1s Ground State Research Articles

Dibenzo-p-dioxin. Twisted and puckered excited state molecular geometries

The title compound is generally acknowledged to have a planar D2h symmetrical molecular geometry in the ground state S0, and previous theoretical and experimental investigations seem to support the assumption of similar planar geometries in the excited singlet and triplet electronic states, S1 and T1. But a variety of theoretical models predict non-planar equilibrium geometries for these states: In the lowest excited singlet state (S1) a twisted, propeller-like geometry with D2 symmetry is predicted, while a strongly puckered, biradicaloid dienediyl-like structure is predicted for the triplet state (T1).

Luminescence properties and optimized structural conformations of the S0, S1 and T1 states of a tetranuclear formamidinate complex based on AuI and AgI metal ions.

N,N'-Bis(pyridin-4-yl)formamidine (4-pyfH) was reacted with AuI and AgI metal salts to form a novel tetranuclear complex, tetrakis[μ-N,N'-bis(pyridin-4-yl)formamidinato]digold(I)disilver(I), [Ag2Au2(C11H9N4)2] or [AuxAg4-x(4-pyf)4] (x = 0-4), 1, which is supported by its metallophilicity. Due to the potential permutation of the coordinated metal ions, six different canonical structures of 1 can be obtained. Complex 1 shows an emission at 501 nm upon excitation at 375 nm in the solid state and an emission at 438 nm upon excitation at 304 nm when dispersed in methanol. Time-dependent density functional theory (TD-DFT) calculations confirmed that these emissions can be ascribed to metal-to-ligand charge transfer (MLCT) processes. Moreover, the calculations of the optimized structural conformations of the S0 ground state, and the S1 and T1 excited states are discussed and suggest a distorted planar conformation for the tetranuclear Au2Ag2 complex.

CI calculations for ground and the lowest core-excited states of Li and Li−

Large scale, accurate non-relativistic configuration interaction (CI) calculations for the ground and core excited states of Li (2S, 4P° and 4P) and Li− (1S, 5P and 5S°) corrected for relativistic and mass polarization effects. Priori selected CI and CI by parts techniques are implemented to approximate both the large scale wavefunction and the solution of large CI eigenvalue problem respectively. Systematic studies of energy and electron affinity convergence with respect to CI excitation level are reported. The calculated value of electron affinity of the ground 1S state is 618.05(5) meV in excellent agreement with experiment. Calculations on excited states reveal more accurate values of the positions of 5P and 5S° states of Li−, lying 505.86(5) meV and 291.20(5) meV below 4P° and 4P states respectively of Li. Transition energies and wavelength of the electric dipole decay 4P → 4P° of Li and 5S° → 5P of Li−, show reasonably good agreement with available theoretical and experimental data.

Free-Electron Screening Mechanism of the Shallow Impurity Breakdown in n-GaAs: Evidences from the Photoelectric Zeeman and Cyclotron Resonance Spectroscopies

A novel breakdown (BD) mechanism of shallow impurity (SI) under the electric field at low temperatures is suggested for n‐GaAs samples with the donor concentrations 1014 cm−3≤ND≤1016 cm−3 and the compensation degree 0.3≤K=NA/ND≤0.8 with acceptors of concentration NA in the external magnetic fields up to H=6.5T, oriented parallel or perpendicular to the external electric field. Diagnosis of the BD mechanism was performed by SI Zeeman (mainly from the ground state 1s to 2p+1 and other excitation states) and cyclotron resonance photoelectric spectroscopy (PES) methods in the wide interval of the electric field including the BD region too. The obtained results reveal that the BD electric field εBD does not correlate with K and the carrier’s mobility μ of the samples, which contradict to the well-known impact ionization mechanism (IIM). A serious discrepancy with IIM is that εBD does not almost depend on the magnetic field up to H=6.5T when εH though the SI ionization energy increases two times. The cyclotron resonance (CR) measurements show that the line width does not depend on the electric field for ε<εBD, indicating the lack of free-carrier (FC) heating in contradiction with IIM. A considerable decrease of the free carriers’ capture cross section (CCS) area by ionized SI centers with a subsequent increase in the FC concentration n is observed by means of PES investigation of the 1s⟶2p+1 and CR lines in the electric fields ε≤εBD and at different magnetic fields, applied along (Hε) the electric field or perpendicular (H⊥ε) to the electric field. The slope of the 1s⟶2p+1 line intensity on the electric field for εH does not depend on the magnetic field, which is valid for εBD too. Various effects determined in the PES measurements at ε=εBD, such as a drastic narrowing of the 1s⟶2p+1 and CR lines, a shift of the CR line to higher magnetic fields, and disappearing of the lines to higher excited SI states, were clarified to be a result of screening of the SI Coulomb potential by free carriers. The FC screening at the BD reduces the potential fluctuation and its influence to the PES line shape of 1s⟶2p+1 and other excited states. It is shown that an increase in the FC concentration reduces the CCS, which can be assumed as the main factor along with the increase in the ionization coefficient for the SI breakdown in the electric field. The screening length rs of the SI Coulomb potential decreases with the increasing FC concentration, reducing the CCS; the latter seems to vanish completely at rs=aB∗ (aB∗ is the effective Bohr radius), when high screening results in vanishing of all the bound states of the Coulomb potential. Note that this limit is similar to the Mott transition. Many experimental facts and our calculation of the CCS support the suggested mechanism for the SI breakdown. The well-known IIM is valid for samples with SI concentrations N≪1013 cm−3 and takes place at very high electric fields.

