Tracing the ultrafast buildup and decay of d-d transitions in FePS3

Excitations between localized 3d states of transition metal ions within crystalline solids, commonly known as d-d transitions, play a pivotal role in diverse phenomena across solid-state physics, materials science, and chemistry. Until now, an experimental method to unravel the complex quasiparticle dynamics associated with d-d transitions has remained elusive. In this study, we bridge this gap by demonstrating that d-d transitions can be distinctly traced both in momentum space and in time using time- and angle-resolved photoelectron spectroscopy (trARPES). As a model system, we select FePS3, a two-dimensional van der Waals antiferromagnet with a rich array of quantum phenomena stemming from d-d transitions. Our investigation reveals the ultrafast dynamics of both a spin-allowed (5T2g → 5Eg) and a spin-forbidden (5T2g → 3T1g) d-d transition within the Fe2+ multiplet. We observe that for off-resonant excitation, the spin-allowed excitation exhibits a quicker buildup compared to the spin-forbidden transition, and it decays with a single exponential behavior over a timescale of 83 fs. For the spin-forbidden transition, we identify two independent decay channels with decay constants of 46 and 1,273 fs that we attribute to an exchange-mediated virtual hopping between neighboring Fe2+ ions and an on-site spin-orbit-mediated spin-flip process, respectively.