Weinkauf R, Lehr L, Schlag E W, Salzmann S, Marian C M
Institut für Physikalische Chemie und Elektrochemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstrasse 1, 40225, Düsseldorf, Germany.
Phys Chem Chem Phys. 2008 Jan 21;10(3):393-404. doi: 10.1039/b710381f. Epub 2007 Nov 28.
A hybrid of a time-of-flight mass spectrometer and a time-of-flight "magnetic-bottle type" photoelectron (PE) spectrometer is used for fs pump-probe investigations of the excited state dynamics of thiophene. A resonant two-photon ionization spectrum of the onset of the excited states has been recorded with a tunable UV laser of 190 fs pulse width. With the pump laser set to the first intense transition we find by UV probe ionization first a small time shift of the maxima in the PE spectrum and then a fast decay to a low constant intensity level. The fitted time constants are 80+/-10 fs, and 25+/-10 fs, respectively. Theoretical calculations show that upon geometry relaxation the electronic state order changes and conical intersections between excited states exist. We use the vertical state order S1, S2, S3 to define the terms S1, S2, and S3 for the characterization of the electron configuration of these states. On the basis of our theoretical result we discuss the electronic state order in the UV spectra and identify in the photoelectron spectrum the origin of the first cation excited state D1. The fast excited state dynamics agrees best with a vibrational dynamics in the photo-excited S1 (80+/-10 fs) and an ultrafast decay via a conical intersection, presumably a ring opening to the S3 state (25+/-10 fs). The subsequently observed weak constant signal is taken as an indication, that in the gas phase the ring-closure to S0 is slower than 50 ps. An ultrafast equilibrium between S1 and S2 before ring opening is not supported by our data.
将飞行时间质谱仪与飞行时间“磁瓶型”光电子（PE）谱仪相结合，用于对噻吩激发态动力学进行飞秒泵浦 - 探测研究。用脉冲宽度为190 fs的可调谐紫外激光记录了激发态起始的共振双光子电离光谱。将泵浦激光设置为第一个强跃迁时，通过紫外探测电离，我们首先发现光电子能谱中最大值出现了小的时间偏移，然后快速衰减到低的恒定强度水平。拟合的时间常数分别为80±10 fs和25±10 fs。理论计算表明，在几何结构弛豫时，电子态顺序发生变化，且激发态之间存在锥形交叉。我们使用垂直态顺序S1、S2、S3来定义这些态的术语S1、S2和S3，以表征它们的电子构型。基于我们的理论结果，我们讨论了紫外光谱中的电子态顺序，并在光电子能谱中确定了第一个阳离子激发态D1的起源。快速的激发态动力学与光激发S1态中的振动动力学（80±10 fs）以及通过锥形交叉（可能是开环到S3态，25±10 fs）的超快衰减最为吻合。随后观察到的微弱恒定信号表明，在气相中闭环到S0态的速度慢于50 ps。我们的数据不支持在开环之前S1和S2之间存在超快平衡。
