  We used optically detected magnetic resonance (ODMR) technique to directly
probe electron-spin resonance transitions in the excited state of
negatively-charged boron vacancy (VB-) defects in hexagonal boron nitride (hBN)
at room temperature. The data showed that the excited state has a zero-field
splitting of ~ 2.1 GHz, a g factor similar to the ground state and two types of
hyperfine splitting ~ 90 MHz and ~ 18.8 MHz respectively. Pulsed ODMR
experiments were conducted to further verify observed resonant peaks
corresponding to spin transitions in the excited state. In addition, negative
peaks in photoluminescence and ODMR contrast as a function of magnetic field
magnitude and angle at level anti-crossing were observed and explained by
coherent spin precession and anisotropic relaxation. This work provided
significant insights for studying the structure of VB- excited states, which
might be used for quantum information processing and nanoscale quantum sensing.
