  We report the optical observation of lead-vacancy (PbV) centers in diamond
fabricated by Pb ion implantation and subsequent high-temperature annealing
(2100 {\deg}C) under high pressure (7.7 GPa). Their optical properties were
characterized by photoluminescence at varying temperatures down to 5.7 K. We
observed intense emission peaks at 550 and 554 nm with a large splitting of
approximately 3900 GHz. The two lines are thought to correspond to the zero
phonon line (ZPL) of PbV centers with split ground and excited states. A cubic
trend of the ZPL width was observed while varying temperature. We performed
polarization measurements of the two lines in a single PbV center, showing
nearly orthogonal dipole polarizations. These optical measurements strongly
indicate that the PbV center possesses D3d symmetry in the diamond lattice. The
observed large ground state splitting significantly suppresses the
phonon-mediated transition, which causes decoherence of the electron spin state
of the group IV color centers in diamond, expecting a long spin coherence time
at a temperature of approximately 8 K.
