Multi-excited States Research Articles

De novo design of single white-emitting polymers based on one chromophore with multi-excited states

Single white-emitting polymers (SWPs) based on multi-chromophores with different colors are believed to show a great potential in solution-processed white light-emitting diodes (WOLEDs). However, they suffer from the extremely low loadings (one ten thousandth to one thousandth) for long-wavelength chromophores when incorporated into SWPs. To solve this bottleneck, we newly propose a one chromophore with multi-excited states strategy for the de novo design of SWPs. By adopting acridine as the donor (D), dibenzothiophene-S,S-dioxide as the acceptor (A) and oxygen as the linker between D and A to constitute the central chromophore fragment, P(DMPAc-O-DBTDO) is successfully developed to possess not only fluorescence and thermally-activated delayed fluorescence from the lowest singlet state (S1), but also room-temperature phosphorescence from the lowest triplet state (T1). As a consequence, a very broad electroluminescence that covers the whole visible region from 400 to 750 nm is realized, leading to an efficient and stable white light with a peak power efficiency of 7.5 lm W−1 (9.5 cd A-1, 3.7%). Meanwhile, the Commission Internationale de l’Eclairage (CIE) coordinates and correlated color temperature (CCT) could be well tuned in a wide doping concentration window (15–100%) for different illumination requirements.

Nonlinear optical spectroscopy and two-photon excited fluorescence spectroscopy reveal the excited states of fluorophores embedded in a beetle's elytra.

Upon illumination by ultraviolet light, many animal species emit light through fluorescence processes arising from fluorophores embedded within their biological tissues. Fluorescence studies in living organisms are however relatively scarce and so far limited to the linear regime. Multiphoton excitation fluorescence analyses as well as nonlinear optical techniques offer unique possibilities to investigate the effects of the local environment on the excited states of fluorophores. Herein, these techniques are applied for the first time to study of the naturally controlled fluorescence in insects. The case of the male Hoplia coerulea beetle is investigated because the scales covering the beetle's elytra are known to possess an internal photonic structure with embedded fluorophores, which controls both the beetle's coloration and the fluorescence emission. An intense two-photon excitation fluorescence signal is observed, the intensity of which changes upon contact with water. A third-harmonic generation signal is also detected, the intensity of which depends on the light polarization state. The analysis of these nonlinear optical and fluorescent responses unveils the multi-excited states character of the fluorophore molecules embedded in the beetle's elytra. The role of form anisotropy in the photonic structure, which causes additional tailoring of the beetle's optical responses, is demonstrated by circularly polarized light and nonlinear optical measurements.

Energy, fine structure, hyperfine structure, and transitions for the high-lying multi-excited 4Pe,o states of B-like ions

The energies of the high-lying multi-excited states 1s22s2pnl and 1s22p2nl 4Pe,o (n ≥ 2) for B-like C+, N2+, F4+, and Mg7+ ions are calculated using Rayleigh–Ritz variation method with multiconfiguration interaction, and the inclusion of mass polarization and relativistic corrections. The fine structure and hyperfine structure for these systems are investigated using first-order perturbation theory. The configuration structure of the high-lying multi-excited series is identified not only by energy, but also by its contribution to normalization of the angular spin components, and it is further tested by the addition of relativistic corrections and fine structure splittings. Transition wavelengths including the quantum electrodynamic effects and higher-order relativistic corrections are determined.

Energy, fine structure, hyperfine structure, and radiative transition rates of the high-lying multi-excited states for B-like neon

The Rayleigh-Ritz variational method with multiconfiguration interaction wave functions is used to obtain the energies of high-lying multi-excited quartet states 1s 22s2pnl and 1s 22p 2 nl 4Pe,o (n ≥ 2) in B-like neon, including the mass polarization and relativistic corrections. The fine structure and hyperfine structure of the excited quartet states for this system are investigated. Configuration structures of the high-lying multi-excited series are further identified by relativistic corrections and fine structure splittings. The transition rates and wavelengths are also calculated. Calculated wavelengths include the quantum electrodynamic effects. The results are compared with other theoretical and experimental data in the literature.

A Multipulse Time-Resolved Fluorescence Method for Probing Second-Order Recombination Dynamics in Colloidal Quantum Dots

The ability to generate and utilize multiexcited electronic states in colloidal quantum dots (QDs) is key to a growing range of QD technologies, but the factors that control their radiative and nonradiative recombination dynamics are not yet fully resolved. A significant barrier toward a greater understanding of these species is the fact that their spectroscopic signatures are energetically close to dominant exciton transitions, making it difficult to separate their decay contributions in inhomogeneously broadened ensembles. Here we describe a multipulse technique wherein a controllable number of 80 MHz laser pulses are used to generate different excited state populations, which are then monitored using time-resolved fluorescence. By changing the number of pulses and using a general data analysis method we are able to separate second-order emission generated by absorption of two or more laser pulses from first-order contributions generated by just one pulse. Furthermore, we show that it is possible to determine the nature of the multiexcited state by comparing the second-order emission intensity to models of QD decay dynamics. We find that in our sample of CdSe/CdS core/shell QDs the second-order emission is dominated by emissive trion states rather than biexcitons. Our spectroscopic technique offers a powerful new way to study multiexcited QDs, and the insights that will be gained from this and future studies could be an important step toward harnessing multiexcitons and other multiexcited states in new QD technologies.

