Time-resolved photoelectron imaging of azulene

在库普曼定理的框架内，我们可以松散地将光电子谱解释为对占据的分子轨道上的电子结合能的测量，人们有兴趣问一问，从通过化学反应过程获得的瞬时光电子信号的测量中可以学到什么。例如，我们能跟踪光化学反应或热开环反应中活性电子的构型变化吗？我们能区分一致与连续键的形成或断裂吗？电子-电子相互作用(组态相互作用)在多大程度上模糊了这一图景？详细研究光能转化为其他形式的电能量和机械能的机制，对于从光生物学(如视觉)到纳米技术(如分子棘轮)等许多重要现象的基本理解具有重要意义。显然，为了能够做到这一点，我们需要一种仪器，能够在化学反应的时间尺度上跟踪光电子光谱的变化；这种变化可能快到几十飞秒(一飞秒是百万分之一秒的千分之一秒)。时间分辨光电子成像光谱(TRPEIS)是研究这种光化学的一种新兴技术。这种方法是基于泵浦-探测光谱学与带电粒子成像的结合。飞秒时间尺度上的短脉冲被用来激发分子的非定态。用第二个延迟的脉冲电离分子来探测光电子的时间演化，并用飞行时间质谱仪末端的位置灵敏探测器探测产生的光电子。电子通过放置在激光-分子相互作用区周围的静电浸没透镜被引导到探测器。该透镜具有将带电粒子的速度矢量聚焦到探测器表面的特性。显然，通过反转萃取电极的偏压，该装置可以很容易地被配置为检测光离子，但光电子能谱中通常有更多的信息。我们已经使用这种方法研究了一些分子(如二氧化氮、吡嗪、天青烯)的光化学，并观察到了一些有趣的现象，如相干波包运动(振动和旋转)。事实证明，蔚蓝呈现出一种特别有趣的电离行为，但到目前为止，我们还没有时间分辨率来真正揭示这种动力学。有了这个提议，我们的目标是将我们实验的时间分辨率提高到25fS或更高。

Within the framework of Koopmans' theorem we can loosely interpret photoelectron spectra as a measure of the binding energy of electrons in occupied molecular orbitals and it is of interest to ask what can one learn from the measurement of transient photoelectron signals obtained through the course of a chemical reaction. Can we, for example, follow configurational changes in the active electrons of a photochemical or thermal ring opening reaction? Can we distinguish concerted from sequential bond formation or rupture? To what extent does electron-electron correlation (the configuration interaction) blur the picture? Detailed investigation into the mechanisms by which optical energy is transformed into other forms of energy, electrical and mechanical, is of great relevance to developing a fundamental understanding of many important phenomena from photobiology (e.g. vision) to nanotechnology (e.g. molecular ratchets). Cleary, in order to be able do this we require an instrument which can follow changes in the photoelectrum spectrum on the time-scale of a chemical reaction; which maybe as fast as tens of femtoseconds (one femtosecond is 1 thousanth of a millionth of a millionth of a second). Time-resolved photo-electron imaging spectroscopy (TRPEIS) is an emerging technique with which to study such photochemistry. The method is based on a marriage of pump-probe spectroscopy to charged particle imaging. A short pulse on the femtosecond time-scale is used to excite a non-stationary state of a molecule. The time evolution is probed by a second time delayed pulse which is used to ionize the molecule, and the resulting photoelectron is detected with a position sensitive detector at the end of a time-of-flight mass spectrometer. The electron is guided to the detector by means of an electrostatic immersion lens placed around the laser-molecule interaction zone. The lens has the property of focusing the charged particle's velocity vector onto the surface of the detector. Obviously by reversing the bias of the extracting electrodes the device can just as easily be configured to detect photoions, but there is generally more information in the photoelectron energy spectrum.We have used this method to study the photochemistry of a number of molecules (e.g. nitrogen dioxide, pryrazine, azulene) and have observed a number of interesting phenomena such as coherent wavepacket motion (both vibrational and rotational). Azulene turns out to exhibit a particularly interesting ionization behaviour but until now we have not had the temporal resolution to really unravel the dynamics. With this proposal our ambition is to push the time resolution of our experiment to 25 fs or better.