Doorway States Research Articles

Strong-Coupling Modification of Singlet-Fission Dynamical Pathways.

We investigate theoretically the influence of strong light-matter coupling on the initial steps of the phototriggered singlet-fission process. In particular we focus on intramolecular singlet fission in a TIPS-pentacene dimer derivative described by a vibronic Hamiltonian including the optically active singlet excited states, doubly excited and charge transfer states, as well as the final triplet-triplet pair state. Quantum dynamics simulations of up to four dimers in the cavity indicate that the modified resonance condition imposed by the cavity strongly quenches the passage through the intermediate charge transfer and double-excitation states, thus largely reducing the triplet-triplet yield in the bare system. Subsequently, we modify the system parameters and construct a model Hamiltonian where the optically active singlet excitation lies below the final triplet-triplet state such that the yield of the bare system becomes insignificant. In this case we find that using the upper polariton as the doorway state for photoexcitation can lead to a much enhanced yield. This pathway is operative provided that the system is sufficiently rigid to prevent vibronic losses from the upper polariton to the dark-states manifold.

Electron Attachment to the Nucleobase Uracil in Diethylene Glycol: The Signature of a Doorway.

The cellular environment plays a significant role in low energy electron-mediated radiation damage to genetic materials. In this study, we have modeled the effect of the bulk medium on electron attachment to nucleobases in diethylene glycol (DEG) using uracil as a test case, in accordance with recent experimental work on the observation of dissociative quasi-free electron attachment to nucleoside via excited anion radical in solution (in DEG). Our EOM-CCSD-based quantum mechanical/molecular mechanical (QM/MM) simulations indicate that the electron scavenging by uracil in DEG is much slower than that observed in the aqueous medium due to its viscosity. This work also establishes that a doorway mechanism exists in uracil microsolvated and bulk solvated with DEG, with the dipole-bound state and solvent-bound state acting as doorway states, respectively.

Compound-Nucleus and Doorway-State Decays of β-Delayed Neutron Emitters ^{51,52,53}K.

We investigated decays of ^{51,52,53}K at the ISOLDE Decay Station at CERN in order to understand the mechanism of the β-delayed neutron-emission (βn) process. The experiment quantified neutron and γ-ray emission paths for each precursor. We used this information to test the hypothesis, first formulated by Bohr in 1939, that neutrons in the βn process originate from the structureless "compound nucleus." The data are consistent with this postulate for most of the observed decay paths. The agreement, however, is surprising because the compound-nucleus stage should not be achieved in the studied β decay due to insufficient excitation energy and level densities in the neutron emitter. In the ^{53}K βn decay, we found a preferential population of the first excited state in ^{52}Ca that contradicted Bohr's hypothesis. The latter was interpreted as evidence for direct neutron emission sensitive to the structure of the neutron-unbound state. We propose that the observed nonstatistical neutron emission proceeds through the coupling with nearby doorway states that have large neutron-emission probabilities. The appearance of "compound-nucleus" decay is caused by the aggregated small contributions of multiple doorway states at higher excitation energy.

Dual Functionality of 6-Methylthioguanine: Synergistic Effects Enhancing the Photolability of DNA Nucleobases.

A small chemical modification of the nucleobase structure can significantly enhance the photoactivity of DNA, which may incur DNA damage, thus holding promising applications in photochemotherapy treatment of cancers or pathogens. However, single substitution confers only limited phototoxicity to DNA. Herein, we combine femtosecond and nanosecond time-resolved spectroscopy with high-level ab initio calculations to disentangle the excited-state dynamics of 6-methylthioguanine (me6-TG) under variable wavelength UVA excitation (310-330 nm). We find that double substitution of nucleobases (thionation and methylation) boosts the photoactivity by introducing more reactive channels. Intriguingly, 1nNπ*, rather than 1nSπ*, acts as the doorway state engendering the formation of the long-lived reactive triplet state in me6-TG. The 1nNπ* induces a low spin-orbit coupling of 8.3 cm-1, which increases the intersystem crossing (ISC) time (2.91 ± 0.14 ns). Despite the slowed ISC, the triplet quantum yield (ΦT) still accounts for a large fraction (0.6 ± 0.1), consistent with the potential energy surface that favors excited-state bifurcation to 1nNπ*min (3.36 ± 0.15 ps) rather than 1ππ*min (5.05 ± 0.26 ps), such that the subsequent ISC to triplet via 1nNπ*min constitutes the main relaxation pathway in me6-TG. Although this ΦT is inferior to its single-substituted predecessor 6-thioguanine (6-TG, 0.8 ± 0.2), the effect of thionation in synergy with methylation opens a unique C-S bond cleavage pathway through crossing to a repulsive 1πσ* state, generating thiyl radicals as highly reactive intermediates that may invoke biological damage. This photodissociation channel is extremely difficult for conventional nucleobases. These findings demonstrate the synergistic effects of double functionality substitution in modulating excited-state dynamics and enhancing the photolabile character of DNA nucleobases, providing inspirations for the rational design of advanced photodynamic and photochemotherapy approaches.

