  A model was developed for the excitation of the UIBs by H atom impacts in the
Interstellar Medium. It builds upon the fact that, in the presence of 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 carbon 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 IR reemission, 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 impacts are generally more efficient
excitation agents than UV absorption because of the overwhelming abundance of
hydrogen 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
impacts and FUV 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.
