Search for a local effect in multiatom resonant core excitation in a surface species: Photoemission and photon-stimulated desorption from N ₂ on Ni(111)

We have investigated photoabsorption, N 1s photoelectron emission, and photon-stimulated desorption sPSDd of N ⁺ ions, from N ₂ molecules perpendicularly chemisorbed on Ni(111) surfaces, in the energy range of the Ni 2p _3/2, _1/2 excitations. For this system, N 1s photoemission monitors single-core-hole production, whereas PSD of N ⁺ is mainly due to excitation of multiply excited N 1s core-hole states. The amplitude variations of these two signals and the kinetic energy distributions (KED's) of the N ⁺ ions were recorded as functions of the photon energy. In addition, we measured the amplitude variations of PE and PSD as a function of the photon incidence angle, which was varied from grazing (7° with respect to the surface) to steeper angles (43° and 50° with respect to the surface). For grazing incidence, strong variations of both the photoelectron and the N ⁺ signals with photon energy and angle of incidence were found in the Ni 2p region which are compatible with x-ray optical sdielectricd effects, one manifestation of multiatom resonant photoemission. The N ⁺ KED's, which are known to depend strongly on the nature of the electronic excitation responsible, did not change across the Ni 2p _3/2 threshold, which excludes any type of state selectivity in the interatomic core-coupling effects observed. For N 1s photoemission, a first analysis of our data suggests a variation of the N 1s signal at the Ni 2p edges also for steeper angles of light incidence, of comparable magnitude to that at grazing incidence. However, more careful x-ray photoelectron spectroscopy experiments and the investigation of electronically stimulated desorption of neutral N ₂ molecules and N atoms reveal that these effects are due to a strong increase of beam damage when passing the Ni 2p edge; these effects could be reduced by rapidly scanning the sample under the beam. We thus conclude that for high angles of incidence most of the Ni 2p-related changes in our N 1s photoemission signal are due to beam damage.