Fingerprints for Spin-Selection Rules in the Interaction Dynamics of ${\mathrm{O}}_2$ at Al(111)

We perform mixed quantum-classical molecular dynamics simulations based on first-principles potential-energy surfaces to demonstrate that the scattering of a beam of singlet ${\mathrm{O}}_2$ molecules at Al(111) will enable an unambiguous assessment of the role of spin-selection rules for the adsorption dynamics. At thermal energies we predict a sticking probability that is substantially less than unity, with the repelled molecules exhibiting characteristic kinetic, vibrational and rotational signatures arising from the nonadiabatic spin transition.

Figure 1

Calculated singlet- and triplet potential energy along the minimum energy path for a triplet ${\mathrm{O}}_2$ molecule impinging side-on over the Al(111) fcc hollow site (cf. the schematic top view shown as the inset). The energy zero corresponds to a free ${\mathrm{O}}_2$ molecule in its triplet ground state. At corresponding distances $Z\to \infty$ far away from the Al(111) surface, the singlet energy is higher by $\Delta E_{\mathrm{TS}}\sim 1\mathrm{eV}$. Additionally shown is the spin coupling matrix element $V_c$ along the path. Obviously, the transitions due to this coupling are strongest at the crossing seam of the singlet and triplet PESs, which is located at rather large distances $Z>2\AA$ away from the surface.

Figure 2

Initial sticking coefficient at normal incidence for singlet oxygen molecules, $S_0^{*}(E)$, as computed for a dynamics confined to the singlet PES (▿) and by accounting for nonadiabatic transitions (□). The dashed line represents the sticking coefficient for triplet oxygen, simply shifted by $\Delta E_{\mathrm{TS}}$ (see text).

Figure 3

Percentage of the electronic potential energy $\Delta E_{\mathrm{TS}}$ redistributed into translational (○), rotational (□), and vibrational (▵) energy of the reflected ${\mathrm{O}}_2$ molecules. Note that all these molecules leave the surface as spin triplets.