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제109회 대한화학회 학술발표회, 총회 및 기기전시회 안내 Electron-Nuclear Coupling Strengths of Two Coumarin Conformers Determined Directly by the Coherent Vibrational Wave Packets

2012년 2월 22일 15시 36분 12초
PHYS.P-501 이곳을 클릭하시면 발표코드에 대한 설명을 보실 수 있습니다.
4월 25일 (수요일) 18:00~21:00
저자 및
이경진, 주태하
포항공과대학교 화학과, Korea
Electronic transition by a light pulse shorter than the vibrational period creates coherent vibrational wave packets. The wave packet motion can be observed by oscillations in various time-resolved spectroscopic signals. Among various methods, time-resolved fluorescence (TRF) has a unique advantage over transient absorption (TA), as it provides information on the dynamics in the electronic excited state exclusively. To launch and to observe vibrational wave packets, however, time resolution must be faster than ~½ of the vibrational period; for example, it has to be close to 30 fs to observe a 500 cm-1 mode, which is extremely hard to achieve in TRF. In this work, we report the observation of the coherent wave packet motions in the excited state of coumarin153 (C153) directly by TRF. We used fluorescence up-conversion following the excitation by two photon absorption. Then we obtained the TRF of C153 with a time resolution better than 30 fs. Oscillations were observed, which arise from the coherent vibrational wave packet motions in the electronic excited state of C153. Oscillation frequencies were retrieved by the linear prediction singular value decomposition (LPSVD) method guided by the Fourier transform to yield five frequency components below 500 cm-1. Analysis of the oscillation components and quantum mechanical ab initio calculations revealed that C153 exists in two conformations, i.e. syn and anti conformers, at room temperature. Normal mode frequencies in the ground and the excited states of two conformers of C153 are calculated, and the electron-nuclear coupling strengths (Huang-Rhys factors) were calculated for the vibrational modes below 500 cm-1 according to the work by Reimers. Especially, considering population ratio of the two conformers of C153 at room temperature, these oscillation frequencies, which have large electron-nuclear coupling strengths matches well with the wave packet oscillations from the experiments. Full theoretical simulation predicts the oscillation spectra reasonably well. The quantum mechanical calculations of the excited state, however, are not accurate enough to reproduce the experiment. Therefore, these results provide a benchmark system to assess a method of quantum mechanical excited state calculation as well as the spectroscopic and dynamical information on the excited state.