Molecular frame photoelectron angular distribution as a sensitive access to electronic structure and dynamics

在光电离和衰变过程中发射的电子的体固定框架角发射分布（所谓的分子框架角分布，MFAD）被认为是分子结构的非常灵敏的探针。当从离子中逸出时，发射的电子积累了关于目标和过程本身动力学的详细信息，从内部照亮了分子势。MFAD由电子部分连续波发射的所有跃迁振幅的相干叠加给出，因此，提供了最完整的信息，这是不可访问的。在过去的二十年里，MFAD被常规地用于研究初始电子态和核心空穴的定位，形状共振和多电子过程，电子相关和不同的电子衰变过程，分子结构和几何形状，以及随后的化学反应的核动力学。在过去的二十年里，我们的小组正在发展和不断改进一种方法，用于分子中电子连续谱的理论描述，这被称为单中心（SC）方法。它允许在分子中的角分辨电离和衰变过程的准确解释。SC方法在最近的一些实验中有许多成功的应用，如：核心空穴定位、双核心空穴产生、原子间库仑衰变、非偶极（延迟）效应、分子分裂的光电子衍射成像、光电离时间延迟等，说明了MFAD的力量，以更深入地了解这些基本的物理过程。拟议的理论项目旨在考虑三个基本和及时的应用，从而刺激新的具有挑战性的实验。对于我们的研究，我们选择了电子结构最简单和常见的双原子分子CO和N2。我们，首先，建议系统地研究由双芯空穴（DCH）生成发射的二次光电子的MFAD，以了解单和双位点DCH后离子势的不同对称性如何印在各自的MFAD中。其次，我们建议调查的非偶极子（延迟）的影响，所造成的高能光子，对前/后不对称性的高能光电子发射的固定在空间的分子的MFAD。最后，我们打算系统地研究电子发射时间延迟在1 s-光电离的电子动能和相对于分子轴的发射角的函数。

The body-fixed frame angular emission distributions of electrons emitted upon photoionization and decay processes (the so-called molecular frame angular distributions, MFADs) are known to be very sensitive probes for molecular structures. When escaping from the ion, the emitted electrons accumulate detailed information on the target and on the dynamics of a process itself, illuminating the molecular potential from within. MFADs are given by a coherent superposition of all transition amplitudes for an emission of the electron partial continuum waves and, as a consequence, provide the most complete information, which is not accessible otherwise. In the last two decades, MFADs were routinely utilized to study properties of initial electronic states and localization of core holes, shape resonances and multi-electron processes, electron correlations and different electronic decay processes, molecular structure and geometry, nuclear dynamics towards following chemical reactions. During the past two decades, our group is developing and permanently improving a method for the theoretical description of electron continuum spectrum in molecules, which is known as a single center (SC) method. It allows for accurate interpretation of angle-resolved ionization and decay processes in molecules. Plenty of successful applications of the SC method to recent experiments on core hole localization, double core hole generation, interatomic Coulombic decay, nondipole (retardation) effects, photoelectron diffractive imaging of molecular breakup, photoionization time-delays, etc., illustrated a power of MFADs to deeper understand those fundamental physical processes. The proposed theoretical project aims at considering three fundamental and timely applications and stimulating thereby new challenging experiments. For our study, we chose simplest and common in the electronic structure diatomic molecules CO and N2. We, first, suggest to systematically study MFADs of the secondary photoelectrons emitted by the double core hole (DCH) generation, in order to understand how different symmetry of the ionic potential after single- and double-site DCHs is imprinted in the respective MFADs. Secondly, we propose to investigate an impact of the nondipole (retardation) effects, caused by high-energy photons, on the forward/backward asymmetry of the MFADs of high-energy photoelectrons emitted from the fixed-in-space molecules. Finally, we intend to systematically investigate electron emission time delays in the 1s-photoionization as a function of the electron kinetic energy and an emission angle with respect to the molecular axis.