Abstract
For the first time, a specific time-delayed peak was registered in the femtosecond transient absorption (TA) spectra of Zn(x)Cd(1-x)S/ZnS (x~0.5) alloy quantum dots (QDs) doped with Mn(2+), which was interpreted as the electrochromic Stark shift of the band-edge exciton. The time-delayed rise and decay kinetics of the Stark peak in the manganese-doped QDs significantly distinguish it from the kinetics of the Stark peak caused by exciton-exciton interaction in the undoped QDs. The Stark shift in the Mn(2+)-doped QDs developed at a 1 ps time delay in contrast to the instantaneous appearance of the Stark shift in the undoped QDs. Simultaneously with the development of the Stark peak in the Mn(2+)-doped QDs, stimulated emission corresponding to (4)T(1)-(6)A(1) Mn(2+) transition was detected in the subpicosecond time domain. The time-delayed Stark peak in the Mn(2+)-doped QDs, associated with the development of an electric field in QDs, indicates the appearance of charge transfer intermediates in the process of exciton quenching by manganese ions, leading to the ultrafast Mn(2+) excitation. The usually considered mechanism of the nonradiative energy transfer from an exciton to Mn(2+) does not imply the development of an electric field in a QD. Femtosecond TA data were analyzed using a combination of empirical and computational methods. A kinetic scheme of charge transfer processes is proposed to explain the excitation of Mn(2+). The kinetic scheme includes the reduction of Mn(2+) by a 1Se electron and the subsequent oxidation of Mn(1+) with a hole, leading to the formation of an excited state of manganese.