In many chemical reactions with more than one possible outcome, the branching ratio is 
nearly constant over a wide range of collision energies. In barrierless systems governed by 
long-range interactions, however, the branching ratio is more sensitive to collision energy, 
and its dependence on it can be useful for better understanding the dynamics and 
reconstructing interaction potentials. Here we present the reaction rates of Penning and 
associative ionisation of metastable neon and helium with argon atoms. We obtain reaction 
rates in merge beam experiments, over a wide range of collision energies corresponding to 
that of room temperature, all the way down to a few millikelvins. We observe a change of 
two orders of magnitude in the branching ratio in the measured collision energy range and 
explain these changes using theoretical calculations.