Finally, we discuss about the optical absorption of S@Ag54 and I@Ag54 (Figure ). Both spectra exhibited shoulder-like absorptions around 400 and 550 nm, and their overall optical absorption spectra were quite similar each other. However, upon closer inspection, the spectrum of I@Ag54 contained a more distinct shoulder-like peak at approximately 500 nm compared with the spectrum of S@Ag54. The heavy atom effect increases the oscillator strength of the spin-forbidden transition from the ground state to the triplet excited state (S0→T1), leading to the S-T absorption peak in the absorption spectrum. In fact, in the optical absorption spectrum of some ligand-protected metal clusters, absorption peak attributable to S-T transition appears with a relatively high oscillator strength at a slightly longer wavelength than the peak attributed to S0→Sn transition. Therefore, we predict that the shoulder peak at approximately 500 nm in the optical absorption spectrum of X@Ag54 is derived from S-T absorption, which is promoted by the heavy atom effect (Figure ).