Theoretical and experimental study of N(D) formation in nitrogen collisions with a metal surface

The dynamics of nitrogen collisions with metals partially covered by alkali atoms is studied both experimentally and theoretically. Our attention focuses on the formation of N⁻(¹D) metastable ions and their interaction with the surface. We present the electron energy spectra induced by slow collisions of N⁺ ions with partially cesiated Pd(111) surfaces under grazing incidence. These spectra display, as a function of Cs coverage, a sharp feature which is due to the autodetachment of N⁻(2p⁴, ¹D) to the N(2p³, ⁴S) ground state. Our calculations, performed with the coupled angular mode (CAM) method on the basis of the resonant electron exchange between the nitrogen atom in states of the 2p³ configuration and the metal surface, consistently explain how negative ions formed close to the surface can survive against electron loss to the metal during the outgoing trajectory and can later decay as free ions. In order to understand the alkali coverage dependence of the N⁻(¹D)-N(⁴S) peak intensity, the local character of the nitrogen interaction with the surface partially covered by adsorbate atoms has been taken into account.