Contribution Of Excited States Research Articles

Solvent tuning of photochemistry upon excited-state symmetry breaking

The nature of the electronic excited state of many symmetric multibranched donor–acceptor molecules varies from delocalized/multipolar to localized/dipolar depending on the environment. Solvent-driven localization breaks the symmetry and traps the exciton in one branch. Using a combination of ultrafast spectroscopies, we investigate how such excited-state symmetry breaking affects the photochemical reactivity of quadrupolar and octupolar A–(π-D)2,3 molecules with photoisomerizable A–π–D branches. Excited-state symmetry breaking is identified by monitoring several spectroscopic signatures of the multipolar delocalized exciton, including the S2 ← S1 electronic transition, whose energy reflects interbranch coupling. It occurs in all but nonpolar solvents. In polar media, it is rapidly followed by an alkyne–allene isomerization of the excited branch. In nonpolar solvents, slow and reversible isomerization corresponding to chemically-driven symmetry breaking, is observed. These findings reveal that the photoreactivity of large conjugated molecules can be tuned by controlling the localization of the excitation.

Discovery and characterization of an acridine radical photoreductant.

Summary:Photoinduced electron transfer (PET) is a phenomenon wherein the absorption of light by a chemical species provides an energetic driving force for an electron transfer reaction.1–4 This mechanism is relevant in many areas of chemistry, including the study of natural and artificial photosynthesis, photovoltaics, and photosensitive materials. In recent years, research in the area of photoredox catalysis has leveraged PET for the catalytic generation of both neutral and charged organic free radical species. These technologies have enabled a wide range of previously inaccessible chemical transformations and have seen widespread utilization in both academic and industrial settings. These reactions are often catalyzed by visible-light absorbing organic molecules or transition-metal complexes of ruthenium, iridium, chromium, or copper.5,6 While a wide variety of closed shell organic molecules have been shown to behave as competent electron transfer catalysts in photoredox reactions, there are only limited reports of PET reactions involving neutral organic radicals as an excited state donor or acceptor. This is perhaps somewhat unsurprising in light of previously reported doublet excited state lifetimes for neutral organic radicals, which are typically several orders of magnitude shorter than singlet lifetimes for known transition metal photoredox catalysts.7–11 Herein we document the discovery, characterization, and reactivity of a neutral acridine radical with a maximum excited state oxidation potential of −3.36 V vs. SCE: significantly more reducing than elemental lithium and marking it as one of the most potent chemical reductants reported.12 Spectroscopic, computational, and chemical studies indicate that the formation of a twisted intramolecular charge transfer species enables the population of higher energy doublet excited states, leading to the observed potent photoreductant behavior. We demonstrate that this catalytically-generated PET catalyst facilitates several chemical reactions that typically require alkali metal reductants and bodes well for the adoption of this system in additional organic transformations requiring dissolving metal reductants.

Reconstruction of smeared spectral functions from Euclidean correlation functions

Abstract We propose a method to reconstruct smeared spectral functions from two-point correlation functions measured on the Euclidean lattice. An arbitrary smearing function can be considered as long as it is smooth enough to allow an approximation using Chebyshev polynomials. We test the method with numerical lattice data of charmonium correlators. The method provides a framework to compare lattice calculation with experimental data including excited-state contributions without assuming quark–hadron duality.

Inclusion of the excited states quark propagator to the nucleon electromagnetic form factors in the perturbative chiral quark model

Electromagnetic form factors of the nucleon are studied within the framework of the Perturbative Chiral Quark Model (PCQM). In a previous analysis of the nucleon electromagnetic form factors, the quark propagator used in the calculation was truncated to quark in the ground state only. The attempt to include the excited states quark propagator contribution to the neutron was performed but not on the proton. Therefore, in this work, we try to be consistent in the nucleonic sector by including the excited states quark propagator up to the second excited states to study the electromagnetic form factors of the nucleon. Our results are in good agreement with experimental data and show the significance of the contribution of the quark excited states propagator to the electromagnetic properties of the nucleon.

Terahertz luminescence and photoconductivity associated with the impurity electron transitions in GaAs/AlGaAs quantum wells

The photoconductivity and photoluminescence spectra of GaAs/AlGaAs quantum wells doped with shallow donors are studied at low lattice temperatures. The optical electron transitions between the first electron subband and donor ground states, as well as between the excited and ground donor states, are revealed in the terahertz photoluminescence and photoconductivity spectra. The temperature evolution of the impurity-related photocurrent in the terahertz spectral range is also studied.