Secondary Electron Attachment-Induced Radiation Damage to Genetic Materials.

Reactions of radiation-produced secondary electrons (SEs) with biomacromolecules (e.g., DNA) are considered one of the primary causes of radiation-induced cell death. In this Review, we summarize the latest developments in the modeling of SE attachment-induced radiation damage. The initial attachment of electrons to genetic materials has traditionally been attributed to the temporary bound or resonance states. Recent studies have, however, indicated an alternative possibility with two steps. First, the dipole-bound states act as a doorway for electron capture. Subsequently, the electron gets transferred to the valence-bound state, in which the electron is localized on the nucleobase. The transfer from the dipole-bound to valence-bound state happens through a mixing of electronic and nuclear degrees of freedom. In the presence of aqueous media, the water-bound states act as the doorway state, which is similar to that of the presolvated electron. Electron transfer from the initial doorway state to the nucleobase-bound state in the presence of bulk aqueous media happens on an ultrafast time scale, and it can account for the decrease in DNA strand breaks in aqueous environments. Analyses of the theoretically obtained results along with experimental data have also been discussed.

Electron Attachment to DNA: The Protective Role of Amino Acids.

We have studied the effect of amino acids on the electron attachment properties of a DNA nucleobase, with cytosine as a model system. The equation of motion coupled cluster theory with an extended basis set has been used to simulate the electron-attached state of the DNA model system. Arginine, alanine, lysine, and glycine are the four amino acids considered to investigate their role in electron attachment to a DNA nucleobase. The electron attachment to cytosine in all the four cytosine-amino acid gas-phase dimer complexes follows a doorway mechanism, where the electron gets transferred from the initial dipole-bound doorway state to the final nucleobase-bound state through the mixing of electronic and nuclear degrees of freedom. When cytosine is bulk-solvated with glycine, the glycine-bound state acts as the doorway state, where the initial electron density is localized on the bulk amino acid and away from the nucleobase, thus leading to the physical shielding of the nucleobase from the incoming electron. At the same time, the presence of amino acids can increase the stability of the nucleobase-bound anionic state, which can suppress the sugar-phosphate bond rupture caused by dissociative electron attachment to DNA.

Electron Attachment to Wobble Base Pairs

We have analyzed the low-energy electron attachment to wobble base pairs using the equation of motion coupled cluster method and extended basis sets. A doorway mechanism exists for the attachment of the additional electron to the base pairs, where the initially formed dipole-bound anion captures the incoming electron. The doorway dipole-bound anionic state subsequently leads to the formation of a valence-bound state, and the transfer of extra electron occurs by mixing of electronic and nuclear degrees of freedom. The formation of the valence-bound anion is associated with proton transfer in hypoxanthine-cytosine and hypoxanthine-adenine base pairs, which happens through a concerted electron-proton transfer process. The calculated rate constant for the dipole-bound to valence-bound transition in wobble base pairs is slower than that observed in the Watson-Crick guanine-cytosine base pair.

Parameterization of Direct and Doorway Processes in R-Matrix Formalism

R-matrix formalism is extended beyond compound nuclear (CN) resonant reactions to include parameterization of direct as well as doorway processes. Direct processes in the R-matrix exterior are parameterized by a unitary matrix that introduces mixing among wave function coefficients of the incoming and outgoing wave function components at the R-matrix channel surface. Doorway processes are parameterized by separating the Hilbert space of the interior R-matrix region into its doorway and CN subspaces, from which doorway state eigenenergies, reduced width amplitudes, and the strengths of their coupling to CN levels appear as new R-matrix parameters. Parameterization of generalized as well as the conventional Reich–Moore approximation for eliminated capture channels in the presence of direct, doorway, and CN processes is presented along with a complex-valued scattering length with contributions from direct, doorway, and CN capture processes. Derivation of Brune’s alternative R-matrix parameters is extended to include doorway states. This work suggests how R-matrix formalism could be extended further by adopting the concepts from related reaction formalisms.

Distinct doorway states lead to triplet excited state generation in epigenetic RNA nucleosides.

