The electron spin resonance (ESR) of in was measured with an X-band frequency and low temperatures. The ions in this crystal show tetragonal symmetry with and . The analysis of the g-tensor indicates that the ground-state wavefunctions have dominant components of , resulting in a negative value of the crystal-field parameter . Three optical absorption bands with peaks at , 248 nm, and 298 nm were observed at room temperature. The 298 nm band is more strongly polarized along the a-axis than along the c-axis, whereas the 248 nm band shows the reverse polarization. The properties of the polarization can be explained by using wavefunctions of the lower excited state of , which is split into and in tetragonal symmetry, being lower in energy.