Studies of Ultrafast Molecular processes by Multiparticle Imaging Techniques

当一个化学键形成时，分子轨道是如何从组成部分的原子轨道形成的，或者反过来说，如果一个化学键断裂，原子轨道是如何从分子轨道形成的？目前的项目旨在为回答这一问题作出重大贡献。我们将使用同步辐射核心激发一个分子到一个陡峭的排斥解离势能曲线，并使用俄歇电子在解离过程中发射的探针提供最详细的信息的演变的核间距离和形状的瞬态轨道在其衰变的瞬间。俄歇电子和碎片离子的能量和方向将同时成像。通过这种方法，我们将获得前所未有的时间（芯孔钟），光谱（窄带宽同步辐射）和空间信息（俄歇电子衍射）的组合。我们将获得第一个电子衍射图像的解离发生。由于符合检测，我们将获得这些衍射图像中的体固定框架的分子。测量的离子碎片的方向产生的分子在瞬间的光激发的取向。在衰变过程中发射的电子在分子势中发生衍射。衰变时的核间距以及电子密度分布被编码在分子坐标系中的电子角分布中。分子在衰变过程中会发光，同时检测到电子和离子，可以测量解离分子的这种自我图像。该项目是DFG-ANR呼吁法国-德国联合项目的一个提案。该财团的法国合作伙伴对这些超快过程做出了开创性的贡献，德国合作伙伴开创了多粒子成像技术（COLTRIMS）。我们的实验将在柏林的两个互补同步辐射源BESSY（低能）和Saclay的SOLEIL（高能）共同进行。我们已经在BESSY上对HCl的L壳层电离进行了联合预备测试实验，这清楚地表明了我们方法的能力。HCl作为基准系统将在HCl K壳上进一步研究。 然后，我们将探索H2S在S 2 p和S 1 s边缘的超快解离。在第三个实验中，我们将研究O2的超快解离。在这里，我们将通过观察俄歇电子的多普勒频移来监测超快运动。我们将激发一个K电子到一个排斥轨道。这会触发分离在解离过程中，首先会发射出一个快速的俄歇电子，然后是一个能量很低（0.5 eV）的原子俄歇电子，我们将同时测量电子和分子轴。慢电子的多普勒频移将揭示在键断裂过程中对称性如何以及何时被破坏。

How are molecular orbitals formed from the atomic orbitals of the constituencies when a chemicalbond is formed or inversely how do the atomic orbitals emerge from the molecular ones if a bond breaks? The current project aims to make a major contribution towards answering this question. We will use synchrotron radiation to core excite a molecule to a steeply repulsive dissociating potential energy curve and use the Auger electrons which are emitted during the dissociation as a probe providing the most detailed information of the evolution of the internuclear distances and the shape of the transient orbitals at the instant of their decay. Energy and direction of the Auger electrons and fragment ions will be imaged in coincidence. With this approach we will gain an unprecedented combination of time (core hole clock), spectral (narrow band width synchrotron) and spatial information (Auger electron diffraction).We will obtain the first electron diffraction images of the dissociation as it occurs. Thanks to the coincidence detection we will obtain these diffraction images in the body-fixed frame of the molecule. The measured direction of the ionic fragments yields the orientation of the molecule at the instant of photo excitations. The electron which is emitted during the decay is diffracted in the molecular potential. The internuclear distances at the time of the decay as well as the electronic density distribution is encoded in the angular distribution of the electron in the molecular frame. The molecule illuminates itself during the decay and the coincident detection of electrons and ions allows measuring this self-image of the dissociating molecule.The project is a submission within the DFG-ANR call for joint French-German projects. The French partner of the consortium has pioneering contributions to these ultrafast processes, the German partner has pioneered multi particle imaging techniques (COLTRIMS).Our experiments will be performed jointly at two complementary synchrotron radiation sources BESSY in Berlin (low energy) and SOLEIL, Saclay (high energy). We have performed a joint preparatory test experiment on the L shell ionization of HCl at BESSY which clearly demonstrates the power of our approach. HCl as a bench mark system will be further studied on the HCl K-shell. We will then explore ultrafast dissociation of H2S at both the S 2p and the S 1s edges. In a third line of experiments we will study ultrafast dissociation of O2. Here we will monitor the ultrafast motion by observing the Doppler shift of the Auger electrons. We will excite a K electron to a repulsive orbital. This will trigger the dissociation. During the dissociation first a fast Auger electron will be emitted and subsequently a very low energy (0.5 eV) atomic Auger electron. We will measure both electrons and the molecular axis in coincidence. The Doppler shift of the slow electron will unveil how and when the symmetry is broken during the bond rupture.