N 6-Hydroxymethyladenosine (hm6A) and N6-formyladenosine (f6A) are two important intermediates during the demethylation process of N6-methyladenosine (m6A), which has been proven to show epigenetic function in mRNA. However, there is no knowledge about how the chemical integrity and stability could be altered when these two nucleosides are exposed to ultraviolet (UV) radiation. Herein, we report the first study on excited state dynamics of hm6A and f6A in solutions by using femtosecond time-resolved spectroscopy and quantum chemistry calculations. Surprisingly, triplet excited species are clearly identified in both hm6A and f6A after UV excitation, which is in sharp contrast to the 10-3 level triplet yield of adenosine scaffolds. Moreover, the doorway states leading to triplet states are found to be an intramolecular charge transfer state and a lower-lying dark nπ* state in hm6A and f6A, respectively. These discoveries pave the way to further study their effects on RNA strands and provide insight for understanding RNA photochemistry.

Low-Energy Shape Resonances of a Nucleobase in Water.

When high-energy radiation passes through aqueous material, low-energy electrons are produced which cause DNA damage. Electronic states of anionic nucleobases have been suggested as an entrance channel to capture the electron. However, identifying these electronic resonances have been restricted to gas-phase electron-nucleobase studies and offer limited insight into the resonances available within the aqueous environment of DNA. Here, resonance and detachment energies of the micro-hydrated uracil pyrimidine nucleobase anion are determined by two-dimensional photoelectron spectroscopy and are shown to extrapolate linearly with cluster size. This extrapolation allows the corresponding resonance and detachment energies to be determined for uracil in aqueous solution as well as the reorganization energy associated with electron capture. Two shape resonances are clearly identified that can capture low-energy electrons and subsequently form the radical anion by solvent stabilization and internal conversion to the ground electronic state. The resonances and their dynamics probed here are the nucleobase-centered doorway states for low-energy electron capture and damage in DNA.

Spectroscopic characterization of singlet-triplet doorway states of aluminum monofluoride.

Aluminum monofluoride (AlF) possesses highly favorable properties for laser cooling, both via the A1Π and a3Π states. Determining efficient pathways between the singlet and the triplet manifold of electronic states will be advantageous for future experiments at ultralow temperatures. The lowest rotational levels of the A1Π, v = 6 and b3Σ+, v = 5 states of AlF are nearly iso-energetic and interact via spin-orbit coupling. These levels thus have a strongly mixed spin-character and provide a singlet-triplet doorway. We here present a hyperfine resolved spectroscopic study of the A1Π, v = 6//b3Σ+, v = 5 perturbed system in a jet-cooled, pulsed molecular beam. From a fit to the observed energies of the hyperfine levels, the fine and hyperfine structure parameters of the coupled states and their relative energies as well as the spin-orbit interaction parameter are determined. The standard deviation of the fit is about 15MHz. We experimentally determine the radiative lifetimes of selected hyperfine levels by time-delayed ionization, Lamb dip spectroscopy, and accurate measurements of the transition lineshapes. The measured lifetimes range between 2 and 200ns, determined by the degree of singlet-triplet mixing for each level.

Parity and time-reversal invariance violation in neutron-nucleus scattering

Parity violating (PV) as well as parity and time-reversal invariance violating (PTRIV) effects are enhanced a million times in neutron reactions near $p$-wave compound resonances. We present the calculation of such effects using a statistical theory based on the properties of chaotic eigenstates and discuss a possibility to extract the strength constants of PTRIV interactions from the experimental data, including nucleon-nucleon and pion-nucleon CP-violating interactions, the QCD $\ensuremath{\theta}$-term, and the quark chromo-EDM. PV effects have random signs for all target nuclei except for $^{232}\mathrm{Th}$, where PV effects of a positive sign have been observed for ten statistically significant $p$-wave resonances, with energy smaller than 250 eV. This may be an indication of a possible regular (nonchaotic) contribution to PV effects. We link this regular effect to the doublets of opposite parity states in the rotation spectra of nuclei with an octupole deformation and suggest other target nuclei where this hypothesis may be tested. We also discuss a permanent sign contribution produced by doorway states. An estimate of the ratio of PTRIV effects to PV effects is presented. Although a polarized target is not needed for the measurement of PV effects, for the interpretation of the results, it may be convenient to do both PV and PTRIV experiments with a polarized target.

