Calculation of electrostatic interaction energies in physisorption studies of ionic surfaces

Abstract

The electrostatic contributions to the adsorption energy for molecules physisorbed at an ionic crystal surface are usually calculated by means of a multipole expansion method in which the series is truncated at the quadrupole term. However, explicit calculation for some typical systems indicate that terms involving higher multipole moments may be relatively large. The actual importance of these higher order terms is found to depend both on the characteristics of the adsorbed molecule and on the detailed shape of the electrostatic potential at a particular adsorption site. The analysis is illustrated for CO, N₂ and H₂ physisorbed on the (100) surface of NaCl; for certain orientations on this surface the sum of the octopole and hexadecapole moment contributions, for CO and N₂, is calculated to be ∼120 % of the quadrupole interaction energy. Some general considerations on the limitations of multipole moment expansions in the calculation of weak molecule-surface interactions are discussed.