Collisional coalescence of photoexcited midgap states in an MX chain compound

The pumping-power dependence of the time decay of the long-lived photoinduc ed midgap absorption has been studied in an MX chain compound [Pt(en)(2)][P t(en)(2)Cl-2](BF4)(4),(en) being ethylenediamine. The 476.5 nm light of a c w Ar-ion laser is used as the pumping light source. In addition to an extre mely nonexponential time decay, the life time of the midgap band is found t o vary as K-0(-2) with the absorption intensity K-0 induced by the laser pu mping. This decay behavior is explained well in terms of the Torney-McConne ll survival-probability function, S(zeta)= e(8 zeta) erfc root 8 zeta with zeta = A(0)(2)Dt, peculiar to the geminate coalescence of unequilibrated st ates in one dimension, where A(0), D, and t are the initial density of the unequilibrated states, diffusion coefficient, and time, respectively. The o bserved midgap states are confirmed from this finding to collapse through d iffusion-limited mutual collisions in locally disordered PtCl chains. Our p revious data on temperature and sample dependencies are reviewed within the framework of this collisional geminate coalescence model. [S0163-1829(99)0 0919-4].