Initial Excited State Research Articles

Semiclassical approximation meets Keldysh\u2013Schwinger diagrammatic technique: scalar varphi ^4

We study the evolution of the non-equilibrium quantum fields from a highly excited initial state in two approaches: the standard Keldysh–Schwinger diagram technique and the semiclassical expansion. We demonstrate explicitly that these two approaches coincide if the coupling constant g and the Plank constant hbar are simultaneously small. Also, we discuss loop diagrams of the perturbative approach, which are summed up by the leading order term of the semiclassical expansion. As an example, we consider shear viscosity for the scalar field theory at the leading semiclassical order. We introduce the new technique that unifies both semiclassical and diagrammatic approaches and open the possibility to perform the resummation of the semiclassical contributions.

Unlocking the Double Bond in Protonated Schiff Bases by Coherent Superposition of S1 and S2.

The primary event occurring during the E-to-Z photoisomerization reaction of retinal protonated Schiff base (rPSB) is single-to-double bond inversion. In this work we examine the nuclear dynamics that occurs when the initial excited state is a superposition of the S1 and S2 electronic excited states that might be created in a laser experiment. The nuclear dynamics is dominated by double bond inversion that is parallel to the derivative coupling vector of S1 and S2. Thus, the molecule behaves as if it were at a conical intersection even if the states are nondegenerate.

Manipulating Energy Transfer in UCNPs@SiO2@Ag Nanoparticles for Efficient Infrared Photocatalysis.

Conventional photocatalysts must be activated by ultraviolet or visible light to meet the energy requirement of populating an initial excited state, while infrared light has a high penetration depth to reaction media but does not have enough photon energy to activate conventional photocatalysts. Here, we report the activation of Ag nanoparticles by upconversion nanoparticles (UCNPs) in UCNPs@SiO2@Ag with manipulated energy transfer for infrared photocatalysis. UCNPs can efficiently convert infrared light to visible and ultraviolet light and are very ideal candidates for bridging the advantage of infrared light and the activation energy requirement of conventional photocatalysts. In the UCNPs@SiO2@Ag nanosystem, we employ the UCNPs to activate conventional Ag nanoparticles under infrared light irradiation. The evanescent field of UCNPs is confined for enhancing the near-field energy-transfer efficiency using a designed core/shell heterostructure, while a SiO2 layer is used for blocking the phonon exchange of thermal vibration between photon upconverters and Ag nanoparticles. Based on the manipulated energy transfer, UCNPs@SiO2@Ag nanoparticles exhibit efficient photocatalytic activity under the irradiation of 980 nm infrared light, while single Ag nanoparticles have negligible catalytic activity under infrared irradiation.

Engineering electron wavefunctions in asymmetrically confined quasi one-dimensional structures

We present results on electron transport in quasi-one dimensional quantum wires in GaAs/AlGaAs heterostructures obtained using an asymmetric confinement potential. The variation of the energy levels of the spatially quantized states is followed from strong confinement through weak confinement to the onset of two dimensionality. An anticrossing of the initial ground and first excited states is found as the asymmetry of the potential is varied, giving rise to two anticrossing events, which occur on either side of symmetric confinement. We present results analyzing this behavior and showing how it can be affected by the inhomogeneity in background potential. The use of an enhanced source-drain voltage to alter the energy levels is shown to be a significant validation of the analysis by showing the formation of double rows of electrons, which correlate with the anticrossing.

Multiscale Conformational Sampling Reveals Excited-State Locality in DNA Self-Repair Mechanism.

Ultraviolet (UV) irradiation is known to be responsible for DNA damage. However, experimental studies in DNA oligonucleotides have shown that UV light can also induce sequence-specific self-repair. Following charge transfer from a guanine adenine sequence adjacent to a cyclobutane pyrimidine dimer (CPD), the covalent bond between the two thymines could be cleaved, recovering the intact base sequence. Mechanistic details promoting the self-repair remained unclear, however. In our theoretical study, we investigated whether optical excitation could directly lead to a charge-transfer state, thereby initiating the repair, or whether the initial excited state remains localized on a single nucleobase. We performed conformational sampling of 200 geometries of the damaged DNA double strand solvated in water and used a hybrid quantum and molecular mechanics approach to compute excited states at the complete active space perturbation level of theory. Analysis of the conformational data set clearly revealed that the excited-state properties are uniformly distributed across the fluctuations of the nucleotide in its natural environment. From the electronic wavefunction, we learned that the electronic transitions remained predominantly local on either adenine or guanine, and no direct charge transfer occurred in the experimentally accessed energy range. The investigated base sequence is not only specific to the CPD repair mechanism but ubiquitously occurs in nucleic acids. Our results therefore give a very general insight into the charge locality of UV-excited DNA, a property that is regarded to have determining relevance in the structural consequences following absorption of UV photons.

