Excited State Dynamics In 1,2,4-Trimethylbenzene And 2-Iodotoluene Investigated With Femtosecond Time-Rsolved Spectroscopy

Excited-state molecular dynamical processes play a fundamental role in the field of environmental science, life science and materials science etc, such as the formation and maintenance of the ozone layer, the photosynthesis, the molecular switch and the energy-storing materials etc. These processes mainly include light emission, vibrational relaxation, internal conversion, intersystem crossing, dissociation, isomerization, etc. Femtosecond time resolved photoelectron imaging and time-of-flight mass spectrometry technique have the ability of tracking excited-state molecular dynamical processes in real time with femtosecond time resolution. In this dissertation, the ultrafast internal conversion dynamics of 1,2,4-trimethylbenzene and the photodissociation dynamics of 2-iodotoluene have been investigated by femtosecond time-resolved photoelectron imaging coupled with time-resolved mass spectrum.1,2,4-trimethylbenzene and 2-iodotoluene are the most common raw materials in chemical and biological synthesis. Understanding the photophysical and photochemical mechanism in these molecules is helpful to give insight into their reaction mechanisms in chemical and biological synthesis. The main contents of this dissertation are shown as follows:(1) The investigation on the ultrafast S2â†'S1 state internal conversion dynamics in 1,2,4-trimethylbenzene. Upon absorption of two 400 nm photons, the 1,2,4-trimethylbenzene molecule is excited to the S2 state. The ultrafast S2â†'S1 state internal conversion dynamics in 1,2,4-trimethylbenzene is observed in real time based on the time-dependent changes of three photoelectron bands in the photoelectron spectroscopy. The lifetimes of initially excited S2 state and the secondly polulated vibrationally hot S1 state have been determined to be～72 fs and ～ 16.4 ps respectively. Comparing the results with previous studies on benzene, toluene and dimethylbenzene, it is observed that the lifetime of the secondly populated S1 state is increased with the increament of numbers of methyl substitution, which is likely due to that the evolution to the S1/S0 conical intersection area is hindered by methyl substitution.(2) The study on the photodissociation of 2-iodotoluene. The bound ππ* state and the repulsive no* state are populated simultaneously following excitation with 266 nm. The non-resonance multi-photon ionization with an intense laser field centered at 800 nm was used to follow the photodissociation dynamics of 2-iodotoluene. A rapid decay occurring timescale of～100 fs is observed on the time-resolved parent ion and fragment ions, which is attributed to the decay of initially populated repulsive no* state. In addition, a rise component with timescale of-380 fs is observed on time-dependent photodissociation fragments, which relects the averaged dissociation time of all dissociative channels. Furthermore, the ground-state iodine atom resonance wavelength of 298.23 nm has been used to selectively detect the dissociative channels leading to ground-state iodine atoms through resonance multi-photon ionization. And an averaged dissociation time ～400 fs is measured for all dissociation channels leading to ground-state iodine atoms.