Excited Manifold Research Articles

Orbital-invariant spin-extended approximate coupled-cluster for multi-reference systems.

We present an approximate treatment of spin-extended coupled-cluster (ECC) based on the spin-projection of the broken-symmetry coupled-cluster (CC) ansatz. ECC completely eliminates the spin-contamination of unrestricted CC and is therefore expected to provide better descriptions of dynamical and static correlation effects, but introduces two distinct problems. The first issue is the emergence of non-terminating amplitude equations, which are caused by the de-excitation effects inherent in symmetry projection operators. In this study, we take a minimalist approach and truncate the Taylor series of the exponential ansatz at a certain order such that the approximation safely recovers the traditional CC without spin-projection. The second issue is that the nonlinear equations of ECC become underdetermined, although consistent, yielding an infinitude of solutions. This problem arises because of the redundancies in the excitation manifold, as is common in other multi-reference approaches. We remove the linear dependencies in ECC by employing an orthogonal projection manifold. We also propose an efficient solver for our method, in which the components are usually sparse but not diagonal-dominant. It is shown that our approach is rigorously orbital-invariant and provides more accurate results than its configuration interaction and linearized CC analogues for chemical systems.

Spectroscopic Properties of LiNbO3:Tm3+ Crystal in the 1650–1970 nm Wavelength Range

Taking into account the Stark structure of optical spectrum of the impurity ion, the theoretical study of spectroscopic properties of LiNbO3:Tm3+ crystal in the wavelength range of 1650–1970 nm was carried out. The wave functions of the Stark sublevels of both ground 3H6 and the first excited 3F4 manifolds of the Tm3+ ion were constructed in the LSJM-representation, the line strengths induced by indirect electric-dipole transitions were calculated, and the main spectroscopic characteristics of emission and absorption spectra of impurity ion were determined.

Unitary coupled-cluster based self-consistent polarization propagator theory: A third-order formulation and pilot applications.

In this article, the development of a third-order self-consistent polarization propagator method based on unitary coupled-cluster (UCC) parametrization of the ground-state wavefunction and the excitation manifold comprising unitary-transformed excitation operators, hereafter referred to as UCC3, is reported. The UCC3 method is designed to provide excitation energies correct up to the third order for excited states dominated by single excitations. An expansion for the UCC transformed Hamiltonian involving Bernoulli numbers as expansion coefficients is adopted in the derivation of UCC3 working equations. Interestingly, UCC-based polarization propagator theory offers an alternative derivation for the strict version of the third-order algebraic diagrammatic construction [ADC(3)-s] method. The UCC3 results for the excitation energies of excited states in H2O, HF, N2, Ne, CH2, BH, and C2 molecules are compared with benchmark full configuration interaction values as well as ADC(3) and equation-of-motion coupled-cluster singles and doubles results to demonstrate the accuracy of the UCC3 method. UCC-based self-consistent polarization propagator theory appears to be a promising framework for developing non-perturbative hermitian formulations for treating electronically excited states.

Connecting bright and dark states through accidental degeneracy caused by lack of symmetry.

Coupled excitonic structures are found in natural and artificial light harvesting systems where optical transitions link different excitation manifolds. In systems with symmetry, some optical transitions are allowed, while others are forbidden. Here we examine an excitonic ring structure and identify an accidental degeneracy between two categories of double-excitation eigenstates with distinct symmetries and optical transition properties. To understand the accidental degeneracy, a complete selection rule between two arbitrary excitation manifolds is derived with a physically motivated proof. Remarkably, symmetry analysis shows that the lack of certain symmetry elements in the Hamiltonian is responsible for this degeneracy, which is unique to rings with size N = 4l + 2 (l being an integer).

