Signatures of nonadiabatic ${\text{O}}_2$ dissociation at Al(111): First-principles fewest-switches study

Recently, spin selection rules have been invoked to explain the discrepancy between measured and calculated adsorption probabilities of molecular oxygen reacting with Al(111). In this work, we inspect the impact of nonadiabatic spin transitions on the dynamics of this system from first principles. For this purpose, the motion on two distinct potential-energy surfaces associated to different spin configurations and possible transitions between them are inspected by means of the fewest-switches algorithm. Within this framework, we especially focus on the influence of such spin transitions on observables accessible to molecular-beam experiments. On this basis, we suggest experimental setups that can validate the occurrence of such transitions and discuss their feasibility.

Figure 1

(Color online) Schematic plot of the potential-energy surfaces along the minimum-energy path in the triplet state for dissociation in parallel alignment over the fcc site. Beside the adiabatic (spin-polarized), the triplet (constrained spin-polarized), and the singlet (adiabatic non-spin-polarized) PESs, which have been determined by means of DFT, also the expected functional form of the potentials associated with the first two excited singlet states and the so-called “ionic” state are shown in (a). In (b), the lower and the upper bounds derived for the coupling of the triplet and the singlet PES are plotted along the minimum-energy path of (a).

Figure 2

(Color online) (a) Sticking coefficient $S_0\left(E\right)$ as computed by molecular dynamics (MD) on the triplet potential-energy surface and by Tully’s surface-hopping (SH) method for both a maximal and a minimal coupling of the triplet and the singlet PES. The respective experimental data[12] are shown as well. (b) Area-normalized histograms of the barriers found on the individual PESs by the two-dimensional nudged elastic band technique (see text). The potentials along the minimum-energy path over the bridge site are shown in the inset.

Figure 3

(Color online) Reflection coefficient at high incident translational energies as computed by MD on the triplet PES and by Tully’s SH method for a minimal and a maximal coupling of the triplet and the singlet PES.

Figure 4

(Color online) (a) Reflection coefficient $R_0$ as function of the incident kinetic energy of singlet oxygen molecules, as calculated by the SH method with a minimal coupling. Additionally, the respective data obtained by MD simulations on the singlet PES alone are shown. (b) Relative triplet yield $R_T/R_0$ for both calculations.

Figure 5

(Color online) Same as Fig. 4, but for maximal coupling.

Figure 6

(Color online) Average kinetic energy gained by singlet molecules when being converted to triplet molecules in the backscattering process at the aluminum surface for a (a) minimal coupling; (b) maximal coupling.

Figure 7

(Color online) Reflection coefficient $R_0$ as function of the incident kinetic energy of singlet oxygen molecules, as calculated by the SH method with a (a) minimal coupling; (b) maximal coupling. The results both within the frozen substrate approximation (dashed lines) and for a three-dimensional surface oscillator (squares and triangles) are shown. In the respective insets, the relative triplet yield $R_T/R_0$ for both calculations is plotted as well.