Thesis or Dissertation Femtosecond spectroscopy studies of incoherent and coherent prosesses in DNA bases and laser dyes

薛, 冰  ,  Bing, Xue

pp.1 - 101 , 2016-03-25 , The University of Electro-Communications
Pump probe spectroscopy is a powerful method to study dynamic processes in materials or chemical compounds. In this thesis, by using the femtosecond laser pulses, we investigated two kinds of photon induced incoherent and coherent phenomena in the different delay time domain: The incoherent phenomena: the dynamics of excited states of RNA/DNA base molecule (uracil and thymine) by sub 10 femtosecond deep ultraviolet laser pulses excitation. From experimental result, two probe photon energy dependent lifetime constants were extracted from the difference absorption spectra measurement in the time range up to 1800 fs. From the lifetime constants, the relaxation processes through conical intersection (CI) are clearly understood. The shortest time constant less than 100 fs is assigned to due to relaxation through CI(S2-S1) from the first excited S2 1(ππ*) state to the S1 1(n?*) state. The second shortest time constant about 1 picosecond is assigned to the relaxation through another CI(S2 d-S0) from the deformed excited S2 d state to the S0 ground state. Specially the location of CI(S2-S1) and CI(S2 d-S0) are first time experimentally clarified at 4.33 eV and 4.36 eV for uracil, 4.45 eV and 4.36 for thymine. With the CI width of 0.21 eV and 0.038 eV for uracil, 0.05 eV and 0.031 eV for thymine, respectively. The coherent phenomena: A new method for non-degenerate two photon absorption cross section measurement was demonstrated and applied for several laser dyes. We improved the traditional method by introducing white light supercontinuum probe and multi-channel lock-in detection. With the advantage of the simplified system structure ensured less error sources, broadband TPA coefficients could be directly acquired in single measurement procedure with high confident data reliability. In this work, several laser dye chromophores are investigated, several of them to be the first reported to our knowledge. Compared to reported degenerate TPA result, an enhancement phenomena was confirmed and discussed with the help of theoretical calculations.

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