Two-dimensional Fano lineshapes: Excited-state absorption contributions

Fano interferences in nanostructures are influenced by dissipation effects as well as many-body interactions. Two-dimensional coherent spectroscopies have just begun to be applied to these systems where the spectroscopic signatures of a discrete-continuum structure are not known. In this article, we calculate the excited-state absorption contribution for different models of higher lying excited states. We find that the characteristic asymmetry of one-dimensional spectroscopies is recovered from the many-body contributions and that the higher lying excited manifolds have distorted lineshapes that are not anticipated from discrete-level Hamiltonians. We show that the Stimulated Emission cannot have contributions from a flat continuum of states. This work completes the Ground-State Bleach and Stimulated Emission signals that were calculated previously [D. Finkelstein-Shapiro et al., Phys. Rev. B 94, 205137 (2016)]. The model reproduces the observations reported for molecules on surfaces probed by 2DIR.

Excited State Magnetic Exchange Interactions Enable Large Spin Polarization Effects.

Excited state processes involving multiple electron spin centers are crucial elements for both spintronics and quantum information processing. Herein, we describe an addressable excited state mechanism for precise control of electron spin polarization. This mechanism derives from excited state magnetic exchange couplings that occur between the electron spins of a photogenerated electron-hole pair and that of an organic radical. The process is initiated by absorption of a photon followed by ultrafast relaxation within the excited state spin manifold. This leads to dramatic changes in spin polarization between excited states of the same multiplicity. Moreover, this photoinitiated spin polarization process can be "read" spectroscopically using a magnetooptical technique that is sensitive to the excited state electron spin polarizations and allows for the evaluation of wave functions that give rise to these polarizations. This system is unique in that it requires neither intersystem crossing nor magnetic resonance techniques to create dynamic spin-polarization effects in molecules.

Cluster decomposition of full configuration interaction wave functions: A tool for chemical interpretation of systems with strong correlation.

Approximate full configuration interaction (FCI) calculations have recently become tractable for systems of unforeseen size, thanks to stochastic and adaptive approximations to the exponentially scaling FCI problem. The result of an FCI calculation is a weighted set of electronic configurations, which can also be expressed in terms of excitations from a reference configuration. The excitation amplitudes contain information on the complexity of the electronic wave function, but this information is contaminated by contributions from disconnected excitations, i.e., those excitations that are just products of independent lower-level excitations. The unwanted contributions can be removed via a cluster decomposition procedure, making it possible to examine the importance of connected excitations in complicated multireference molecules which are outside the reach of conventional algorithms. We present an implementation of the cluster decomposition analysis and apply it to both true FCI wave functions, as well as wave functions generated from the adaptive sampling CI algorithm. The cluster decomposition is useful for interpreting calculations in chemical studies, as a diagnostic for the convergence of various excitation manifolds, as well as as a guidepost for polynomially scaling electronic structure models. Applications are presented for (i) the double dissociation of water, (ii) the carbon dimer, (iii) the π space of polyacenes, and (iv) the chromium dimer. While the cluster amplitudes exhibit rapid decay with an increasing rank for the first three systems, even connected octuple excitations still appear important in Cr2, suggesting that spin-restricted single-reference coupled-cluster approaches may not be tractable for some problems in transition metal chemistry.

Excited states manifold of 2,2′-bithiophene: basis set dependence study

ABSTRACTThe 2,2′-bithiophene energy spectra computed using a sequence of cc-pVnZ and aug-cc-pVnZ basis sets reveal surprisingly strong dependence of the excited states manifold on the quality of the basis set. The observed computational artefacts include: numerous missing states, wrong order of states and considerable shifts in the energy spectrum. The presented results suggest that the cc-pVnZ basis sets are completely unsuitable for modelling optical spectra of organic molecules and that the aug-cc-pVnZ basis sets are capable of predicting only the lowest portion of the energy spectrum. A simple and inexpensive remedy for the observed problems is suggested: an additional, molecule-centred, Rydberg basis should be rudimentarily used in quantum chemical calculations aiming at modelling optical spectra of molecules. The main conclusions of the presented study seem to be general and independent of the chemical identity of the studied system and of details of the employed computational methodology.

Coupled Cluster Studies of Ionization Potentials and Electron Affinities of Single-Walled Carbon Nanotubes.

