We report optical measurements of the internal and kinetic energies of the electronically excited molecules SiN(B2Σ) and SiH(A2Δ) produced by ion bombardment of silicon. Both molecules are observed to have Boltzmann-like vibrational and rotational energy distributions with average internal energies ~ 0.3 eV, and average kinetic energies ~ 3.0 eV. A simple classical model of vibrational excitation of diatomic molecules due to direct collisional ejection is presented. The model calculations are in qualitative agreement with experimental results, and they illustrate the role of an important dynamical factor - the ratio of the collision duration to the vibrational period of the molecule. There is a propensity for the molecule to respond to collisions as a single unit when this ratio is large, whereas impulsive momentum transfer to the constituent atoms is obtained when this ratio is small. © 1985.