Ruby is chromium -doped corundum (Al 2 O 3). The Cr in corundum's lattice forms an octahedra with surrounding oxygen ions. The octahedral crystal field together with spin-orbital interaction results in different energy levels. Once 3d electrons in Cr are energized by lasers, the excited electrons would go to T 2 and T 2 levels. Later they return to E levels and the R 1, R 2 lines come from luminescence from E levels to A 2 ground level. The energy difference of E levels are 29 cm, corresponding to the splitting of R 1, R 2 lines at 1.39 nm.