In this paper, we apply equation-of-motion coupled cluster (EOM-CC) methods in the studies of the vertical ionization potentials (IPs) and electron affinities (EAs) for a series of single-walled carbon nanotubes (SWCNT). The EOM-CC formulations for IPs and EAs employing excitation manifolds spanned by single and double excitations (IP/EA-EOM-CCSD) are used to study the IPs and EAs of the SWCNTs as a function of the nanotube length. Several armchair nanotubes corresponding to C20nH20 models with n = 2-6 have been used in benchmark calculations. In agreement with previous studies, we demonstrate that the electronegativity of C20nH20 systems remains, to a large extent, independent of the nanotube length. We also compare IP/EA-EOM-CCSD results with those obtained with coupled cluster models with single and double excitations corrected by perturbative triples, CCSD(T), and density functional theory (DFT) using global and range-separated hybrid exchange-correlation functionals.

The Photochemical Branching Ratio in 1,6-Dinitropyrene Depends on the Excitation Energy.

Nitropolycyclic aromatic hydrocarbons constitute one of the most disconcerting classes of pollutants. Photochemical degradation is thought to be a primary mode of their natural removal from the environment, but the microscopic mechanism leading to product formation as a function of excitation wavelength is poorly understood. In this Letter, it is revealed that excitation of 1,6-dinitropyrene with 425, 415, or 340 nm radiation leads to an increasing amount of radical production through photodissociation at the expense of triplet-state population-the two primary reaction pathways in this class of pollutants. Radical formation requires overcoming an energy barrier in the excited singlet manifold. This activation energy explains the large fraction of the initial singlet-state population that intersystem crosses to a doorway triplet state, instead of leading overwhelmingly to photodissociation. The unforeseen excitation wavelength dependence of this branching process is expected to regulate the photochemistry of 1,6-dinitropyrene and possibly of other nitroaromatic pollutants in the environment.

Looseness diagnosis method for connecting bolt of fan foundation based on sensitive mixed-domain features of excitation-response and manifold learning

Looseness diagnosis for connecting bolt of fan foundation is an important task for ensuring proper operation of fan and safe traffic. Aiming at solving the key problems of bolt looseness diagnosis including looseness feature extraction, looseness feature set construction, non-sensitive or poor sensitive feature interference and feature set nonlinear dimension reduction, a looseness diagnosis method for connecting bolt of fan foundation based on sensitive mixed-domain features of excitation response and manifold learning is proposed. Firstly, the response signal is collected by applying a pulse excitation signal to the fan, and the frequency response function is calculated. The looseness of fan foundation is characterized by response signal and the frequency response function. Then, the looseness mixed-domain feature set is constructed through fusion time-domain feature and frequency-domain feature of response signal and frequency response function. Secondly, the looseness sensitivity index is calculated based on scatter matrix for sensitive feature selection to avoid the interference of non-sensitive and poor sensitive feature, thus the looseness sensitive feature set is constructed. Moreover, orthogonal neighborhood preserving embedding (ONPE), an effective manifold learning algorithm with non-linear dimensionality reduction capability, is applied to compress the high-dimensional looseness sensitive feature set into the low-dimensional one. Finally, the low-dimensional looseness sensitive feature set is imported into weight K nearest neighbor classifier (WKNNC) as input to recognize different loosening of connecting bolt, and the stability of recognition accuracy rate is ensured. The feasibility and performance of the proposed method were proved by successful looseness diagnosis application on a tunnel fan foundation's connecting bolt.

Direct observation of subpicosecond vibrational dynamics in photoexcited myoglobin.

Determining the initial pathway for ultrafast energy redistribution within biomolecules is a challenge, and haem proteins, for which energy can be deposited locally in the haem moiety using short light pulses, are suitable model systems to address this issue. However, data acquired using existing experimental techniques that fail to combine sufficient structural sensitivity with adequate time resolution have resulted in alternative hypotheses concerning the interplay between energy flow among highly excited vibrational levels and potential concomitant electronic processes. By developing a femtosecond-stimulated Raman set-up, endowed with the necessary tunability to take advantage of different resonance conditions, here we visualize the temporal evolution of energy redistribution over different vibrational modes in myoglobin. We establish that the vibrational energy initially stored in the highly excited Franck-Condon manifold is transferred with different timescales into low- and high-frequency modes, prior to slow dissipation through the protein. These findings demonstrate that a newly proposed mechanism involving the population dynamics of specific vibrational modes settles the controversy on the existence of transient electronic intermediates.

