Wavefunction overlap effects in the collisional excitation of NH₃ by slow molecular ions

Collisional excitation of ammonia from the ground ¹A₁ electronic state to various optically-allowed and dipole-forbidden states has been studied in 1.8 keV collisions with H₂⁺, N₂⁺, O₂⁺ and CO₂⁺ projectiles by energy loss spectrometry using a high-resolution ion translational energy spectrometer. The collisional excitation cross section is found to be sensitively dependent on whether the projectile ion is of Sigma or Pi symmetry. This dependence is interpreted as a manifestation of wavefunction overlap effects involving the highest occupied molecular orbitals of the target and projectile ions. Results of concomitant theoretical calculations of overlap integrals for different orientations of each projectile-target system provide support for the wavefunction overlap picture.