Studies of Ultrafast Molecular processes by Multiparticle Imaging Techniques

当化学键形成时，分子轨道是如何从分子轨道中形成的？反之，如果化学键断裂，原子轨道又是如何从分子轨道中产生的？当前的项目旨在为回答这个问题做出重大贡献。我们将使用同步辐射将一个分子核心激发到一个陡峭的排斥解离势能曲线，并使用在解离过程中发射的俄歇电子作为探针，提供核间距离演变的最详细信息和瞬态轨道在其衰变瞬间的形状。俄歇电子和碎片离子的能量和方向将重合成像。通过这种方法，我们将获得前所未有的时间（核心洞钟），光谱（窄带同步加速器）和空间信息（俄歇电子衍射）的组合。我们将得到解离发生时的第一张电子衍射图像。通过符合性检测，我们将在分子的体固定框架内得到这些衍射图像。离子碎片的测量方向产生了分子在光激发瞬间的方向。在衰变过程中发射的电子在分子电位中衍射。衰变时的核间距离以及电子密度分布都被编码在分子框架中电子的角分布中。分子在衰变过程中发光，电子和离子的同步检测允许测量解离分子的自我图像。该项目是在DFG-ANR呼吁中提交的法德联合项目。该联盟的法国合作伙伴在这些超快工艺方面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BESSY（低能）和萨克莱的SOLEIL（高能）两个互补的同步辐射源上联合进行。我们已经在BESSY对HCl的L壳电离进行了联合预备测试实验，这清楚地表明了我们的方法的力量。HCl作为基准体系将在HCl - k壳上进一步研究。然后我们将探索H2S在S 2p和S 1s边的超快解离。在第三组实验中，我们将研究氧的超快解离。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电子到一个排斥性轨道。这将触发解离。在解离过程中，首先会发射一个快速的俄歇电子，随后会发射一个非常低能量（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键断裂过程中对称性是如何以及何时被打破的。

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.