M1 transitions induced by twisted photons with an effective perturbation V̂=-μ̂B

Photons carrying orbital angular momentum, also known as twisted photons, hold the potential to reveal new insights in atomic and molecular spectroscopy measurements. In this work, we explored magnetic dipole (M1) transitions induced by twisted photons for their application in electron paramagnetic resonance spectroscopy. We calculated the corresponding transition matrix elements and compared them with those of conventional plane wave photons. Our study showed that the matrix elements of twisted photons qualitatively differ from those of plane wave photons and linked this difference to the fact that twisted photon states are a superposition of states with photon spin projections onto the direction of propagation of -1, 0, and 1. According to our study, M1 excitations induced by twisted photons exhibit a spatial dependence on the perpendicular momentum, the radial position of the sample, and the total angular momentum projection of the photon. In addition, we illustrated the characteristic features of M1 transitions induced by twisted photons using model systems with total spin S = 1/2 and S = 3/2, which are the representative of typical organic radicals and high-spin transition metal complexes, respectively. In particular, the S = 3/2 system with zero-field splitting, where the energy levels increase or decrease depending on the spin projection (from -3/2 to +3/2), clearly highlights the qualitative distinctions in excitation behavior introduced by twisted light. Finally, numerical simulations performed for point-like and pellet-like samples under experimentally relevant THz beam conditions corroborate the theoretical predictions and provide a practical framework for future experimental validation.