Effects of quantum fluctuations on the photoinduced midgap absorption in an MX chain complex [Pt(en${)}_2$][Pt(en${)}_2$${\mathrm{Cl}}_2$](${\mathrm{BF}}_4$${)}_4$ where en=ethylenediamine

Long-lived midgap states are found to be induced efficiently in a substance of the mixed-valence MX chain complex [Pt(en${)}_2$][Pt(en${)}_2$${\mathrm{Cl}}_2$](${\mathrm{BF}}_4$${)}_4$, where en=ethylenediamine, by irradiation with the 488-nm line of an Ar laser polarized perpendicular to the Pt-Cl chain bonds. If the temperature is changed between 10 and 250 K in the dark after laser irradiation, the near-infrared absorption bands due to the photoinduced states show pronounced changes in the spectral position, line shape, and intensity. The behaviors of the mean energy and the second and third moments of the A band at low temperatures can be interpreted well in terms of the quantum fluctuation with an eigenfrequency of the order of 0.01 eV. Above about 100 K, the mean energy and the second and third moments show anomalous temperature dependencies, suggesting that the midgap states walk randomly in shallow and periodic potential wells of the chainlike bonds.