  We demonstrate for the first time coherent control of bond making, a
milestone on the way to coherent control of photo-induced bimolecular chemical
reactions. In strong-field multiphoton femtosecond photoassociation
experiments, we find the yield of detected magnesium dimer molecules to be
enhanced for positively chirped pulses and suppressed for negatively chirped
pulses. Our ab initio model shows that control is achieved by purification via
Franck-Condon filtering combined with chirp-dependent Raman transitions.
Experimental closed-loop phase optimization using a learning algorithm yields
an improved pulse that utilizes vibrational coherent dynamics in addition to
chirp-dependent Raman transitions. Our results show that coherent control of
binary photo-reactions is feasible even under thermal conditions.