Quantum speed limit based on the bound of Bures angle

In this paper, we investigate the unified bound of quantum speed limit time in open systems based on the modified Bures angle. This bound is applied to the damped Jaynes-Cummings model and the dephasing model, and the analytical quantum speed limit time is obtained for both models. As an example, the maximum coherent qubit state with white noise is chosen as the initial states for the damped Jaynes-Cummings model. It is found that the quantum speed limit time in both the non-Markovian and the Markovian regimes can be decreased by the white noise compared with the pure state. In addition, for the dephasing model, we find that the quantum speed limit time is not only related to the coherence of initial state and non-Markovianity, but also dependent on the population of initial excited state.

Nanoscale Metal-Organic Frameworks and Metal-Organic Layers with Two-Photon-Excited Fluorescence.

Metal-organic frameworks (MOFs) based on 9,10-diphenylanthracene-derived ligands had been reported to exhibit upconverted fluorescence through triplet-triplet annihilation. We found that zirconium MOFs based on 9,10-diphenylanthracene can also give upconverted fluorescence via two-photon absorption without adding a triplet photosensitizer when a femtosecond pulsed laser is used as the excitation source. By tuning the synthetic condition, we obtained nanoscale MOFs of UiO structure in both octahedral and hexagonal nanoplate shapes, as well as a hexagonal nanoplate of MOFs of hcp-UiO structure and two-dimensional metal-organic layers. All of them, as well as a homogeneous solution of the 9,10-diphenylanthracene ligand, exhibit upconverted fluorescence upon excitation using a laser pulse of 60 fs with a pulse energy of ∼1.1 × 106 nJ/cm2 (unfocused). Moreover, we observed different emission spectra by two-photon excitation compared to those by one-photon excitation, which indicates access to a unique initial excited state via two-photon excitation. This phenomenon is not observed for a homogeneous solution of the ligand. These nanoscale MOFs may find application in two-photon fluorescence imaging.

Non-monotonous behavior of the number variance, Mandel factor, invariant uncertainty product and purity for the quantum damped harmonic oscillator

We consider the relaxation of the quantum harmonic oscillator in the framework of the standard master equation. Analyzing the known solutions, we have found some new interesting effects. Namely, we show that the evolution of some parameters, such as the energy fluctuations, the Mandel factor, the invariant uncertainty product and quantum purity, can exhibit non-monotonous behavior with big deviations from the initial and final values. Also, we show that highly excited initial Fock states maintain their non-classical nature longer than low excited ones. During the evolution, the quantum purity of these states stays at almost constant low level during a long period, after the fast decrease initially and before the fast return to the final value at the end.

Parameter estimation in cosmic string spacetime by using the inertial and accelerated detectors

Since cosmic string spacetime is locally flat but with nontrivial global topology characterized by a deficit angle, we devote to address the quantum bound to the estimation of the deficit angle parameter by using a two-level atom as a detector which is coupled to a massless scalar field. We show that the initial excited state of the detector is the optimal state, and quantum precision always obtains the maximum value when the detector evolves for a limited time. We find that the sensitivity in the predictions for the deficit angle parameter decreases with the increase of . We also note that a uniform, rectilinear accelerated motion does not improve the estimation accuracy, so we obtain the inertial detector is better than the uniform, rectilinear accelerated detector. Therefore, we provide a possibility for detecting the nontrivial global topology in the spacetime.

Trans-Planckian censorship, inflation, and excited initial states for perturbations

The recently proposed trans-Planckian censorship conjecture (TCC) seems to require that the energy scale of inflation is significantly lower than the Planck scale $(H_\text{inf}<10^{-20} \Mpl)$. This, in turn, implies that the tensor-to-scalar ratio for inflation is negligibly small, \textit{independent} of assumptions of slow-roll or even of having a single scalar field, thus ruling out inflation if primordial tensor modes are ever observed. After demonstrating the robustness and generality of these bounds, we show that having an excited initial state for cosmological perturbations seems to be a way out of this problem for models of inflation.

Solvent-dependent ultrafast optical response of conjugated push-pull chromophores.

