Two-photon photoemission from NO adsorbed on Cu(111)

Abstract

Two-photon photoemission is observed for unoccupied and occupied states of molecularly adsorbed NO on Cu(111) at 130 K. Excitation and photoemission are performed with laser light of 5 ns duration in the photon energy range from 3.7 to 4.5 eV. Two photoemission structures are observed for the NO-adsorbed surface: a surface state structure and a NO-induced structure. The surface state structure arises from the occupied metal surface state via an intermediate image potential state. The image potential state shows substantial broadening by the NO adsorption. The NO-induced structure originates from a two-step process in which photoexcitation between the occupied and normally unoccupied states of adsorbed NO was followed by relaxation and photoemission. We find that the energy level of the occupied state of NO is 2.3 ± 0.1 eV below the Fermi level. The bottom of the broad unoccupied state of NO is 1.3 ± 0.1 eV above the Fermi level. Relaxation to bulk states becomes significant at an energy higher than 0.6 eV from the bottom of the unoccupied NO state.