Theoretical calculations on ground and excited electronic states of N-2 adsorbed on Pt(100) are carried out to investigate the properties of photoinduced electron attachment states. The metal-adsorbate system is described by an embedded cluster method, and configuration interaction theory is used to resolve the electronic states. The N-2 molecule is found to adsorb at an atop Pt atom site with an adsorption energy of 0.18 eV and with a vibrational frequency 70 cm(-1) lower than in the gas phase. Only the perpendicular orientation is stable. An electron attachment state corresponding to excitation of an electron from the Fermi level of the metal to a pi* orbital of N-2 is found to occur at a vertical excitation energy of 4.2 eV (triplet state) and 4.3 eV (singlet state). The NN internuclear distance increases in the excited state and this leads to a range of excitation energies, from 3.6 eV to the minimum of the excited triplet state potential surface to 4.2 eV for the vertical excitation. The corresponding range for singlet states is 3.8 to 4.3 eV. The equilibrium distance of nitrogen from the surface is 2.09 Angstrom for the ground state and 1.96 Angstrom for the electron attachment state. Deexcitation from the excited state to the repulsive region of the ground-state potential can lead to desorption from the surface. Nitrogen does not desorb or dissociate in the excited electronic state.