Vibronic relaxation dynamics of o-dichlorobenzene in its lowest excited singlet state

Vibronic dynamics of o-dichlorobenzene in its lowest excited singlet state, S-1, is investigated in real time by using femtosecond pump-probe method, combined with time-of-flight mass spectroscopy and photoelectron velocity mapping technique. Relaxation processes for the excitation in the range of 276-252 nm can be fitted by single exponential decay model, while in the case of wavelength shorter than 252 nm two-exponential decay model must be adopted for simulating transient profiles. Lifetime constants of the vibrationally excited S-1 states change from 651 +/- 10 ps for 276 nm excitation to 61 +/- 1 ps for 242 nm excitation. Both the internal conversion from the S-1 to the highly vibrationally excited ground state S-0 and the intersystem crossing from the S-1 to the triplet state are supposed to play important roles in deexcitation processes. Exponential fitting of the de-excitation rates on the excitation energy implies such de-excitation process starts from the highly vibrationally excited S-0 state, which is validated, by probing the relaxation following photoexcitation at 281 nm, below the S-1 origin. Time-dependent photoelectron kinetic energy distributions have been obtained experimentally. As the excitation wavelength changes from 276 nm to 242 nm, different cationic vibronic vibrations can be populated, determined by the Franck-Condon factors between the large geometry distorted excited singlet states and final cationic states. (C) 2017 Elsevier B.V. All rights reserved.