Femtosecond UV-pump/mid-infrared-probe spectroscopy was used to explore in detail the primary photochemical events of the free radical initiator, (2,4,6-trimethylbenzoyl)diphenylphosphine oxide, in liquid dichloromethane solution at room temperature. Following electronic excitation of its lowest excited singlet state, S, the radical initiator undergoes an intersystem crossing to the triplet ground state, T, with a time constant of 135 ps. A subsequent α-cleavage occurs from the triplet state with a time constant of 15 ps and yields a trimethylbenzoyl radical together with a diphenylphosphinoyl radical. Transient absorptions from the S and T states are observed that can be assigned to the P═O stretching mode and the symmetric in-plane deformation mode of the trimethylphenyl moiety of the radical initiator.
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