SU(2) uncertainty limits

Although progress has been made recently in defining nontrivial uncertainty limits for the SU(2) group, a description of the intermediate states bound by these limits remains lacking. In this paper we enumerate possible uncertainty relations for the SU(2) group that involve all three observables and that are, moreover, invariant under SU(2) transformations. We demonstrate that these relations however, even taken as a group, do not provide sharp, saturable bounds. To find sharp bounds, we systematically calculate the variance of the SU(2) operators for all pure states belonging to the $N=2$ and $N=3$ polarisation excitation manifold (corresponding to spin 1 and spin 3/2). Lastly, and perhaps counter to expectation, we note that even pure states can reach the maximum uncertainty limit.

Parsing polarization squeezing into Fock layers

We investigate polarization squeezing in squeezed coherent states with varying coherent amplitudes. In contrast to the traditional characterization based on the full Stokes parameters, we experimentally determine the Stokes vector of each excitation manifold separately. Only for states with a fixed photon number do the methods coincide; when the photon number is indefinite, our approach gives a richer and deeper description. By capitalising on the properties of the Husimi Q function, we map this notion onto the Poincare space, providing a full account of the measured squeezing.

Excited state relaxation pathways of 4-dimethylamino-β-nitrostyrene: Effect of solvent polarity and donor–acceptor conjugation

Abstract Solvent polarity dependent relaxation pathways of the excited state of 4-dimethylamino-β-nitrostyrene (DANS) have been investigated using steady state and transient absorption spectroscopic techniques in femtosecond and nanosecond timescale. The molecule undergoes ultrafast twisted intramolecular charge transfer (TICT) relaxation involving the rotation of dimethylaniline group in polar solvents. In the medium and low polarity solvents, TICT relaxation rate is retarded due to larger barrier of twisting along TICT coordinate. In nonpolar solvents, the S1 state population undergo efficient intersystem crossing (ISC) to triplet surface without any conformational change. However, trans → cis isomerisation in nonpolar solvents takes place from the excited triplet manifold, as monitored by steady state photo-irradiation experiments. The ISC efficiency decreases with the increase in solvent polarity, which is explained by invoking solvent polarity dependent stabilization of charge transfer S1 state relative to a receiver triplet state. Comparison of the excited state dynamics of DANS with less conjugated para-dialkylamino nitrobenzene derivatives, reported previously, suggests that additional π-conjugation between the donor and acceptor groups stabilizes planar ICT state resulting to slower TICT relaxation.

State-Selective Excitation of Quantum Systems via Geometrical Optimization

We lay out the foundations of a general method of quantum control via geometrical optimization. We apply the method to state-selective population transfer using ultrashort transform-limited pulses between manifolds of levels that may represent, e.g., state-selective transitions in molecules. Assuming that certain states can be prepared, we develop three implementations: (i) preoptimization, which implies engineering the initial state within the ground manifold or electronic state before the pulse is applied; (ii) postoptimization, which implies engineering the final state within the excited manifold or target electronic state, after the pulse; and (iii) double-time optimization, which uses both types of time-ordered manipulations. We apply the schemes to two important dynamical problems: To prepare arbitrary vibrational superposition states on the target electronic state and to select weakly coupled vibrational states. Whereas full population inversion between the electronic states only requires control at initial time in all of the ground vibrational levels, only very specific superposition states can be prepared with high fidelity by either pre- or postoptimization mechanisms. Full state-selective population inversion requires manipulating the vibrational coherences in the ground electronic state before the optical pulse is applied and in the excited electronic state afterward, but not during all times.