Two new difluoroboron β-carbonyl cyclic ketonate complexes C2B and DC2B were investigated using several spectroscopic methods. Relative to the absorption spectra, the fluorescence spectra were more affected by the polarity of the solvent. Also, compound C2B showed a more pronounced Stokes' shift after solvent polarity increased. Transient absorption measurements then demonstrated the relaxation behaviour of the excited state compound molecule. The kinetic results showed that the excited state C2B in tetrahydrofuran (THF) can return from the intramolecular charge-transfer (ICT) state and the initial excited state to the ground state. The kinetic relaxation pathway after THF was replaced by dimethyl sulfoxide became single. When the carbazole unit was introduced, DC2B also exhibited an ICT state but there was no significant difference in the excited state relaxation path after solvent polarity was changed. The results indicated that C2B is more susceptible to solvent polarity regulation. The global fit results revealed that an increase in the solvent polarity prolonged the lifetime of the ICT state of compound C2B and had the opposite effect on compound DC2B. These results provide guidance for understanding the relationship between solvent polarity and the designing and synthesizing advanced compound materials.

Entanglement in a fermion chain under continuous monitoring

We study the entanglement entropy of the quantum trajectories of a free fermion chain under continuous monitoring of local occupation numbers. We propose a simple theory for entanglement entropy evolution from disentangled and highly excited initial states. It is based on generalized hydrodynamics and the quasi-particle pair approach to entanglement in integrable systems. We test several quantitative predictions of the theory against extensive numerics and find good agreement. In particular, the volume law entanglement is destroyed by the presence of arbitrarily weak measurement.

Transient Two-Dimensional Electronic Spectroscopy: Coherent Dynamics at Arbitrary Times along the Reaction Coordinate.

Recent advances in two-dimensional electronic spectroscopy (2DES) have enabled identification of fragile quantum coherences in condensed-phase systems near the equilibrium molecular geometry. In general, traditional 2DES cannot measure such coherences associated with photophysical processes that occur at times significantly after the initially prepared state has dephased, such as the evolution of the initial excited state into a charge transfer state. We demonstrate the use of transient two-dimensional electronic spectroscopy (t-2DES) to probe coherences in an electron donor-acceptor dyad consisting of a perylenediimide (PDI) acceptor and a perylene (Per) donor. An actinic pump pulse prepares the lowest excited singlet state of PDI followed by formation of the PDI•--Per•+ ion pair, which is probed at different times following the actinic pulse using 2DES. Analysis of the observed coherences provides information about electronic, vibronic, and vibrational interactions at any time along the reaction coordinate for ion pair formation.

Intramolecular Long-Range Charge-Transfer Emission in Donor-Bridge-Acceptor Systems.

Charge recombination to the electronic ground state typically occurs nonradiatively. We report a rational design of donor-bridge-acceptor molecules that exhibit charge-transfer (CT) emission through conjugated bridges over distances of up to 24 Å. The emission is enhanced by intensity borrowing and extends into the near-IR region. Efficient charge recombination to the initial excited state results in recombination fluorescence. We have established the identity of CT emission by solvent dependence, sensitivity to temperature, femtosecond transient absorption spectroscopy, and unique emission polarization patterns. Large excited-state electronic couplings and small energy gaps enable the observation of intramolecular long-range CT emission over the unprecedented long distance. These results open new possibilities of using intramolecular long-range CT emission in molecular electronic and biomedical imaging probe applications.

Initial Excited State and Divergence of Wave Beams as Factors Responsible for the Realization of the Wave Process that Controls the Growth of Transformation Twins

In the dynamic theory of martensitic transformations the wave mechanism of growth of martensite crystals is controlled by the overlap of wave beams of quasi-longitudinal (or longitudinal) waves that carry deformation of the “tension–compression” type in orthogonal directions. The appearance of wave beams is considered a consequence of the appearance of the initial excited (vibrational) states. The presence of the transformation twins is interpreted as the result of the matched propagation of relatively long-wave (𝓁 waves) and short-wave (s waves) displacements. With the aid of the Fourier transforms, an analysis was performed of the influence of the diffraction divergence of a pair of orthogonal wave beams on the formation of a region of their superposition, including the limiting case of narrow beams with one of the transverse dimensions of the front less than λ/2, where λ is the wavelength. The connection of the distribution of the amplitudes of spatial harmonics with the configuration of the initial excited state is discussed.

