Asynchronous Photoexcited Electronic and Structural Relaxation in Lead-Free Perovskites

Vacancy-ordered lead-free perovskites with more-stable crystalline structures have been intensively explored as the alternatives for resolving the toxic and long-term stability issues of lead halide perovskites (LHPs). The dispersive energy bands produced by the closely packed halide octahedral sublattice in these perovskites are meanwhile anticipated to facility the mobility of charge carriers. However, these perovskites suffer from unexpectedly poor charge carrier transport. To tackle this issue, we have employed the ultrafast, elemental-specific X-ray transient absorption (XTA) spectroscopy to directly probe the photoexcited electronic and structural dynamics of a prototypical vacancy-ordered lead-free perovskite (Cs3Bi2Br9). We have discovered that the photogenerated holes quickly self-trapped at Br centers, simultaneously distorting the local lattice structure, likely forming small polarons in the configuration of Vk center (Br2– dimer). More significantly, we have found a surprisingly long-lived, structural distorted state with a lifetime of ∼59 μs, which is ∼3 orders of magnitude slower than that of the charge carrier recombination. Such long-lived structural distortion may produce a transient “background” under continuous light illumination, influencing the charge carrier transport along the lattice framework

Precise Measurements of Branching Fractions for Ds+D_s^+ Meson Decays to Two Pseudoscalar Mesons

We measure the branching fractions for seven $D_{s}^{+}$ two-body decays to pseudo-scalar mesons, by analyzing data collected at $\sqrt{s}=4.178\sim4.226$ GeV with the BESIII detector at the BEPCII collider. The branching fractions are determined to be $\mathcal{B}(D_s^+\to K^+\eta^{\prime})=(2.68\pm0.17\pm0.17\pm0.08)\times10^{-3}$, $\mathcal{B}(D_s^+\to\eta^{\prime}\pi^+)=(37.8\pm0.4\pm2.1\pm1.2)\times10^{-3}$, $\mathcal{B}(D_s^+\to K^+\eta)=(1.62\pm0.10\pm0.03\pm0.05)\times10^{-3}$, $\mathcal{B}(D_s^+\to\eta\pi^+)=(17.41\pm0.18\pm0.27\pm0.54)\times10^{-3}$, $\mathcal{B}(D_s^+\to K^+K_S^0)=(15.02\pm0.10\pm0.27\pm0.47)\times10^{-3}$, $\mathcal{B}(D_s^+\to K_S^0\pi^+)=(1.109\pm0.034\pm0.023\pm0.035)\times10^{-3}$, $\mathcal{B}(D_s^+\to K^+\pi^0)=(0.748\pm0.049\pm0.018\pm0.023)\times10^{-3}$, where the first uncertainties are statistical, the second are systematic, and the third are from external input branching fraction of the normalization mode $D_s^+\to K^+K^-\pi^+$. Precision of our measurements is significantly improved compared with that of the current world average values

Long-range Stacking Effects of Nucleobases in Charge Transfer

Base-stacked structure is an important feature of DNA molecules. Previous studies on the stacking effect concerning DNA-mediated hole transfer have revealed the influence of neighboring bases on onsite energies. But the neighboring base effect acts only in a short-distance. Besides it, a long-range (longer than three base pairs) stacking effect called squeezing effect in this paper has not yet been reported. Such a squeezing effect causes the bases near the middle of a sequence consisting of same type base pairs have lower onsite energies than the bases near the terminals. We predict it by H ̈uckelanalysis in an unconventional way and confirmed it by semiempirical calculations combinated with molecular dynamics simulations. The results suggest that in order to obtain a reasonable onsite energy map when study charge transfer on DNA, the stacking effects should be considered in a long-distance as possible. The consideration of squeezing effect also provides a new suggestion on the driving force of fluctuation-assisted DNA charge transfer. The method used to calculate the onsite energies in abase stack can be generalized to other π-stacked systems.<br /

Mapping of Charge Delocalization Domain in Fluctuating DNA

In this manuscript, we presented a theoretical method which allows one to gain the snapshot of the transient charge delocalization structure of the fluctuating DNA. It avoided the subjective observation on the graphical representations of charge density or molecular orbital. Instead, it is formulaic and programmable. By virtue of it, we built a model for the concept of the so-called delocalized domain, which has been proposed by different experimental scientists so as to interpret the confusing non-monotonic distance dependence of DNA-mediated charge transfer events. We reproduced the non-monotonicity in terms of the delocalized domain and revealed its origin. Importantly, the model is not complicated at all. One who has basic quantum chemistry knowledge could understand it without difficulty. This makes it easy to be applied on other π-stack systems besides DNA </p

