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

Abstract Ultrafast electronic relaxation processes following two photoexcitation of 400 nm 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 2 state are directly imaged in real time and well characterized by two distinct time constants of ~ 85 ± 10 fs and 2.4 ± 0.3 ps. The rapid component corresponds to the lifetime of the initially excited S 2 state, including the structure relaxation from the Franck-Condon region to the conical intersection of S 2 /S 1 and the subsequent internal conversion to the highly excited S 1 state. While, the slower relaxation constant is attributed to the further internal conversion to the high levels of S 0 from the secondarily populated S 1 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 2 and the evidently faster decay of S 1 , 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.