S1/S0 Potential Energy Surfaces Experience Different Types of Restricted Rotation: Restricted Z/ E Photoisomerization and E/ Z Thermoisomerization by an Out-of-Plane Benzyl Group or In-Plane m-Pyridinium Group?

Any method that can enhance the fluorescence of fluorophores is highly desirable. Fluorescence enhancement accomplished by restricted Z/ E photoisomerization through intramolecular steric hindrance or relatively high bond order of a C═C double bond in a S1 excited state has rarely been studied. In this article, we used green fluorescent protein (GFP) chromophore analogues as a model to get new physical insights into the restricted Z/ E photoisomerization and E/ Z thermoisomerization phenomena. We found that the S1 and S0 potential energy surfaces (PESs) of the GFP chromophore analogues experience two dramatically different types of restricted rotation, and 2b can be a representative example. In its S1 PES, it is not the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group but the relatively high bond order of the I-bond in the S1 excited state of 2b that makes it have a higher barrier for the Z/ E photoisomerization, a smaller Z/ E photoisomerization quantum yield, and a higher fluorescence quantum yield. In its S0 PES, it is not the reduced bond order of the I-bond in the S0 ground state of 2b but the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group that makes it have an extra higher barrier for E/ Z thermoisomerization and a much smaller E/ Z thermoisomerization rate constant.

Investigation of the two-color polarization spectroscopy between the excited states based on cesium atoms

Two-color polarization spectroscopy (TCPS) of cesium 6S<sub>1/2</sub>-6P<sub>3/2</sub>-8S<sub>1/2</sub> (852.3 nm + 794.6 nm) ladder-type system in a room-temperature vapor cell are investigated. The frequency of 852.3 nm laser used as a pump beam is locked on one of the hyperfine transitions between the ground state 6S<sub>1/2</sub> and excited state 6P<sub>3/2</sub> by the saturated absorption spectroscopy technique, which can populate some atoms on the 6P<sub>3/2</sub> excited state and induce anisotropy in the atomic medium. The frequency of 794.6 nm laser serving as a probe beam is scanned across the whole 6P<sub>3/2</sub>→8S<sub>1/2</sub> transition to ascertain this anisotropy, and thus the TCPS is obtained. In experiment, we measure and analyse the influence of frequency detuning of 852.3 nm pump laser on TCPS, and especially reveal that some of hyperfine energy levels of intermediate excited state 6P<sub>3/2</sub>, which has no direct interaction with the 852.3 nm pump laser, are also populated by a small fraction of atoms with a specific speed in the direction of pump laser beam due to Doppler effect, so they also have contribution to the TCPS when the 794.6 nm probe laser is scanned to the resonance transition line between the 6P<sub>3/2</sub> and 8S<sub>1/2</sub> states after the Doppler frequency shift has been considered. In addition, we prove that the atomic coherence like electromagnetically induced transparency effect obviously results in a narrower line width of TCPS in the case of counter-propagating experimental configuration than that in the case of pump beam co-propagating with the probe beam in the Cs vapor cell. Finally, we apply the TCPS with dispersive shaped feature to frequency stabilization with no modulation, and the frequency fluctuations of 794.6 nm laser are ～0.5 MHz and ～9.2 MHz for the frequency-locking and free running in ～225 s, respectively. The above research work is expected to play a role in precisely measuring the atomic energy level structure and its related hyperfine structure constant (magnetic dipole and electric quadrupole coupling constants), and also in stabilizing the laser frequency to the excited state transition especially for the optical fiber communication, two-color laser cooling/trapping neutral atoms, optical filter, etc.