High-pressure tuning of primary photochemistry in bacterial photosynthesis: membrane-bound versus detergent-isolated reaction centers.

While photosynthesis thrives at close to normal pressures and temperatures, it is presently well known that life is similarly commonplace in the hostile environments of the deep seas as well as around hydrothermal vents. It is thus imperative to understand how key biological processes perform under extreme conditions of high pressures and temperatures. Herein, comparative steady-state and picosecond time-resolved spectroscopic studies were performed on membrane-bound and detergent-purified forms of a YM210W mutant reaction center (RC) from Rhodobacter sphaeroides under modulating conditions of high hydrostatic pressure applied at ambient temperature. A previously established breakage of the lone hydrogen bond formed between the RC primary donor and the protein scaffold was shown to take place in the membrane-bound RC at an almost 3 kbar higher pressure than in the purified RC, confirming the stabilizing role of the lipid environment for membrane proteins. The main change in the multi-exponential decay of excited primary donor emission across the experimental 10 kbar pressure range involved an over two-fold continuous acceleration, the kinetics becoming increasingly mono-exponential. The fastest component of the emission decay, thought to be largely governed by the rate of primary charge separation, was distinctly slower in the membrane-bound RC than in the purified RC. The change in character of the emission decay with pressure was explained by the contribution of charge recombination to emission decreasing with pressure as a result of an increasing free energy gap between the charge-separated and excited primary donor states. Finally, it was demonstrated that, in contrast to a long-term experimental paradigm, adding a combination of sodium ascorbate and phenazine methosulfate to the protein solution potentially distorts natural photochemistry in bacterial RCs.

Influence of uniaxial stress on phonon-assisted relaxation in bismuth-doped silicon

The relaxation of electrons bound to bismuth donors in silicon and the effect of uniaxial stress have been studied using the time-resolved single color pump-probe technique. In unstressed Si:Bi, an excited 2p0 donor state is resonantly coupled with the donor 1s(A1) ground state via an intervalley f-TO optical phonon. This results in a very short lifetime (a few ps) of the excited state. Even a slight deformation of the silicon crystal leads to modification of particular interstate energies of the donor, which resolves the resonant coupling to intervalley phonons whose energies remain unchanged. We have shown that once the energy gap between the lower stress-split component of the 2p0 state and the ground state becomes less than the energy of the f-TO phonon, the relaxation of electrons from the excited state slows down to about 300 ps. The experimental data are compared with theoretical calculations of the 2p0 state relaxation rate, which are performed assuming that it is dominated by the emission of intervalley phonons.

Coherent intradimer dynamics in reaction centers of photosynthetic green bacterium Chloroflexus aurantiacus

Early-time dynamics of absorbance changes (light minus dark) in the long-wavelength Qy absorption band of bacteriochlorophyll dimer P of isolated reaction centers (RCs) from thermophilic green bacterium Chloroflexus (Cfx.) aurantiacus was studied by difference pump-probe spectroscopy with 18-fs resolution at cryogenic temperature. It was found that the stimulated emission spectrum gradually moves to the red on the ~100-fs time scale and subsequently oscillates with a major frequency of ~140 cm−1. By applying the non-secular Redfield theory and linear susceptibility theory, the coherent dynamics of the stimulated emission from the excited state of the primary electron donor, bacteriochlorophyll dimer P*, was modeled. The model showed the possibility of an extremely fast transition from the locally excited state P1* to the spectrally different excited state P2*. This transition is clearly seen in the kinetics of the stimulated emission at 880 and 945 nm, where mostly P1* and P2* states emit, respectively. These findings are similar to those obtained previously in RCs of the purple bacterium Rhodobacter (Rba.) sphaeroides. The assumption about the existence of the second excited state P2* helps to explain the complicated temporal behavior of the ΔA spectrum measured by pump-probe spectroscopy. It is interesting that, in spite of the strong coupling between the P1* and P2* states assumed in our model, the form of the coherent oscillations is mainly defined by pure vibrational coherence in the excited states. A possible nature of the P2* state is discussed.

On unified contribution of pair Coulomb excitations in three-body reactions

SynopsisIn the present work we obtain the unified contribution of pair excited states to the asymptotics (at infinity in configuration space) of the three-body absolutely continuous spectrum eigenfunctions. Note that it is the case that is most interesting and rich from the physical point of view, as it is connected in particular with re-clustering of charged quantum complexes.

