  In the rich phases of van der Waals (vdW) materials featuring intertwined
electronic order and collective phenomena, characterizing elementary dynamics
that entail the low-energy Hamiltonian and electronic degrees of freedom is of
paramount importance. Here we performed resonant inelastic X-ray scattering
(RIXS) to elaborate the spin-orbital ground and excited states of the vdW
antiferromagnetic insulator FePS$_{3}$, as well as their relation to magnetism.
We observed the spectral enhancement of spin-orbital multiplet transitions
about $\sim$ 100 and $\sim$ 220 meV, as well as quasielastic response, when
entering the zig-zag antiferromagnetic phase, where the spectral changes
develop an order-parameter-like evolution with temperature. By comparing with
ligand field theory calculations, we discovered the essential role of trigonal
lattice distortion and negative metal-ligand charge-transfer to account for
these emergent excitations. Such spectral profiles are further examined upon
confinement by mechanical exfoliation. We reveal their spectral robustness down
to the few atomic layer limit, in accordance with the persistent
antiferromagnetic state previously reported in optical measurements. Our study
demonstrates the versatile RIXS capability that resolves magneto-crystalline
anisotropy and charge-transfer energetics. These provide the crucial insight to
understand how the spontaneous magnetic symmetry-breaking stabilizes in the
quasi-two-dimensional limit for the vdW magnet FePS$_{3}$.
