A novel detection scheme for pump–repump–probe spectroscopy is presented, where the use of modulation and referencing allows to record the efficiency of a photochemical reaction as a function of the pump–repump delay in a single measurement. This new technique is applied to investigate the ring-opening reaction of an indolylfulgide after pre-excitation. Here the reaction efficiency doubles, when the excitation pulse is very close to the pre-excitation pulse, and the improvement of the efficiency decays with increasing pump–repump delay on the time scale of about 5 ps.