Effect of magnetic anisotropy on direct chiral discrimination in paramagnetic NMR spectroscopy.

We present and analyse a complete analytic expression for the generalized shielding polarizability third-rank tensor Φαβγ, whose isotropic average, the pseudoscalar [capital Phi, Greek, macron], is proportional to a chiral macroscopic electric polarization P measurable in a pulsed nuclear magnetic resonance (NMR) experiment probing molecules with an arbitrarily degenerate ground state, in solution. We have recently predicted that [capital Phi, Greek, macron] can be measurable at room temperature in chiral paramagnetic molecules, and identified strong magnetic anistropy (as featured e.g. in lanthanide complexes) as a crucial molecular property to achieve room temperature chiral discrimination using NMR spectroscopy. As previously proposed, the components of Φαβγ are obtained as analytical third derivatives of the electronic free energy. Here we present the explicit calculation of these derivatives, which provide working expressions for the explicit accurate ab initio calculation of Φαβγ. We apply our theory by performing ab initio multiconfigurational calculations of all contributions to Φαβγ, for a set of ten DyIII complexes, characterized by a strongly axial ground Kramers doublet, but also by thermally accessible excited Kramers doublets at room temperature. The results show that the thermally populated excited state contributions, while generally reducing the value of [capital Phi, Greek, macron] calculated on the assumption of a thermally isolated ground state, still confirm the room temperature detectability of this property for all ten studied complexes. Trends on the relative sign of dominant contributions are then discussed on the basis of a crystal field model electrostatic potential splitting a ground spin-orbit multiplet, which provides an insight into the properties of the generalized shielding polarizability tensor for open shell species.

Stark shift of hydrogenic and non-hydrogenic donor impurity excited states in parabolic quantum dot: Under the effect of electric field and temperature

Abstract In the present study, we discuss theoretically the influence of lateral and normal electric field on the hydrogenic donor impurity (HDI) and non-hydrogenic donor impurity (NHDI) excited state energies in a cylindrical quantum dot (CQD) with parabolic potential. The variational approach is employed to obtain the donor impurities ground and excited state energies in the framework of the effective mass approximation; consequently, the binding energy is calculated. The comparisons between the influence of the lateral and the normal electric fields on the Stark shift of the electron (ΔE1) and Stark shift of the HDI (ΔEimp) are given. It has been found that the direction of the electric field has a considerable effect on ΔEimp. Our results showed that ΔEimp has shown a blue shift for some HDI excited states and the other states showed a red shift. The results of the combined effect of the lateral electric field and the temperature on the HDI and NHDI binding energy have been presented. It has been indicated that the HDI states in small CQDs are very sensitive to the weak lateral electric field strength than both the normal electric field strength and the temperature values. More, the Stark shift in HDI ground state energy is more affected by the temperature in large CQDs than in small CQDs.

Strained, Photoejecting Ru(II) Complexes that are Cytotoxic Under Hypoxic Conditions.

A series of strained Ru(II) complexes were studied for potential anticancer activity in hypoxic tissues. The complexes were constructed with methylated ligands that were photolabile and an imidizo[4,5-f][1,10]phenanthroline ligand that contained an appended aromatic group to potentially allow for contributions of ligand-centered excited states. A systematic variation of the size and energy of the aromatic group was performed using systems containing 1-4 fused rings, and the photochemical and photobiological behaviors of all complexes were assessed. The structure and nature of the aromatic group had a subtle impact on photochemistry, altering environmental sensitivity, and had a significant impact on cellular cytotoxicity and photobiology. Up to 5-fold differences in cytotoxicity were observed in the absence of light activation; this rose to 50-fold differences upon exposure to 453nm light. Most significantly, one complex retained activity under conditions with 1% O2 , which is used to induce hypoxic changes. This system exhibited a photocytotoxicity index (PI) of 15, which is in marked contrast to most other Ru(II) complexes, including those designed for O2 -independent mechanisms of action.

Quantum interference of a time-dependent wave packet of atom irradiated by an ultra-short laser pulse**Project partially supported by the National Key Research and Development Program of China (Grant Nos. 2019YFA0307700 and 2017YFA0403300), the National Natural Science Foundation of China (Grant Nos. 11627807, 11534004, C11975012, and 11774129), the Jilin Provincial Research Foundation for Basic Research, China (Grant No. 20170101153JC), and the

The wave packet evolution of an atom irradiated by an intense laser pulse is systematically investigated by using the numerical solution of the time-dependent Schrödinger equation. There are two types of spatial interference structures in the time-dependent evolution of the atomic wave packet. With the increasing of the evolution time, the interference fringe spacing for type I (type II) becomes larger (smaller). As the wavelength of the incident laser increases, the interference of the wave packet is changed from type II to type I, and the shift of interference type can be attributed to the contribution of excited states by using the energy analysis of the time-dependent wave function.

