  Vanadium dioxide is of broad interest as a spin-1/2 electron system that
realizes a metal-insulator transition near room temperature, due to a
combination of strongly correlated and itinerant electron physics. Here,
resonant inelastic X-ray scattering is used to measure the excitation spectrum
of charge, spin, and lattice degrees of freedom at the vanadium L-edge under
different polarization and temperature conditions. These spectra reveal the
evolution of energetics across the metal-insulator transition, including the
low temperature appearance of a strong candidate for the singlet-triplet
excitation of a vanadium dimer.
