Photoexcitation

About: Photoexcitation is a research topic. Over the lifetime, 5874 publications have been published within this topic receiving 134733 citations.

Suppression of Trion Formation in CsPbBr3 Perovskite Nanocrystals by Postsynthetic Surface Modification

Abstract: Lead halide perovskite nanocrystals (NCs) are one of the most anticipated and promising materials for light-emitting diodes and lasers because of their high photoluminescence quantum yields (PLQYs) However, the formation of trions (charged excitons) in the NCs reduces their PLQYs Here, we clarify the trion formation mechanism in perovskite CsPbBr3 NCs by analyzing the excitation fluence dependence of transient absorption signals Under weak photoexcitation, trions are formed by charge carrier trapping at surface states In contrast, biexciton Auger recombination dominates the trion formation under strong photoexcitation We found that the postsynthetic surface treatment suppresses the extrinsic surface-related formation of trions The thorough understanding of the trion formation mechanisms is essential for the PLQY improvement of perovskite NCs and helps to reduce ionization of NCs in solid-state devices

Spectroscopy of Cs attached to helium nanodroplets.

Abstract: Cesium oligomers are formed on helium nanodroplets which are doped with one or a few Cs atoms. The monomer absorption of the first electronic p←s transition upon laser excitation is probed. Spectra employing laser-induced fluorescence, beam depletion, and resonant photoionization are compared. In particular, mass-resolved photoionization allows us to specifically probe excitation induced processes such as, e.g., the formation of cesium-helium exciplexes. Absorption spectra of Cs dimers and trimers are recorded in the spectral region accessible by a Ti:sapphire laser. Assignment of dimer spectra is achieved by comparison with model calculations based on ab initio potentials. Electronic absorption lines of Cs trimers are attributed to transitions in the quartet manifold.

Photodissociation dynamics of indole in a molecular beam

Abstract: Photodissociation of indole at 193 and 248 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. H atom elimination was found to be the only dissociation channel at both wavelengths. The photofragment translational energy distribution obtained at 193 nm contains a fast and a slow component. Fifty-four percent of indole following the 193 nm photoexcitation dissociate from electronically excited state, resulting in the fast component. The rest of 46% indole dissociate through the ground electronic state, giving rise to the slow component. A dissociation rate of 6×105s−1, corresponding to the dissociation from the ground electronic state, was determined. Similar two-component translational energy distribution was observed at 248 nm. However, more than 80% of indole dissociate from electronically excited state after the absorption of 248 nm photons. A comparison with the potential energy surfaces from the ab initio calculation has been made.

Tuning the reorganization energy of electron transfer in supramolecular ensembles – metalloporphyrin, oligophenylenevinylenes, and fullerene – and the impact on electron transfer kinetics

Abstract: Oligo(p-phenylenevinylene) (oPPV) wires of various lengths featuring pyridyls at one terminal and C60 moieties at the other, have been used as molecular building blocks in combination with porphyrins to construct a novel class of electron donor–acceptor architectures. These architectures, which are based on non-covalent, directional interactions between the zinc centers of the porphyrins and the pyridyls, have been characterized by nuclear magnetic resonance spectroscopy and mass spectrometry. Complementary physico-chemical assays focused on the interactions between electron donors and acceptors in the ground and excited states. No appreciable electron interactions were noted in the ground state, which was being probed by electrochemistry, absorption spectroscopy, etc.; the electron acceptors are sufficiently decoupled from the electron donors. In the excited state, a different picture evolved. In particular, steady-state and time-resolved fluorescence and transient absorption measurements revealed substantial electron donor–acceptor interactions. These led, upon photoexcitation of the porphyrins, to tunable intramolecular electron-transfer processes, that is, the oxidation of porphyrin and the reduction of C60. In this regard, the largest impact stems from a rather strong distance dependence of the total reorganization energy in stark contrast to the distance independence seen for covalently linked conjugates.

Thermal effects in the ultrafast photoinduced electron transfer from a molecular donor anchored to a semiconductor acceptor

Abstract: A nonadiabatic molecular dynamics (MD) simulation of the photoin- duced electron transfer (ET) from a molecular electron donor to the TiO 2 semicon- ductor acceptor is carried out in a microcanonical ensemble with an average tem- perature of 350 K. The electronic structure of the dye-semiconductor system and the adiabatic dynamics are simulated by ab initio MD, while the nonadiabatic (NA) effects are incorporated by a quantum-classical mean-field approach. The ET dy- namics are driven by thermal fluctuations that dominate ionic motions at the simu- lated temperature. The ground and excited state ion dynamics are similar; therefore, the change in the quantum force due to the electronic photoexcitation can be neglected, and the final analysis is greatly simplified. The simulated ET occurs on a 5-fs timescale, in agreement with recent ultrafast experimental data. Vibrational motions of the chromophore ring carbons induce an oscillation of the photoexcited state energy, resulting in a bimodal distribution of the initial conditions for ET. At low energies the photoexcited state is localized primarily on the chromophore, while at high energies the photoexcited state is substantially delocalized into the first 3 surface layers of the TiO 2 surface. Thermally driven adiabatic transfer is the domi- nant ET mechanism. Compared to the earlier simulation at 50 K, the rate of NA transfer at 350 K remains almost unchanged, whereas the rate of adiabatic ET increases substantially.