Cotunneling spectroscopy and the properties of excited-state spin manifolds ofMn12single molecule magnets

We study charge transport through single molecule magnet (SMM) junctions in the cotunneling regime as a tool for investigating the properties of the excited-state manifolds of neutral ${\mathrm{Mn}}_{12}$ SMs. This study is motivated by a recent transport experiment [S. Kahle et al., Nano Lett. 12, 518 (2012)] that probed the details of the magnetic and electronic structure of ${\mathrm{Mn}}_{12}$ SMMs beyond the ground-state spin manifold. A giant spin Hamiltonian and master equation approach is used to explore theoretically the cotunneling transport through ${\mathrm{Mn}}_{12}$-Ac SMM junctions. We identify SMM transitions that can account for both the strong and weak features of the experimental differential conductance spectra. We find the experimental results to imply that the excited spin-state manifolds of the neutral SMM have either different anisotropy constants or different $g$ factors in comparison with its ground-state manifold. However, the latter scenario accounts best for the experimental data.

Roles of allene-group in an intramolecular charge transfer character of a short fucoxanthin homolog as revealed by femtosecond pump-probe spectroscopy

Ultrafast excited state dynamics of the short fucoxanthin homolog with four conjugated double bonds and its allene-modified analog in various solvents have been studied by femtosecond pump-probe spectroscopy. The results clarified that the short fucoxanthin homolog exhibits a strong intramolecular charge transfer character in an excited manifold even in a nonpolar solvent. Then, the data show that a modification of the allene-group induced the lifetime shortening of the lowest singlet state of the short fucoxanthin homolog. This implies that the allene-group enhances the ICT character of the short fucoxanthin homolog.

A pair natural orbital based implementation of ADC(2)-x: Perspectives and challenges for response methods for singly and doubly excited states in large molecules

At the example of the extended algebraic-diagrammatic construction through second order, ADC(2)-x, we study the performance of a pair natural orbital (PNO) expansion for the description of partially or dominantly doubly excited states within a response theory framework. We find that with the presence of dominantly doubly excited states in the ADC(2)-x spectra the PNO truncation errors are between a factor of two and five larger than for strict second-order methods, where such excitations are only described in a zeroth-order approximation, but that otherwise the rate of convergence with the PNO selection threshold is similar.Furthermore, we analyze the reason for the red shift of excitation energies in ADC(2)-x and trace it back to an unbalanced account of electron correlation effects in the ground and the excited state by the term added in the extended method to describe double excitations correct through first order. The balance can be restored by including the contributions from one additional commutator which accounts in lowest order for the coupling between double excitations in the ground and the excited state. A perturbative correction to ADC(2) which includes both correction terms for doubly excited configurations gives excitation energies close to results from CCSD with, however, much lower computational expenses.The truncated pair natural orbital expansion for the ground and excited state doubles amplitudes opens a route to reduce the steep O(N6) cost-scaling of the extended ADC(2) and other higher-order response methods without significant degradation of their accuracy or a priori restriction of the double excitation manifold. Already our proof-of-principles implementation exhibits a cost-scaling between O(N3) and O(N4), which can be further reduced with a stringent use of integral screening and local density fitting to a quadratic cost-scaling.

N‐Alkoxyheterocycles As Irreversible Photooxidants

Irreversible photooxidation based on N-O bond fragmentation is demonstrated for N-methoxyheterocycles in both the singlet and triplet excited state manifolds. The energetic requirements for bond fragmentation are studied in detail. Bond fragmentation in the excited singlet manifold is possible for ππ* singlet states with energies significantly larger than the N-O bond dissociation energy of ca 55 kcal mol(-1). For the nπ* triplet states, N-O bond fragmentation does not occur in the excited state for orbital overlap and energetic reasons. Irreversible photooxidation occurs in the singlet states by bond fragmentation followed by electron transfer. Irreversible photooxidation occurs in the triplet states via bimolecular electron transfer to the donor followed by bond fragmentation. Using these two sensitization schemes, donors can be irreversibly oxidized with oxidation potentials ranging from ca 1.6-2.2 V vs SCE. The corresponding N-ethylheterocycles are characterized as conventional reversible photooxidants in their triplet states. The utility of these sensitizers is demonstrated by irreversibly generating the guanosine radical cation in buffered aqueous solution.