Ab Initio Molecular Dynamics Study of Hot Atom Dynamics after Dissociative Adsorption of ${\mathbf{H}}_2$ on Pd(100)

The relaxation of hot hydrogen atoms upon the dissociative adsorption of ${\mathrm{H}}_2$ on Pd(100) was studied by ab initio molecular dynamics simulations based on density functional theory, modeling the full dissociative adsorption process in a consistent manner. In spite of the nonlinear dependence of every single trajectory on the run conditions, on the average it is the energy dissipation to the substrate that determines the mean distance of the two H atoms after adsorption which amounts to three to four lattice units and provides an upper bound for heavier species such as oxygen.

Figure 1

Calculated trajectories of hydrogen atoms upon the dissociative adsorption on clean Pd(100) within a ($6\times 6$) surface unit cell. The initial kinetic energy was 0.2 eV. The total run time was 2.5 ps. Three sets of trajectories with the same initial conditions are shown. The full lines correspond to the joint motion of the two hydrogen atoms while for the lighter dash-dotted lines the trajectories were computed individually after the two hydrogen atoms reached a separation of 2.5 Å. The dotted lines correspond to a trajectory with a fixed substrate but dissipation included via velocity rescaling. The surface unit cell of the simulations is indicated by the dashed blue line.

Figure 2

Mean lateral distance of the two hydrogen atoms upon the ${\mathrm{H}}_2$ dissociative adsorption on Pd(100) as a function of the run time determined by averaging over AIMD trajectories for different computational setups (see text). The basic AIMD results denoted by full MD were obtained by averaging over 100 trajectories whereas the other results are based on at least 75 trajectories in each case.

Figure 3

Calculated mean total kinetic energy of the Pd substrate atoms and the impinging ${\mathrm{H}}_2$ molecule as a function of the run time for initial kinetic energies of 200 meV and 500 meV. The substrate was modeled as a three-layer slab with the uppermost Pd layer allowed to move. In addition, results for five-layer slab calculations and $E_{\mathrm{kin}}({\mathrm{H}}_2)=200\mathrm{meV}$ with the two uppermost Pd layer allowed to move are included.