Electromagnetic processes of nuclear excitation: from direct photoabsorption to free electron and muon capture

In the vast expanse of the Universe and on our planet, nuclei exist in a state of excitement. These excited nuclear states (isomers) can persist for varying periods, from fractions of a second to billions of years and beyond, before decaying to their ground state. If harnessed, feeding and depleting these isomers could represent a clean and high-density way to store and release energy on demand. The quest for efficient dynamical population control of nuclear isomer has long captivated the imagination of physicists, yet this elusive goal remains beyond our grasp. In this dissertation, I examine the potential of employing nuclear excitation mechanisms as viable tools for achieving such manipulation. Three processes of nuclear excitations from both theoretical and experimental perspectives are explored: direct photoabsorption, nuclear excitation by electron capture (NEEC), and nuclear excitation by free muon capture (NEuC). This thesis begins by delving into the historical framework of nuclear excitation by electron capture (NEEC), a process that was proposed in 1976 and is yet to be comprehended. A recently claimed observation has sparked new interest in nuclear excitation processes as a way to release the energy trapped in isomers. However, the irreconcilability between the first observation, the theoretical framework, and the recent repetition of the experiment reveals that there is still much to learn. Regardless of the specific process being examined, the primary goal is to increase the likelihood of their occurrence. One such possibility involves NEEC taking place in excited ions, where the screening effect of other electrons provides nearly resonant orbitals where capture can occur. This process was initially proposed to mitigate the discrepancy between the experimental finding and the theoretical prediction. In this new setting, three orders of magnitude increase in the NEEC cross-sections for 73Ge is found theoretically. Another approach enabling the manipulation of the NEEC cross-section involves engineering the electron wavefunction that undergoes capture. This technique not only demonstrates an increased occurrence of NEEC but also highlights the potential to alter the shell where the highest capture takes place. The second mechanism, NEuC, occurs in exotic muonic atoms. The process is introduced as a counterpart to NEEC, with the electron being replaced by a muon. It follows a presentation of the framework within which this process has emerged and how it changes the paradigm in comparison to NEEC. Owing to the increased proximity of muons to the nucleus, this process has been found to exhibit cross-sections that are several orders of magnitude higher than NEEC for excitations in the MeV range. By examining the unique properties of NEuC, insights into the process and its potential applications are provided, including muon-induced fission. Lastly, nuclear excitations are studied in the context of a laser-generated plasma scenario, where nuclei might be excited through the resonant absorption of a photon, together with other competing processes. The design and implementation of a tabletop setup for generating keV-hot plasma upon femtosecond laser irradiation are presented. The experimental work has been conducted on a 181Ta target using a time-dependent X-ray spectroscopic technique. The absence of a clear decay signal raises the question of whether the excitation of the 181mTa isomer has ever been observed in this context.