Understanding Surface Photochemistry from First Principles: The Case of CO-NiO(100)

The excitation mechanism in the CO-NiO(100) system induced by a uv-laser pulse has been investigated from first principles. For the laser-driven process, the relevant electronically excited states are identified, and it is shown that a transition within the CO molecule is the crucial excitation step rather than substrate mediated processes. A new mechanism is proposed, in which the formation of a genuine C-Ni bond in the excited state is the driving force for photodesorption rather than electrostatic interactions, as has been found in similar systems. This results in very high velocities of CO molecules desorbing from the NiO(100) surface after electronic relaxation.

Figure 1

Minimum energy geometry of CO on the ${\mathrm{NiO}}_5{\mathrm{Mg}}_{13}^{18+}$ cluster in the ground state. The point charge field used is not shown.

Figure 2

Active natural molecular orbitals of $\mathrm{CO}\mathrm{\text{−}}{\mathrm{NiO}}_5{\mathrm{Mg}}_{13}^{18+}/\mathrm{PCF}$ from $\mathrm{CASSCF}(4,5)$ of the ${\tilde{A}}^{3}E$ excited state. The geometry corresponds to the adsorption minimum of the ground state; occupation numbers of the natural orbitals are in parentheses. Illustrations were obtained using the MOLEKEL program [21, 22].

Figure 3

Potential energy curves along the desorption coordinate $Z$ for the ground state ${\tilde{X}}^3{B}_1$ (squares) and the excited states ${\tilde{a}}^{5}E$ (asterisks), ${\tilde{A}}^{3}E$ (plus signs), and ${\tilde{a}}^{1}E$ (crosses) of $\mathrm{CO}\mathrm{\text{−}}{\mathrm{NiO}}_5{\mathrm{Mg}}_{13}^{18+}/\mathrm{PCF}$ computed at the CASPT2 level. The C-O distance has been kept fixed to $R_{\mathrm{CO}}=1.143\AA$. The curves have not been corrected for the BSSE.