Using a first-principles approach, we assign N 1s core-level shifts at ammonium exposed Si(001)2x1 surfaces to definite bonding configurations of N atoms. Model structures are obtained by fully relaxing the atomic positions of N atoms in different bonding configurations. Calculated values of N 1s core-level shifts of N-Si-3, N-Si2H, and N-SiH2 structural units show a linear dependence on the number of nearest-neighbor H atoms, in good agreement with data from photoemission experiments. Our results support the picture in which NH3 is adsorbed dissociatively as NH2 and H. (C) 2000 American Institute of Physics. [S0003-6951(00)00305-3].