  We explore coherent control of Penning and associative ionization in cold
collisions of metastable He$^*({2}^3\text{S})$ atoms via the quantum
interference between different states of the He$_2^*$ collision complex. By
tuning the preparation coefficients of the initial atomic spin states, we can
benefit from the quantum interference between molecular channels to maximize or
minimize the cross sections for Penning and associative ionization. In
particular, we find that we can enhance the ionization ratio by 30% in the cold
regime. This work is significant for the coherent control of chemical reactions
in the cold and ultracold regime.
