechanisms driven by the interaction that promotes physisorption. This investigation contributes also to a broader understanding of molecular scattering from surfaces by relating observed effects to phenomena controlled by rotational alignment and rotational rainbow, already well-characterized both experimentally and theoretically, in gas phase collisions and for molecular scattering from metallic surfaces.

Stereodynamic Effects of CO Molecules Scattered from a Graphite Surface

“…The basic features of the scattering of carbon monoxide molecules in well-defined initial rotovibrational states ( j i , v i ) from a graphite surface have been studied by exploiting MD calculations. The used method treats collision events semiclassically and has been successfully applied over the last years to highlight the main selectivity and peculiarities of hydrogen, oxygen, and nitrogen molecules interacting with the same graphite surface. − In the past, it has been adopted to describe the dynamics of elementary gas-surface processes involving atoms/molecules with different surfaces (see for instance refs − and references therein). The method, based on the assumption that each collision event can be characterized by solving the relevant three-dimensional (3D) Hamilton’s equations of motion self-consistently with the dynamics of the lattice phonons, also includes the evaluation of the molecular state-to-state transitions associated with the internal degree of freedom rearrangement.…”

“…Before proceeding in the integration of Hamilton’s equation, it is necessary to determine both the dynamics of phonons for the considered graphite 3D crystal and an accurate PES driving the reaction under study. Here, we consider the same graphite crystal and, consequently, the same phonon frequency distribution of refs − . Therefore, the graphite 3D crystal lattice used in the calculations consists of 186 carbon atoms, disposed on three layers according to the appropriate lattice symmetry .…”

“…f sw ( r C–O ) is a weight function that switches the interaction potential between V CO–graphite and V C/O–graphite as r C–O , the intramolecular distance, increases. The analytical expression of f sw ( r C–O ) is the same used in the building up of reactive PES in the previous study on the interaction of the molecules with graphite. , This weight function is chosen to provide a quick switching between 1 and 0, preserving the values of V CO–graphite and V C/O–graphite for r C–O lower and higher, respectively, of the intramolecular distance considered critical for CO molecule dissociation ( r diss = 3.5 Å) . The potential is obtained as a sum of both components, weighted according to the f sw ( r C–O ) value, in the range of distances at which the switch occurs.…”

“…The criteria adopted in the present analysis, leading to the assignment of a given trajectory to one of the previously listed reaction channels, are similar to those used in refs − . So, molecular scattering occurs if, after the interaction with the graphite surface, the intramolecular distance ( r C–O ) is lower than r diss and at the same time the distance between its CM and the surface is larger than 8.0 Å.…”

“…For this reason, we have undertaken the study of the interaction of light molecules, with well-defined internal energy, with a graphite surface. In the previous investigations − some selectivity and peculiarities, together with stereodynamic effects, have been pointed out for H 2 (and its isotopologue molecules), O 2 , and N 2 molecules using chemical molecular dynamics (MD) simulations in conjunction with an accurate formulation of the interaction potential energy surface (PES), which drives the collision dynamics. In particular, the adopted formulation exploits an improved Lennard-Jones (ILJ) model that represents the long-range interaction, playing a key role in the trapping of gaseous molecules on surfaces, with a high degree of accuracy based on the intrinsic physical properties of interacting species.…”