Influence of plasma shielding effect on ground state and excited state energies of Ar<sup>16+</sup>

A systematical knowledge of the atomic properties in plasma is of great interest for various research areas, such as the explanation of the X-ray radiation from universe, plasma diagnostics, extreme ultraviolet (EUV) and X-ray sources and so on. Among these researches, the detailed information about how the plasma influences the atomic energy level and transition spectrum are crucial for understanding the X-ray emission mechanism and the state of plasma. An analytic calculation method of treating the non-relativistic energy and its relativistic corrections for the multi-electron atoms embedded in weakly coupled plasma is developed based on the Rayleigh-Ritz variation method. The systematical investigations are performed for the ground state 1s<sup>2</sup> <sup>1</sup>S, single excited states 1sns <sup>1,2</sup>S (<i>n</i> = 2−5), 1s<i>n</i>p <sup>1,3</sup>P (<i>n</i> = 2−5) and double excited state 2s2p <sup>1</sup>P of Ar<sup>16+</sup> ion in weak coupled plasma. The analytic formulas for calculating the non-relativistic energy and its relativistic correction energy are derived, which include mass correction, one and two-body Darwin correction, spin-spin contact interaction and orbit-orbit interaction. All the angular integration spin sums involved in the problem are worked out explicitly by using the irreducible theory. The influence of plasma on non-relativistic energy and relativistic correction energy are discussed. The results show that the mass correction and the one-body Darwin correction are the main ones among the terms of relativistic correction, and are three orders of magnitude greater than the other relativistic terms. The plasma shielding effect mainly affects the non-relativistic energy, and has little effect on the relativistic correction. At the same time, it has a more significant selectivity for the electronic configuration. Further research shows that the influence of plasma on the energy of the outer shell electron is greater than that of the inner shell electron. With the increase of the plasma shielding parameters, the outer shell electron extends outward, and the higher the excited state, the greater the degree of extension is. This work should be useful for astrophysical applications where such a plasma environment exists.

Excited States of Weak Interacting Complexes of Formaldehyde and Alkali Metal Ions

The electronically excited states of formaldehyde and its complexes with alkali metal ions are investigated with the time-dependent density functional theory (TD DFT) method. Vertical transition energies for several singlet and triplet excited states, adiabatic transition energies for the first singlet and triplet excited states S1 and T1, the adiabatic geometries and vibrational frequencies of the ground state S0 and the first singlet and triplet excited states S1 and T1 for formaldehyde and its complexes are calculated. Better agreement with the experiment than that of the CIS method is obtained for CH2O at the TD DFT level. The nonlinear C=O⋯M+ interaction in the excited states S1 and T1 is weaker than the linear interaction in the ground state. In the S0 and S1 states, the C=O bond is elongated by cation complexation and its stretching frequency is red-shifted, but in the T1 state the C=O bond is shortened and its frequency is blue-shifted.

Trajectory surface hopping study of propane photodissociation dynamics at 157 nm.

