Collisional coalescence of photoexcited midgap states in an MX chain compound

The pumping-power dependence of the time decay of the long-lived photoinduced midgap absorption has been studied in an MX chain compound $\left[{\mathrm{Pt}\left(\mathrm{en}\right)\mathrm{}}_2\right]{\left[\mathrm{Pt}\right(\mathrm{en})\mathrm{}}_2{\mathrm{Cl}}_2{\left]\right(\mathrm{B}\mathrm{F}}_4{)}_4,\left(\mathrm{en}\right)\mathrm{}$ being ethylenediamine. The 476.5 nm light of a cw Ar-ion laser is used as the pumping light source. In addition to an extremely nonexponential time decay, the life time of the midgap band is found to vary as $K_0^{-2}$ with the absorption intensity $K_0$ induced by the laser pumping. This decay behavior is explained well in terms of the Torney-McConnell survival-probability function, $S(\zeta {)=e}^{8\zeta }\mathrm{erfc}\sqrt{8\zeta }$ with $\zeta {=A}_0^{2}Dt,$ peculiar to the geminate coalescence of unequilibrated states in one dimension, where $A_0,$ D, and t are the initial density of the unequilibrated states, diffusion coefficient, and time, respectively. The observed midgap states are confirmed from this finding to collapse through diffusion-limited mutual collisions in locally disordered PtCl chains. Our previous data on temperature and sample dependencies are reviewed within the framework of this collisional geminate coalescence model.