Optical absorption and Raman scattering in the title substance are studied. An intragap absorption band at 1.67 eV is enhanced efficiently by the irradiation with 488.0 nm light even at room temperature. Under hydrostatic pressures, although a phase transition occurs at 0.6 GPa, this band follows the midpoint of the charge-transfer gap well. The Raman scattering by the Pt-Cl stretching mode shows that the original CDW phase coexists with the high-pressure CDW phase under pressures up to 3.0 GPa.