Energies, fine structures, and radiative lifetimes for the multiexcited quartet states of B‐like oxygen

AbstractEnergies for the multiexcited states 1s22s2pnl and 1s22p2nl 4Pe,o (n ≥ 2) of B‐like oxygen are calculated using Rayleigh–Ritz variation method with configuration interaction. The mass polarization and relativistic corrections are obtained with first‐order perturbation theory. Configuration structures of the high‐lying multiexcited series are identified by energies and contribution to normalization of angular‐spin components. These structures are further checked by calculations of relativistic corrections and fine structure splittings. Hyperfine parameters and hyperfine coupling constants are calculated for the first time. Wavelengths including quantum electrodynamic effect and higher‐order relativistic corrections and lifetimes are also calculated. These results are compared with available results in the literature. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Power density and temperature dependent multi-excited states in InAs/GaAs quantum dots

Self-assembled InAs/GaAs (001) quantum dots (QDs) were grown by molecular beam epitaxy using ultra low-growth rate. A typical dot diameter of around 28 ± 2 nm and a typical height of 5 ± 1 nm are observed based on atomic force microscopy image. The photoluminescence (PL) spectra, their power and temperature dependences have been studied for ground (GS) and three excited states (1–3ES) in InAs QDs. By changing the excitation power density, we can significantly influence the distribution of excitons within the QD ensemble. The PL peak energy positions of GS and ES emissions bands depend on an excitation light power. With increasing excitation power, the GS emission energy was red-shifted, while the 1–3ES emission energies were blue-shifted. It is found that the full width at half maximum of the PL spectra has unusual relationship with increasing temperature from 9 to 300 K. The temperature dependence of QD PL spectra shown the existence of two stages of PL thermal quenching and two distinct activation energies corresponding to the temperature ranges I (9–100 K) and II (100–300 K).

Carrier dynamics in InAs quantum dots embedded in InGaAs/GaAs multi quantum well structures

Ground and multi excited state photoluminescence, as well as its temperature dependence, in InAs quantum dots embedded in symmetric InxGa1-xAs/GaAs (x = 0.15) quantum wells (DWELL) have been investigated. The solution of the set of rate equations for exciton dynamics (relaxation into QWs or QDs and thermal escape) solved by us earlier is used for analysis the variety of thermal activation energies of photoluminescence thermal quenching for ground and multi excited states of InAs QDs. The obtained solutions were used at the discussion of the variety of activation energies of PL thermal quenching in InAs QDs. It is revealed three different regimes of thermally activated quenching of the QD PL intensity. These three regimes were attributed to thermal escape of excitons: i) from the high energy excited states of InAs QDs into the WL with follows exciton re-localization; ii) from the InxGa1-xAs QWs into the GaAs barrier and iii) from the WL into the GaAs barrier with their subsequent nonradiative recombination in GaAs barrier.

Photoluminescence energy trend for ground and excited states in InAs quantum dots in a well InGaAs/GaAs structures

AbstractThis paper presents the photoluminescence study at 80K and scanning photoluminescence spectroscopy investigation of the ground and multi excited states at 80 and 300K in InAs quantum dots (QDs) inserted in symmetric In0.15Ga0.85As/GaAs QW structures created at different QD growth temperatures. It is shown that some of the structures investigated exhibit a spatial long range variation of the average QD size in the QD ensemble across the sample. This long range QD size inhomogeneity was used for an investigation of the multi excited state energy trend versus ground state energy (or QD sizes). Experimental results were compared with the electron‐hole level energy trend versus QD size predicted theoretically on the base of the 8‐band k·p approach. Some anti correlation between experimental and theoretical results has been revealed and discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal ionisation of ground and multiply excited states in InAs quantum dots embedded into InGaAs/GaAs MQW

The photoluminescence (PL) spectra of highly uniform self-assembled InAs quantum dots (QDs) embedded in In 0.15Ga 0.85As multi-quantum-well (MQW) heterostructures have been investigated at variable temperatures. This paper presents the PL bands, connected with ground (GS) and multi-excited states (ES) in QDs. Not equidistant optical transitions have been revealed. Spectral peak shifts and PL intensity variations in the temperature range 12–220 K for all PL bands are analyzed. The activation energy of the temperature quenching processes for GS and 4 ES optical transitions in InAs QDs are measured. The mechanism of these processes and the positions of the energy levels in QDs are discussed as well.

Coincidence measurements between photons, projectiles and recoil ions in low energy Kr18+ + Kr collisions. Auto-ionizing and radiative effect of multi-excited states

We present results of experiments using coincidence methods between visible photons of Rydberg transitions, projectiles and recoil ions arising from the low energy collision of Kr 18+ on Kr. For each observed Rydberg transition, we have determined the charge state distributions for both the recoil ions and the projectiles. From these results we have deduced some information on the creation of multi-excited states in the collision and on their evolution after the collision.