Comparison of third-order nonlinear optical properties of benzothiazolium salt and neutral benzothiazole derivative: Broadband absorption response and transient dynamic analysis

Two benzothiazole derivatives, neutral benzothiazole derivative and the corresponding benzothiazolium salt, were designed and synthesized. Broadband reverse saturable absorption (RSA) were observed for both compounds in different solvents under femtosecond Z-scan, and the range of RSA of benzothiazolium salt showed larger redshift compared with that of the neutral benzothiazole. However the variations of absorption properties of these samples in femtosecond and picosecond time regimes were different. The transient absorption spectrum reveal the conversion process from the initial local excited (LE) state to the charge transfer (CT) state and the nonlinear optical response of two compounds in different time regimes were analysed.

Initial Excited-State Structural Dynamics of dT and dA Oligonucleotide Homopentamers Using Resonance Raman Spectroscopy.

Photochemical damage of DNA is initiated by absorption of ultraviolet light, and the photoproducts are formed as a result of excited-state structural and electronic dynamics. We have used UV resonance Raman spectroscopy to measure the initial excited-state structural dynamics of homopentamers of adenosine monophosphate (3'-dApdApdApdApdAp-5') and thymidine monophosphate (3'-dTpdTpdTpdTpdTp-5') and compare them to those of the monomeric nucleobases. The resonance Raman spectra of the homopentamers are similar to those of the corresponding monomers. Initial excited-state slopes, homogeneous and inhomogeneous broadening, and other excited-state parameters were extracted by self-consistent simulation of the resonance Raman excitation profiles and absorption spectra with a time-dependent formalism and are also similar to the initial excited-state slopes and broadening in the nucleotide monomers. The lack of differences between the initial excited-state structural dynamics of the nucleotides within the pentamer and the isolated nucleobases is consistent with a model in which the formation of photochemical products in oligonucleotides and DNA is dependent on the formation of the transition-state structure within these polymers, dictated by their large-scale dynamics. These results are discussed in light of the known photochemistry of DNA and the nucleobases.

Triple autoionization of atomic ions

A time-dependent close-coupling method for four-electron atomic systems is formulated to calculate the triple autoionization of atomic ions. Initial excited states are obtained by relaxation of the Schrodinger equation in imaginary time using a Schmidt orthogonalization method involving interior subshells. Autoionization rates are obtained by propagation in real time using projections onto products of bound and continuum single particle states. For the () excited configuration of the atomic ion, we obtain total, double, and triple autoionization rates. The total autoionization rates are in reasonable agreement with single autoionization rates obtained using configuration-average distorted-wave theory.

Rescuing single field inflation from the swampland

The difficulty of building metastable vacua in string theory, has led some to conjecture that in the string theory landscape, potentials satisfy |∇V/V|≥c∼O(1). This condition, which is supported by different explicit constructions, suggests that the EFTs which lead to metastable de-Sitter vacua belong to what is dubbed as swampland. This condition endangers the paradigm of single field inflation. In this paper, we show how scalar excited initial states cannot rescue single field inflation from the swampland, as they produce large local scalar non-gaussianity, which is in conflict with the Planck upper bound. Instead, we demonstrate that one can salvage single field inflation using excited initial states for tensor perturbations, which in this case produce only large flattened non-gaussianity in the tensor bispectrum. We comment on the possible methods one can prepare such excited initial conditions for the tensor perturbations.

Visible Light Driven Hydrogen Evolution by Molecular Nickel Catalysts with Time-Resolved Spectroscopic and DFT Insights.

Hydrogen (H2) is a clean fuel that can potentially be a future solution for the storage of intermittent renewable energy. However, current H2 production is mainly dominated by the energy intensive steam reforming reaction, which consumes a fossil fuel, methane, and emits copious amounts of carbon dioxide as one of the byproducts. To address this challenge, we report a molecular catalyst that produces H2 from aqueous solutions, is composed of affordable, earth-abundant elements such as nickel, and has been incorporated into a system driven by visible light. Under optimized conditions, we observe a turnover number of 3880, among the best for photocatalytic H2 evolution with nickel complexes from water-methanol solutions. Through nanosecond transient absorption, electron paramagnetic resonance, and UV-vis spectroscopic measurements, and supported by density functional theory calculations, we report a detailed study of this photocatalytic H2 evolution cycle. We demonstrate that a one-electron reduced, predominantly ligand-centered, reactive Ni intermediate can be accessed under visible light irradiation using triethylamine as the sacrificial electron donor and reductive quencher of the initial photosensitizer excited state. In addition, the computational calculations suggest that the second coordination sphere ether arms can enhance the catalytic activity by promoting proton relay, similar to the mechanism among [FeFe] hydrogenases in nature. Our study can form the basis for future development of H2 evolution molecular catalysts that incorporate both ligand redox noninnocence and alternative second coordination sphere effects in artificial photosynthetic systems driven by visible light.