The photodissociation dynamics of propane molecules has been studied using the quasiclassical trajectory surface hopping (TSH) method in conjunction with Tully's fewest switches algorithm. The trajectories are propagated on potential energy surfaces computed on-the-fly using the multiconfiguration and multireference ab initio method starting in the lowest excited singlet state (HOMO → 3s Rydberg state) of propane at 157 nm with the emphasis on the site specificity of atomic hydrogen elimination, molecular hydrogen elimination, and their product branching ratios. Our dynamics simulation revealed that there are three primary dissociation channels: the atomic hydrogen elimination, the molecular hydrogen elimination, and the C-C bond scission. The trajectories indicate that the H2 elimination from the internal carbon atom (2,2-H2 elimination) and terminal carbon atom (1,1-H2 elimination) is the major process and follows a three centred synchronous concerted mechanism. 1,2-H2 and 1,3-H2 eliminations on the other hand are minor processes and exclusively follow the roaming mediated nonadiabatic dynamics. The probability of elimination of the hydrogen atom from two terminal groups (terminal hydrogen elimination) is greater than that from the internal CH2 group (internal hydrogen elimination). Almost 83% of atomic hydrogen elimination occurs through the asynchronous concerted mechanism from the terminal carbon atom via triple dissociation leading to CH3 + C2H4 + H products. This finding is in good agreement with a recent experimental observation. The present TSH study indicates that approximately one-third of the trajectories those resulted in a triple dissociation channel, CH3 + C2H4 + H completed in the ground singlet state following a nonadiabatic path (hopping from the first excited singlet S1 to the ground state S0) via the C-C and C-H dissociation coordinate conical intersection S1/S0. The products CH3(1 2A2″) + C2H4(1Ag) + H, obtained are ground state methyl radicals and ground state ethylene. The trajectories those ended in a triple dissociation channel CH3 + C2H4 + H adiabatically in the S1 state lead to CH3(1 2A2″) + C2H4 (1 3B1) + H, where singlet methyl radicals and triplet ethylene are formed in their corresponding lowest electronic state via a spin conserving route. Two channels, CH4 + CH3CH and C2H6 + CH2, are found to have minor contributions. In the case of methane elimination, the trajectories that follow an adiabatic path lead to CH3CH(1 1A″) + CH4,(1 1A1), where ethylidene is in the excited state and methane is in the ground state. Methane elimination via nonadiabatic path leads to CH3CH(11A') + CH4(1 1A1), where both ethylidene and methane are in the ground electronic state. Ethane eliminations follow the adiabatic path leading to C2H6(1 1A1g) + CH2(1 1B1) where ethane is in the ground state and methylene is in the first excited state.

Two-Dimensional-Layered Perovskite ALaTa2O7:Bi3+ (A = K and Na) Phosphors with Versatile Structures and Tunable Photoluminescence.

Topological chemical reaction methods are indispensable for fabricating new materials or optimizing their functional properties, which is particularly important for two-dimensional (2D)-layered compounds with versatile structures. Herein, we demonstrate a low-temperature (∼350 °C) ion exchange approach to prefabricate metastable phosphors ALa1- xTa2O7: xBi3+ (A = K and Na) with RbLa1- xTa2O7: xBi3+ serving as precursors. The as-prepared ALa0.98Ta2O7:0.02 Bi3+ (A = Rb, K, and Na) share the same Dion-Jacobson type 2D-layered perovskite phase, and photoluminescence analyses show that ALa0.98Ta2O7:0.02 Bi3+ (A = Rb, K, and Na) phosphors exhibit broad emission bands peaking at 540, 550, and 510 nm, respectively, which are attributed to the nonradiative transition of Bi3+ from excited state 3P1 or 3P0 to ground state 1S0. The various Bi3+ local environments at the crystallographic sites enable the different distributions of emission and excitation spectra, and the photoluminescence tuning of ALa0.98Ta2O7:0.02 Bi3+ (A = Rb, K, and Na) phosphors are realized through alkali metal ion exchange. Notably, the combination of superior trivalent bismuth emission and low-temperature ion exchange synthesis leads to a novel yellow-emitting K(La0.98Bi0.02)Ta2O7 phosphor which is successfully applied in a white LED device based on a commercially available 365 nm LED chip. Our realizable cases of this low-temperature ion exchange strategy could promote exploration into metastable phosphors with intriguing properties.

Spectral and photophysical properties of cytisine in acetonitrile – Theory and experiment

Spectral and photophysical properties of (−)-cytisine that is used as a smoking cessation aid, and which derivatives are promising tools in a treatment of neurological diseases, were investigated in acetonitrile, non-specifically interacting solvent with a polarity similar to water. The two chair conformers of cytisine were found the most stable in the ground state S0 and the lowest excited singlet state S1(π,π*), wherein axial one was characterized by a significantly larger abundance, fluorescence lifetime 0.15 ns and fluorescence quantum yield 0.008. The S1(π,π*) excited state of both cytisine conformers deactivated almost exclusively via internal conversion.

Evidence of Electron Capture of an Outgoing Photoelectron Wave by a Nonvalence State in (C6F6) n.

