A molecular plasma generated by a particle beam generally departs strongly from a Maxwellian distribution and exhibits a pronounced depletion of electrons as soon as cold electrons cross the first vibrational barrier. We show that this situation produces screened Coulomb interactions that enlarge the Coulomb cross-section in the neighbourhood of this first vibrational threshold. Including these screened interactions through the Balescu–Lenard operator with all excitation transitions leads to a modification of the distribution at the first vibrational barrier. An application is given to the case of low-pressure nitrogen lasers.