Femtosecond dynamics of excited states ofCOadsorbed onMgO(001)‐(1×1)

Abstract

We have investigated the time development of excited electronic states of $\mathrm{C}\mathrm{O}$ molecules adsorbed on the $\mathrm{Mg}\mathrm{O}(001)\text{--}(1\ifmmode\times\else\texttimes\fi{}1)$ surface using a state-of-the-art first principles approach. Density-functional theory is used to calculate the ground state geometry of the clean surface and of the molecules adsorbed on the surface. Thereafter, the quasiparticle band structures of bulk MgO, of the $\mathrm{Mg}\mathrm{O}(001)\text{--}(1\ifmmode\times\else\texttimes\fi{}1)$ surface, and of $\mathrm{C}\mathrm{O}$ adsorbed on the surface are calculated within the GW approximation. Taking the electron-hole interaction into consideration the electron-hole excitations and their optical spectrum are obtained from the solution of the Bethe-Salpeter equation for the electron-hole two-particle Green function. The optical spectra of bulk MgO, the $\mathrm{Mg}\mathrm{O}(001)\text{--}(1\ifmmode\times\else\texttimes\fi{}1)$ surface, and $\mathrm{C}\mathrm{O}$ adsorbed on the surface are calculated, yielding good agreement with available experimental data. Finally, based on the solution of the BSE for the adsorbate system $\mathrm{C}\mathrm{O}:\mathrm{Mg}\mathrm{O}(001)\text{--}(1\ifmmode\times\else\texttimes\fi{}1)$, the time propagation of molecular excitations is studied employing the time-dependent Schr\odinger equation. An initial $\mathrm{C}\mathrm{O}$ excitation exhibits a very fast decay due to its coupling to charge-transfer exciton states between the substrate and the adsorbate. The decay is characterized by a lifetime of about 1.6 fs, which is a factor of 5 faster than the decay of single-particle states.