Relaxation Times and Population Inversion of Excited States of Arsenic Donors in Germanium

The relaxation times of excited states of arsenic dopant in germanium at cryogenic temperatures T < 15 K have been experimentally studied by the optical pump-probe method using radiation of a free-electron laser. Two variants of the excitation of impurity centers have been used in the experiment: (i) from the 1s(A1) ground state of the dopant and (ii) from the 1s(T2) first excited state having a finite thermal population. In the former variant, it has been shown that the decay times of the 2p0 and 3p± states are about 0.8 and 0.6 ns, respectively. In the latter variant, a single measurement can simultaneously provide the relaxation times of two 2p± and 1s(T2) states about 0.6 and no more than 0.16 ns, respectively. The data obtained have indicated the possibility of forming population inversion and the gain of terahertz radiation at the 2p± → 1s(T2) and 2p0 → 1s(T2) transitions at the optical excitation of the mentioned impurity centers.

Excited-state effects in nucleon structure on the lattice using hybrid interpolators

It would be very useful to find a way of reducing excited-state effects in lattice QCD calculations of nucleon structure that has a low computational cost. We explore the use of hybrid interpolators, which contain a nontrivial gluonic excitation, in a variational basis together with the standard interpolator with tuned smearing width. Using the clover discretization of the field strength tensor, a calculation using a fixed linear combination of standard and hybrid interpolators can be done using the same number of quark propagators as a standard calculation, making this a cost-effective option. We find that such an interpolator, optimized by solving a generalized eigenvalue problem, reduces excited-state contributions in two-point correlators. However, the effect in three-point correlators, which are needed for computing nucleon matrix elements, is mixed: for some matrix elements such as the tensor charge, excited-state effects are suppressed, whereas for others such as the axial charge, they are enhanced. The results illustrate that the variational method is not guaranteed to reduce the net contribution from excited states except in its asymptotic regime, and suggest that it may be important to use a large basis of interpolators capable of isolating all of the relevant low-lying states.

Small atomic clusters: quantum chemical research of isomeric composition and physical properties

In this review, we give a brief summary of methodological and computational aspects of determination of structure and different size-dependent properties of small atomic clusters by means of computational quantum chemistry. Particular attention is paid to the accurate calculation of thermodynamic properties of clusters with allowance for the vibrational anharmonicity and contribution of excited electronic states. We describe in detail the problems and consequences related to accounting for the contribution of isomeric forms to the observable physical and thermodynamic characteristics. The physical size-dependent properties discussed in this review are the binding energy, zero-point energy, collision diameter, polarizabilities, ionization potential, electron affinity, specific heat capacity, and reduced Gibbs energy. The major part of the review deals with the methodology for physically sound extrapolating the multitude of size-dependent physical and thermodynamic properties of small clusters toward significantly larger (nanometer- and micrometer-sized) particles.

Extraction of bare form factors for Bs →Kℓν decays in nonperturbative HQET

We discuss the extraction of the ground state [Formula: see text] matrix elements from Euclidean lattice correlation functions. The emphasis is on the elimination of excited state contributions. Two typical gauge-field ensembles with lattice spacings 0.075, 0.05 fm and pion masses 330, 270 MeV are used from the O[Formula: see text]-improved CLS 2-flavor simulations and the final state momentum is [Formula: see text] GeV. The b-quark is treated in HQET including the [Formula: see text] corrections. Fits to two-point and three-point correlation functions and suitable ratios including summed ratios are used, yielding consistent results with precision of around 2% which is not limited by the [Formula: see text] corrections but by the dominating static form factors. Excited state contributions are under reasonable control but are the bottleneck towards precision. We do not yet include a specific investigation of multi-hadron contaminations, a gap in the literature which ought to be filled soon.

