Studies On Excited State Dynamics Of2,6-Lutidine, Benzene And Dimethylformamide

The dynamics processes of excited molecules are divided into photophysical and photochemical processes, including molecular radiation, internal conversion, intersystem crossing, vibrational relaxation, dissociation, isomerization, etc. These dynamics processes play a key role in the area of life science, environmental science, etc, such as the most basic process in nature-nonradiative energy transfer in photosynthesis, the cis-trans isomerization of retinal radical located in the center of bacteriorhodopsin, the DNA damage in the ultraviolet radiant environment, etc. Femtosecond time resolved photoelectron imaging technique has the property of femtosecond time resolution and can follow the dynamics processes of excited molecules in real time. In this dissertation, the excited state dynamics of2,6-lutidine, benzene and dimethylformamide have been studied by femtosecond time resolved photoelectron imaging coupled with mass spectroscopy.2,6-lutidine, benzene and dimethylformamide are the most common molecules in drug, chemistry and biology synthesis. Specifically,2,6-lutidine is the main synthesized material for the treatment of high blood pressure medicine and emergency medicine, benzene is the basic materials of oil industry, and dimethylformamide is the simple model of peptide bonds in protein. The study of excited state dynamics in these molecules is conducive to understand their reaction mechanisms in drug, chemistry and biology synthesis. The main contents of this dissertation are shown follows(1) The study on the ultrafast internal conversion in2,6-lutidine.2,6-lutidine is excited to the S2state by one266nm photon, the S2state dynamics processes of2,6-lutidine is confirmed as the internal conversion through S2/S1and S2/S0conical intersections of potential energy surface by time-dependent changes of four photoelectron bands in the photoelectron spectroscopy. The timescale of internal conversion through S2/S1conical intersections is635fs, while the timescale of internal conversion through S1/S0conical intersections is4.37ps. The internal conversions are the dominated decay channels in2,6-lutidine, differs from the isomerization in pyridine. This is due to the increase of charge density with two methyl groups, making the ring more stable.(2) The study on the ultrafast nonradiative dynamics of S2state in benzene. Benzene is excited to the S2state by two400nm photons, two decay channels are observed:a rapid decay pathway with43fs, and a longer lived channel with1.06ps. The fast decay is ascribed to the internal conversion through the S2-S1conical intersections, and the slower one is attributed to the intersystem crossing from the S2state. It’s possibe that the vibrational excitation modes (9101611,510, and910) from S2state enhance the spin-orbit coupling greatly, which leads to the S2-T3intersystem crossing. The photoelectron kinetic energy distributions between1.1eV and1.7eV exhibit a rapid energy transfer, revealing nuclear motion on the S2adiabatic surface.(3) The investigation on the ultrafast internal conversion of S2state in dimethylformamide. Following two-photon400nm excitation, we found that the population of S2(ππ*) state undergoes ultrafast internal conversion to the highly vibrationally excited Si (nπ*) state within99fs by very fast C-N stretching. While the nonradiative deactivation of Si (nπ*) state occurs in2.4ps, and it is to a large extent due to the C-N bond cleavage from the S1potential energy surface, which would be able to efficiently compete with the internal conversion of S1-So. Because of dimethylformamide is the simple model of peptide bonds, our work provides help for studying the energy transfer and conversion of peptide bond in protein.(4) The preliminary study on the photodissociation of iodobenzene in the pump-control-probe laser field. The pump laser pulse,200nm, is used to pump the C6H5I molecules to an excited state, the control laser pulse is1400nm, which is used to change the potential energy surface of iodobenzene, and the third probe laser pulse is used to ionize the I/I*fragment with304nm wavelength. The resonance-enhanced multiphoton ionization spectrums show the pathway of I+is from the ionization of neutral iodine atom, and the time profiles of iodobenzene reflect the ac stark effect from infrared laser.(5) The study on simulated work of hexapole state selection and oriented molecules. We write the Monte Carlo program and assign the single quantum rotational state. Then, the orientational probability distribution functions of single quantum rotational states are acquired. Finally, the ion lens is improved and the angular distributions of photo fragments in the photodissociation of oriented molecules are simulated.