Frequency-resolved photoelectron spectra are presented for (C6F6) n- with n = 1-5 that show that C6F6- is solvated by neutral C6F6 molecules. Direct photodetachment channels of C6F6- are observed for all n, leaving the neutral in the S0 ground state or triplet states, T1 and T2. For n ≥ 2, an additional indirect electron loss channel is observed when the triplet-state channels open. This indirect emission appears to arise from the electron capture of the outgoing photoelectron s-wave by a neutral solvent molecule through an anion nonvalence state. The same process is not observed for the S0 detachment channel because the outgoing electron wave is predominantly a p-wave. Our results show that anion nonvalence states can act as electron-accepting states in cluster environments and can be viewed as precursor states for diffuse states of liquid C6F6.

Geometric Energy Derivatives at the Complete Basis Set Limit: Application to the Equilibrium Structure and Molecular Force Field of Formaldehyde.

Geometric energy derivatives which rely on core-corrected focal-point energies extrapolated to the complete basis set (CBS) limit of coupled cluster theory with iterative and noniterative quadruple excitations, CCSDTQ and CCSDT(Q), are used as elements of molecular gradients and, in the case of CCSDT(Q), expansion coefficients of an anharmonic force field. These gradients are used to determine the CCSDTQ/CBS and CCSDT(Q)/CBS equilibrium structure of the S0 ground state of H2CO where excellent agreement is observed with previous work and experimentally derived results. A fourth-order expansion about this CCSDT(Q)/CBS reference geometry using the same level of theory produces an exceptional level of agreement to spectroscopically observed vibrational band origins with a MAE of 0.57 cm-1. Second-order vibrational perturbation theory (VPT2) and variational discrete variable representation (DVR) results are contrasted and discussed. Vibration-rotation, anharmonicity, and centrifugal distortion constants from the VPT2 analysis are reported and compared to previous work. Additionally, an initial application of a sum-over-states fourth-order vibrational perturbation theory (VPT4) formalism is employed herein, utilizing quintic and sextic derivatives obtained with a recursive algorithmic approach for response theory.

Miniaturization of physics system in Sr optical clock

The compactness and robustness of the vacuum setup are the major limitations to develop transportable and space-borne optical clocks. The first step in the engineering challenge is to reduce volume and weight with respect to a stationary system. In this paper, we present the realization of a miniaturized vacuum system by building two anti-Helmholtz coils inside the vacuum magneto-optical-trap (MOT) chamber. The built-in coils are specially designed to minimize the distance between the coils, and in this way it is possible to reduce the current needed to realize a typical magnetic gradient of 40 Gs/cm required for blue MOT. When the MOT coil current is 2 A, an axial magnetic field gradient of 43 Gs/cm is obtained in the center of the MOT, which is enough for the first stage Doppler cooling. This novel design allows us to reduce size, weight and power consumption with respect to a traditional laser cooling apparatus, and simultaneously avoid complicating the water cooling equipment. Our vacuum system has a size of 60 cm×20 cm×15 cm, about 1/10 of the original system in the laboratory. In addition, the circularly polarized Zeeman slowing laser is sent to counter propagating atomic beam, and atoms at a few hundred meters per second are now routinely slowed down to velocities of tens of meters per second. As a result, about 16.4% of the atoms are actually trapped into the blue MOT. The final temperature of the blue MOT is approximately 10.6 mK, and the internal diameter is 1.5 mm by observing the expansion of the atoms from the MOT. The populations of cold atoms finally trapped in the MOT are 1.6×106 of 88Sr and 1.5×105 of 87Sr. The 1S0 → 1P1 transition used for the blue MOT is not perfectly closed due to the decay channel of the 5p1P1 → 4d1D2, and a part of atoms are stored in the 3P2 and 3P0 states. To prevent the atoms from losing, 707 and 679 nm repumping lasers are employed to recycle these atoms in the 3P1 state, and then the atoms decay to the ground state 1S0. Now the typical number of loaded atoms dramatically increases by 5 times compared with before. The slowing efficiency of Zeeman slower is also optimized for the operation with deceleration related to the parameter of magnet length, which uses more effectively available magnetic field distribution, in contrast to the usual constant deceleration mode. Our future work will focus on constructing a Zeeman slower combined with permanent magnets or an electric magnet for better tuning of the magnetic field.