Stacking Effects on Charge Transfer Dynamics in Fluctuating DNA

Base-stacked structure is an important feature of DNA molecules. But dynamics study on the influences of the stacking effects on charge transfer in DNA is yet rare.In this article, a general rule about the relationship of onsite energies of same bases in a stack is derived by H ̈uckel theory. It is found that the base in the middle position of the stack has lower onsite energy than the bases at the terminals due to squeezing effect, which is different from previous studied neighboring base effect. The former is along-range effect while the latter acts in a short range. Semiempirical MNDO calculations on (A:T) n (n=1∼10) systems verfied the H ̈uckel analysis. From this perspective,the so-called incoherent hopping mechanism is actually somewhat coherent due to the squeezing effect. To understand these stacking effects on charge transfer in DNA,a cross-scale method which combines classical MD simulations, quantum mechanism calculations, Marcus electron transfer theory and kinetic Monte Carlo simulations is developed and applied on hole dynamics in (A:T) n (G:C) (n=1∼10) systems. Although no superexchange mechanism is explicitly involved in the studied systems, a crossover from strong to weak distance-dependency of hole arrival rates, which is an experimentally observed property of hole dynamics in DNA and is thought an evidence of the conversion from superexchange to hopping mechanism, also appears. We attribute it to the stacking effects. Such a result provides a new idea on explaining the crossover of different distance-dependencies of charge transfer rates in DNA. In addition, the squeezing effect may be a new driving force for long-range charge transfer. At the same time, some technical methods developed in the dynamics, e.g. calculations of onsite energies and electronic couplings in a stack, and simulated hole dynamics, etc.,can be generalized to other complex molecular systems with charge transfer behaviors.<br /

Electron scattering from molecule CH4 at 10-5000eV

Electron scattering from molecules in the intermediate- and high-energy range is investigated employing the developed semi-empirical formula for electron scattering from diatomic molecules. Total cross sections of e-CH4 scattering are obtained over an incident energy range of 10-5000eV. The results agree well with other available experimental and theoretical data. According to our formula, some quantitative information of single Yukawa potential are also obtained

Effects of Solvent Dielectric Constant and Viscosity on Two Rotational Relaxation Paths of Excited 9‑(Dicyanovinyl) Julolidine

The understanding of the interplay between microenvironment and molecular rotors is helpful for designing and developing of molecular sensors of local physical properties. We present a study on the two rotational relaxation paths of excited 9-(dicyanovinyl) julolidine in several solvents. One rotational path (C–C single-bond rotation, τ_b) quickly leads to the formation of a twisted state. The other path (CC double-bond rotation, τ_c) shows that the populations go back to the ground state directly via a conical intersection between the S₁ and ground state. The increase in the solvent dielectric constant shows little effect on the τ_b lifetime for its small energy barrier (<0.01 eV), but τ_c lifetime is increased in larger dielectric constant solvents due to the larger energy gap at conical intersection. Both τ_b and τ_c are increased greatly with the increased solvent viscosity. τ_b is more sensitive to viscosity than τ_c may be due to its larger rotational moiety

Molecular dynamics simulation of SRP GTPases: Towards an understanding of the complex formation from equilibrium fluctuations

Signal recognition particle (SRP) and its receptor (SR) play essential role in the SRP-dependent protein targeting pathway. They interact with one another to precisely regulate the targeting reaction. The mechanism of this interaction consists of at least two discrete conformational states: complex formation and GTPase activation. Although structural studies have provided valuable insights into the understanding of the SRP-SR interaction, it still remains unclear that how SRP and SR GTPases use their intrinsic conformational flexibilities to exert multiple allosteric regulations on this interaction process. Here, we use computational simulations to present the dynamic behavior of the SRP GTPases at an atomic level to gain further understanding of SRP-SR interaction. We show that: (i) equilibrium conformational fluctuations contain a cooperative inter- and intradomain structural rearrangements that are functionally relevant to complex formation, (ii) a series of residues in different domains are identified to correlate with each other during conformational rearrangements, and (iii) α3 and α4 helices at domain interface actively rearrange their relative conformation to function as a bridge between the N domain and the core region of the G domain. These results, in addition to structural studies, would harness our understanding of the molecular mechanism for SRP and SR interactio