A model was developed for the excitation of the unidentified infrared bands (UIBs) by H atom impacts in the interstellar medium. It builds upon the fact that, in the presence of far-ultraviolet (far-UV) radiation and hydrocarbon grains, the hydrogen gas will be partially dissociated and the grain surface will be partially hydrogenated and partially covered with free C bonds. Under such a statistical equilibrium, H atoms from the gas will recombine with C atoms at the grain surface at some rate. At each recombination, the H atom deposits an energy of about 5 eV in the grain. Half of this is directly converted into vibrational excitation, always distributed in the same way among the most tightly coupled vibration modes of the grain. Absent frequent grain–grain collisions, the only outlet for this energy is infrared re-emission, part of it in the UIBs, provided the chemical structure of the grains is adequate, and the other part in the continuum. The partition only depends upon the grain size, all grains being assumed to have the same constitution. Only a fraction, about 0.25, of the grains (among the smallest ones) will contribute significantly to the UIBs.
It is shown quantitatively that H atom impacts are generally more efficient excitation agents than UV absorption because of the overwhelming abundance of H relative to UV photons. Only very close to young bright stars is this no longer true because photon flux then largely exceeds H atom flux. Thus, H atom impacts and far-UV absorption are both necessary to understand the variety of observed UIB spectra.
The model translates into a small number of equations enabling a quantitative comparison of its predictions with available astronomical observations, which have become exquisitely rich and accurate in the last two decades.