Vibronic relaxation dynamics of o-dichlorobenzene in its lowest excited singlet state

Vibronic dynamics of o-dichlorobenzene in its lowest excited singlet state, S1, is investigated in real time by using femtosecond pump-probe method, combined with time-of-flight mass spectroscopy and photoelectron velocity mapping technique. Relaxation processes for the excitation in the range of 276–252 nm can be fitted by single exponential decay model, while in the case of wavelength shorter than 252 nm two-exponential decay model must be adopted for simulating transient profiles. Lifetime constants of the vibrationally excited S1 states change from 651 ± 10 ps for 276 nm excitation to 61 ± 1 ps for 242 nm excitation. Both the internal conversion from the S1 to the highly vibrationally excited ground state S0 and the intersystem crossing from the S1 to the triplet state are supposed to play important roles in de-excitation processes. Exponential fitting of the de-excitation rates on the excitation energy implies such de-excitation process starts from the highly vibrationally excited S0 state, which is validated, by probing the relaxation following photoexcitation at 281 nm, below the S1 origin. Time-dependent photoelectron kinetic energy distributions have been obtained experimentally. As the excitation wavelength changes from 276 nm to 242 nm, different cationic vibronic vibrations can be populated, determined by the Franck-Condon factors between the large geometry distorted excited singlet states and final cationic states.

DFT Investigation of Ligand Photodissociation in [RuII(tpy)(bpy)(py)]2+ and [RuII(tpy)(Me2bpy)(py)]2+ Complexes.

Photoinduced ligand dissociation of pyridine occurs much more readily in [Ru(tpy)(Me2bpy)(py)]2+ than in [Ru(tpy)(bpy)(py)]2+ (tpy = 2,2':6',2″-terpyridine; bpy = 2,2'-bipyridine, Me2bpy = 6,6'-dimethyl-2,2'-bipyridine; py = pyridine). The S0 ground state and the 3MLCT and 3MC excited states of these complexes have been studied using BP86 density functional theory with the SDD basis set and effective core potential on Ru and the 6-31G(d) basis set for the rest of the atoms. In both complexes, excitation by visible light and intersystem crossing leads to a 3MLCT state in which an electron from a Ru d orbital has been promoted to a π* orbital of terpyridine, followed by pyridine release after internal conversion to a dissociative 3MC state. Interaction between the methyl groups and the other ligands causes significantly more strain in [Ru(tpy)(Me2bpy)(py)]2+ than in [Ru(tpy)(bpy)(py)]2+, in both the S0 and 3MLCT states. Transition to the dissociative 3MC states releases this strain, resulting in lower barriers for ligand dissociation from [Ru(tpy)(Me2bpy)(py)]2+ than from [Ru(tpy)(bpy)(py)]2+. Analysis of the molecular orbitals along relaxed scans for stretching the Ru-N bonds reveals that ligand photodissociation is promoted by orbital mixing between the ligand π* orbital of tpy in the 3MLCT state and the dσ* orbitals that characterize the dissociative 3MC states. Good overlap and strong mixing occur when the Ru-N bond of the leaving ligand is perpendicular to the π* orbital of terpyridine, favoring the release of pyridine positioned in a cis fashion to the terpyridine ligand.

TADF Material Design: Photophysical Background and Case Studies Focusing on CuI and AgI Complexes.

