Search for a local effect in multiatom resonant core excitation in a surface species: Photoemission and photon-stimulated desorption fromN2on Ni(111)

Abstract

We have investigated photoabsorption, N $1s$ photoelectron emission, and photon-stimulated desorption (PSD) of ${\mathrm{N}}^{+}$ ions, from ${\mathrm{N}}_{2}$ molecules perpendicularly chemisorbed on Ni(111) surfaces, in the energy range of the Ni $2{p}_{3∕2,1∕2}$ excitations. For this system, N $1s$ photoemission monitors single-core-hole production, whereas PSD of ${\mathrm{N}}^{+}$ 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 ${\mathrm{N}}^{+}$ 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\ifmmode^\circ\else\textdegree\fi{} with respect to the surface) to steeper angles (43\ifmmode^\circ\else\textdegree\fi{} and 50\ifmmode^\circ\else\textdegree\fi{} with respect to the surface). For grazing incidence, strong variations of both the photoelectron and the ${\mathrm{N}}^{+}$ signals with photon energy and angle of incidence were found in the Ni $2p$ region which are compatible with x-ray optical (dielectric) effects, one manifestation of multiatom resonant photoemission. The ${\mathrm{N}}^{+}$ KED's, which are known to depend strongly on the nature of the electronic excitation responsible, did not change across the Ni $2{p}_{3∕2}$ 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 ${\mathrm{N}}_{2}$ 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.