Polycrystalline tetracene films have been explored using weak $\sim 30\mathrm{fs}$ visible laser pulses that excite the lowest singlet exciton as well as coherent vibrational motion. Transient difference spectra show a triplet absorption which arises following singlet fission (SF) and persists for 1.6 ns without decay. Adaptive pulse shaping identifies multipulse optimal fields which maximize this absorption feature by $\sim 20\%$. These are comprised of subpulses separated by time delays well correlated with the period of lattice vibrations suggesting such modes control the yield of SF photochemistry.

• Received 21 August 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.257403