Real-time observation of cascaded electronic relaxation processes in p-Fluorotoluene.

Ultrafast electronic relaxation processes following two photoexcitation of 400nm in p-Fluorotoluene (pFT) have been investigated utilizing time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. Cascaded electronic relaxation processes started from the electronically excited S state are directly imaged in real time and well characterized by two distinct time constants of ~85±10fs and 2.4±0.3ps. The rapid component corresponds to the lifetime of the initially excited S state, including the structure relaxation from the Franck-Condon region to the conical intersection of S/S and the subsequent internal conversion to the highly excited S state. While, the slower relaxation constant is attributed to the further internal conversion to the high levels of S from the secondarily populated S locating in the channel three region. Moreover, dynamical differences with benzene and toluene of analogous structures, including, specifically, the slightly slower relaxation rate of S and the evidently faster decay of S, are also presented and tentatively interpreted as the substituent effects. In addition, photoelectron kinetic energy and angular distributions reveal the feature of accidental resonances with low-lying Rydberg states (the 3p, 4s and 4p states) during the multi-photon ionization process, providing totally unexpected but very interesting information for pFT.