The development of organic light emitting diodes (OLEDs) and the use of emitting molecules have strongly stimulated scientific research of emitting compounds. In particular, for OLEDs it is required to harvest all singlet and triplet excitons that are generated in the emission layer. This can be achieved using the so-called triplet harvesting mechanism. However, the materials to be applied are based on high-cost rare metals and therefore, it has been proposed already more than one decade ago by our group to use the effect of thermally activated delayed fluorescence (TADF) to harvest all generated excitons in the lowest excited singlet state S1 . In this situation, the resulting emission is an S1 →S0 fluorescence, though a delayed one. Hence, this mechanism represents the singlet harvesting mechanism. Using this effect, high-cost and strong SOC-carrying rare metals are not required. This mechanism can very effectively be realized by use of CuI or AgI complexes and even by purely organic molecules. In this investigation, we focus on photoluminescence properties and on crucial requirements for designing CuI and AgI materials that exhibit short TADF decay times at high emission quantum yields. The decay times should be as short as possible to minimize non-radiative quenching and, in particular, chemical reactions that frequently occur in the excited state. Thus, a short TADF decay time can strongly increase the material's long-term stability. Here, we study crucial parameters and analyze their impact on the TADF decay time. For example, the energy separation ΔE(S1 -T1 ) between the lowest excited singlet state S1 and the triplet state T1 should be small. Accordingly, we present detailed photophysical properties of two case-study materials designed to exhibit a large ΔE(S1 -T1 ) value of 1000 cm-1 (120 meV) and, for comparison, a small one of 370 cm-1 (46 meV). From these studies-extended by investigations of many other CuI TADF compounds-we can conclude that just small ΔE(S1 -T1 ) is not a sufficient requirement for short TADF decay times. High allowedness of the transition from the emitting S1 state to the electronic ground state S0 , expressed by the radiative rate kr (S1 →S0 ) or the oscillator strength f(S1 →S0 ), is also very important. However, mostly small ΔE(S1 -T1 ) is related to small kr (S1 →S0 ). This relation results from an experimental investigation of a large number of CuI complexes and basic quantum mechanical considerations. As a consequence, a reduction of τ(TADF) to below a few μs might be problematic. However, new materials can be designed for which this disadvantage is not prevailing. A new TADF compound, Ag(dbp)(P2 -nCB) (with dbp=2,9-di-n-butyl-1,10-phenanthroline and P2 -nCB=bis-(diphenylphosphine)-nido-carborane) seems to represent such an example. Accordingly, this material shows TADF record properties, such as short TADF decay time at high emission quantum yield. These properties are based (i) on geometry optimizations of the AgI complex for a fast radiative S1 →S0 rate and (ii) on restricting the extent of geometry reorganizations after excitation for reducing non-radiative relaxation and emission quenching. Indeed, we could design a TADF material with breakthrough properties showing τ(TADF)=1.4 μs at 100 % emission quantum yield.

Circular polarization of X-ray radiation emitted by longitudinally polarized electron impact excitation: Under a screened Coulomb interaction

Longitudinally polarized electron impact excitation from the ground state 1s2 to the excited state 1s2l (l =s,p) levels of highly charged He-like Fe24+ ions in weakly coupled hot-dense plasmas is investigated using a fully relativistic distorted-wave method. The Debye-Hückel potential is used to describe the plasma screening. Benchmark results such as the total cross sections, the magnetic sublevels cross sections, and the circular polarizations of corresponding X-ray radiations are presented. For the excitation process, results show that the plasma screening has an effect in reducing both the total and magnetic sublevels cross sections. For the de-excitation process, it is found that while the plasma screening as a slightly effect on the circular polarizations of radiations for the 1s2s 3S1 → 1s2 1S0,1s2p 3P2 → 1s2 1S0, and 1s2p 1P1 → 1s2 1S0 transition lines, it gives a substantial contribution for the same properties of the 1s2p 3P1 → 1s2 1S0 line.

Molecular Dynamics of Oxazole Yellow Dye in its Ground and First Excited Electronic States in Solution and when Intercalated in dsDNA.

Oxazole yellow (YOPRO), a cyanine dye consisting of benzoxazole and quinoline rings connected by a linker, is almost nonfluorescent in water, but its fluorescence is greatly enhanced after intercalation in double-stranded DNA, forming the basis of DNA concentration assays. To explore this difference, new potential energy surfaces for the two linker dihedral angles in the ground S0 and first excited S1 electronic states are developed. Umbrella sampling molecular dynamics is used to obtain the free energy of rotation around the two dihedral angles of the linker. The two-dimensional free energy surface of the S1 state, spanning the Franck-Condon transition point from the S0 electronic state minimum (dihedral 1 around 180°, dihedral 2 around 0°) to the S1 state minimum (∼90, ∼0), is obtained in water and when intercalated. In water, YOPRO's S1 free energy surface is completely downhill from the Franck-Condon point, whereas when intercalated, there is a barrier on the path. Thus, when intercalated in DNA, S1 YOPRO is more constrained than in water, supporting the hypothesis that intercalation does inhibit ring rotational motion around the linker and therefore strongly reduces the nonradiative relaxation, resulting in higher fluorescence intensity.

State-dependent interactions in ultracold 174Yb probed by optical clock spectroscopy

We report on the measurement of the scattering properties of ultracold 174Yb bosons in a three-dimensional optical lattice. Site occupancy in an atomic Mott insulator is resolved with high-precision spectroscopy on an ultranarrow, metrological optical clock transition. Scattering lengths and loss rate coefficients for 174Yb atoms in different collisional channels involving the ground state 1S0 and the metastable 3P0 states are derived. These studies set important constraints for future experimental studies of two-electron atoms for quantum-technological applications.