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

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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.