A theoretical consideration of collisional processes for excited atoms within an atomic beam is made. The results are presented of calculations of rate constants for the associative ionisation reaction (AI) when resonantly excited atoms of sodium collide. It was shown that the output of charged particles in AI reactions in atomic beam experiments may greatly differ from that in gas cell conditions due to the inherent features of the distribution function of the beam atoms over their relative velocities. The parameters of the model of the ionisation process are derived from a comparison of experimental data obtained in atomic beam and gas cell conditions.