Inter-Kramers electric quadrupole transitions in high-spin systems induced by resonant alternating inhomogeneous electric field

This study presents a theoretical analysis of the interaction of high-spin systems with inhomogeneous alternating electric field and homogeneous alternating magnetic field that induce electric quadrupole (QT, E2) and magnetic dipole transitions (MT, M1), respectively. In order to distinguish QTs from MTs, an analytical expression for the intensities and selection rules for a model system with a half-integer total spin S=3/2 was derived using the spin Hamiltonian and operator-equivalent approaches. The direct comparison of the absorption patterns for the QT and MT of a model high-spin Co(II) system was performed in a frequency domain corresponding to Frequency Domain Fourier Transform Terahertz Electron Paramagnetic Resonance spectroscopy. This type of systems often exhibits the properties of a single molecular magnet at helium temperatures and is characterized by a large zero field splitting. Despite more flexible selection rules for electric quadrupole transitions, the powder spectra of QT and MT were shown to be similar, emphasizing the need for precise spectral measurements to determine the dominant transition type in high-spin systems. The approach developed in the paper not only solves a rather complex quantum mechanical problem that includes the estimation of the quadrupole moment of unpaired electrons, but also demonstrates a possible way for advanced manipulation of spin states, a capability crucial for the development of quantum computing and information storage technologies.