Transient absorption spectra of gold nanoparticles (AuNPs) embedded in mesoporous TiO2 film were studied by a femtosecond laser photolysis pump-probe technique using 25 fs pulses at 740 nm (1.68 eV) and a low fluence of 24 μJ cm(-2). The shift of the bleaching peak in transient spectra by ∼100 meV is detected in the AuNP-TiO2 system, whereas the bleaching peak shift of the same AuNPs in aqueous colloids is not more than ∼5 meV. In addition to the thermal mechanism of the nonlinear response of AuNPs connecting with the electron gas heating and the smearing effect of the Fermi distribution, the electron transfer between AuNPs and TiO2 becomes important for the nonlinear response, in addition to the electron heating mechanism. The electron transfer can explain both the spectral shift and widening of the SPR band of AuNPs in TiO2.

No abstract

For the first time, a specific time-delayed peak was registered in the femtosecond transient absorption (TA) spectra of ZnxCd1−xS/ZnS (x~0.5) alloy quantum dots (QDs) doped with Mn2+, 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 Mn2+-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 Mn2+-doped QDs, stimulated emission corresponding to 4T1-6A1 Mn2+ transition was detected in the subpicosecond time domain. The time-delayed Stark peak in the Mn2+-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 Mn2+ excitation. The usually considered mechanism of the nonradiative energy transfer from an exciton to Mn2+ 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 Mn2+. The kinetic scheme includes the reduction of Mn2+ by a 1Se electron and the subsequent oxidation of Mn1+ with a hole, leading to the formation of an excited state of manganese.

No abstract

No abstract