Using H2-N2 mixed gases, high-electric-field-induced formation of ions such as N2 + and N2H+ on a surface of gold has been investigated in the pulsed-laser time-of-flight atom-probe field ion microscope. The N2H+ ions are found from the lattice steps of the surface in the range (30-40) V/nm. The formation of the ions depends not only on the applied field strength, but also on the atomic structure of the substrate. By applying theoretical values of the binding energy and ionization potentials of N2H, as calculated by the ab initio method and using measurements of the energy distributions of the N2 + and N2H+ ions, several conclusions can be drawn.