Excited heavy quarkonium production in Higgs boson decays

The rare decay channels of Higgs boson to heavy quarkonium offer vital opportunities to explore the coupling of Higgs to heavy quarks. We study the semi-exclusive decay channels of Higgs boson to heavy quarkonia, i.e., $H^0\to |(Q\bar{Q^{\prime}})[n]\rangle+\bar{Q}Q^{\prime}$ ($Q^{(\prime)}=c~\text{or}~b$ quark) within the NRQCD framework. In addition to the lower-level Fock states $|(Q\bar{Q'})[1S]\rangle$ continent, contributions of high excited states $|(Q\bar{Q'})[2S]\rangle$, $|(Q\bar{Q'})[3S]\rangle$, $|(Q\bar{Q'})[4S]\rangle$, $|(Q\bar{Q'})[1P]\rangle$, $|(Q\bar{Q'})[2P]\rangle$, $|(Q\bar{Q'})[3P]\rangle$ and $|(Q\bar{Q'})[4P]\rangle$ are also studied. According to our study, the contributions of high excited Fock states should be considered seriously. Differential distributions of total decay width with respect to invariant-mass and angles, as well as uncertainties caused by non-perturbative hadronic non-perturbative matrix elements are discussed. If all excited heavy quarkonium states decay to the ground spin-singlet state through electromagnetic or hadronic interactions, we obtain the decay widths for $|(Q\bar{Q'})\rangle$ quarkonium production through $H^0$ semi-exclusive decays: $25.10^{+11.6\%}_{-51.6\%}$ keV for $|(b\bar{c})[n]\rangle$ meson, $3.23^{+0\%}_{-62.2\%}$ keV for $|(c\bar{c})[n]\rangle$ and $2.36^{+0\%}_{-57.1\%}$ keV for $|(b\bar{b})[n]\rangle$, where uncertainties are caused by adopting different non-perturbative potential models. At future high energy LHC ($\sqrt{s}=27$ TeV), numerical results show that sizable amounts of events for those high excited states can be produced, which implies that one could also consider exploring the coupling properties of Higgs to heavy quarks in these high excited states channels, especially for the charmonium and bottomonium.

Engineering opposite electronic polarization of singlet and triplet states increases the yield of high-energy photoproducts

Efficient photosynthetic energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states. However, energies of high-lying excited states are rarely extracted, in part because the congested density of states in the excited-state manifold leads to rapid deactivation. Conventional photosystem designs promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor ("one-way" ET), a form of positive design. Curiously, negative design strategies that explicitly avoid unwanted side reactions have been underexplored. We report here that electronic polarization of a molecular chromophore can be used as both a positive and negative design element in a light-driven reaction. Intriguingly, prudent engineering of polarized excited states can steer a "U-turn" ET-where the excited electron density of the donor is initially pushed away from the acceptor-to outcompete a conventional one-way ET scheme. We directly compare one-way vs. U-turn ET strategies via a linked donor-acceptor (DA) assembly in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor. Ultrafast spectroscopy of DA pinpoints the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing. These results demonstrate that oppositely polarized electronically excited states can be employed to steer photoexcited states toward useful, high-energy products by routing these excited states away from states that are photosynthetic dead ends.

Strong-field ionization dynamics of asymmetric equilateral triatomic model molecules in circularly polarized laser fields

The strong-field-driven ionization dynamics of larger, nondiatomic molecules still constitutes a major challenge for a theoretical and numerical description. While a full numerical treatment of all electronic and nuclear degrees of freedom is computationally prohibitive, several approaches exist to model photoelectron spectra employing certain approximations. Among them are classical trajectory methods or the so-called strong-field approximation (SFA). Here we investigate in detail the strong-field ionization dynamics of simple, noncollinear triatomic model systems. For a detailed characterization of the ionization process of our system, we combine quantum dynamical simulations, modified classical trajectory calculations, and extensions of the SFA in order to analyze the complex pattern of the photoelectron momentum distribution. We aim at disentangling the contributions of excited states and the long-range character of the potential. We show that upon interaction with circularly polarized laser fields, the long-range character merely induces a small shift in the spectra, while the contribution of excited states is in several cases essential: in particular in near-infrared laser fields, compared to mid-infrared drivers, and for systems with larger internuclear distances, when excited electronic states are energetically closer. Key modifications are for our trajectory model to explicitly incorporate the numerically obtained tunnel barrier width directly into the ionization rate, thereby enabling a spatially anisotropic ionization rate, and an approximate treatment of the contribution of excited states via the Stark shift. In contrast, for the SFA, it is important to include electronically excited states for the description of the initial state. We also discuss the effect of the carrier-envelope phase on the ionization dynamics, as well as the influence of molecular averaging.