A general theory of excitation of a diatomic molecule due to collision with an atom (or a molecule), for example the He-HF system, in the presence of an infrared laser beam is presented. The effect of collision on the radiative excitation process is investigated by changing the various collision parameters, for example the impact parameter, collision velocity etc., with different values of molecule-field detunings.For a complete description, the quantum dynamics of excitation of a diatomic molecule due to a laser field is investigated in detail by using a quasi-energy (non-perturbative) formalism.