DOORWAY-модели в обратной задаче для сложного вибронного аналога резонанса Ферми

We solve the inverse problem for the complex Fermi resonance or its vibronic analogue, and to this end we use the matrix XEXt, where E = diag({Ek}) is a diagonal matrix, Ek are the energies of the observed “conglomerate” of lines, and the intensities of these lines Ik determine the first row of the matrix X, (X1k)2 = Ik, k = 1, 2, … , n, n  3. Hamiltonian matrix of the direct model, HDIR, whose parameters are the energies of pre-diagonalized “dark” states, Ai, and the matrix elements of their coupling to the “bright” state, Bi, (i = 1, 2, … , n – 1), is obtained after the diagonalization of the XEXt block, which belongs to the “dark” states. We show that Hamiltonian matrix with the single doorway state (DW), HDW1, can be obtained from the matrices HDIR or XEXt by first step of the Householder triangularization, i.e. by similarity transformation with a reflection matrix constructed by quantities Bi or Di = (XEXt)1,i+1. For the energy of the first DW1 state, g1, and the matrix element of its coupling to the “bright” state, w1, the use of the Householder transformation gives: g1=Sigman-1i=1B2iAi/(Sigman-1j=1B2j)= Sigmank=1E3kIk/Sigmanl=1E2lIl, |w1|=(Sigman-1i=1B2i)1/2= Sigmank=1E2kIk)1/2. In similar way, using the Householder transformation, the Hamiltonians for the models with several doorway states, HDW2, HDW3,…, HDW(n-1), are successively obtained. The Hamiltonian of the DW(n-2) model is represented by a symmetric tridiagonal matrix HDW(n-1), its diagonal elements gi determine the energies of the DW1, DW2,…, DW(n-1) states, and the off-diagonal elements wi determine the corresponding coupling between them.

DOORWAY models in the inverse problem for a complex vibronic analogue of the fermi resonance

We solve the inverse problem for the complex Fermi resonance or its vibronic analogue, and to this end we use the matrix XEXt, where E=diag(\Ek\) is a diagonal matrix, Ek are the energies of the observed "conglomerate" of lines, and the intensities of these lines Ik determine the first row of the matrix X, (X1k)2=Ik k=1,2,...,n,n≥3. Hamiltonian matrix of the direct model, HDIR, whose parameters are the energies of pre-diagonalized "dark" states, Ai, and the matrix elements of their coupling to the "bright" state, Bi, (i=1,2,...,n-1), is obtained after the diagonalization of the XEXt block, which belongs to the "dark" states. We show that Hamiltonian matrix with the single doorway state (DW), HDW1, can be obtained from the matrices HDIR or XEXt by first step of the Householder triangularization, i.e. by similarity transformation with a reflection matrix constructed by quantities Bi or Di=(XEXt)1,i+1. For the energy of the first DW1 state, g1, and the matrix element of its coupling to the "bright" state, w1, the use of the Householder transformation gives: g1=Sigman-1i=1B2iAi/(Sigman-1j=1B2j)= Sigmank=1E3kIk/Sigmanl=1E2lIl, |w1|=(Sigman-1i=1B2i)1/2= Sigmank=1E2kIk)1/2. In similar way, using the Householder transformation, the Hamiltonians for the models with several doorway states, HDW2,HDW3,...,HDW(n-1), are successively obtained. The Hamiltonian of the DW(n-2) model is represented by a symmetric tridiagonal matrix HDW(n-1), its diagonal elements gi determine the energies of the DW1-,DW2-,...,DW(n-1) states, and the off-diagonal elements wi determine the corresponding coupling between them. Keywords: vibronic coupling, complex vibronic analogue of the Fermi resonance, inverse problem, Householder transformation, doorway models.

Analysis of Doorway States in a Graphene Structure

It is often important that systems absorb energy as efficiently as possible. One way to accomplish this is by modifying systems to have doorway states (DWS). These types of systems have been developed in both classical and quantum physics. In article number 2100065, Enrique Arturo Carrillo and co-workers show what kind of modifications must be made to graphene to admit the presence of such DWS.

Spectroscopic Manifestations of Indirect Vibrational State Mixing: Novel Anharmonic Effects on a Prereactive H Atom Transfer Surface.

The NH stretch region of the IR spectrum of methyl anthranilate is modeled in the S1 state to understand the connection between the absence of this fundamental in the fluorescence-dip infrared spectra of Blodgett et al. [Phys. Chem. Chem. Phys. 2020, 22, 14077] and its relevance to the H atom dislocation that occurs upon electronic excitation. A set of coordinates are chosen that highlight the role of certain low-frequency modes. A Hamiltonian is developed in which a large-amplitude two-dimensional surface describing the H-bonded H atom is linearly and quadratically coupled to the remaining degrees of freedom which are treated at the harmonic level. The surface is calculated within the time-dependent density functional theory framework by using the B3LYP/6-311++(d, p) level of theory with dispersion. Our spectral results show that indirect couplings lead to massive intensity sharing over hundreds of wavenumbers. This sharing is predicted to be dramatically reduced upon deuteration. The spectral broadening mechanism is found to involve off-resonant doorway states that are themselves strongly coupled to states nearly degenerate with the NH stretch fundamental and represents a complementary mechanism to previous explanations based on Fermi resonance or the presence of Franck-Condon like combination bands with low-frequency motions. Consistent with the spectra predictions, time-dependent calculations show that if the NH stretch fundamental were excited with an ultrafast laser, it would decay within 40 fs. The competition between H atom dislocation and vibrational relaxation is discussed.

Analysis of Doorway States in a Graphene Structure

Doorway states (DWSs) are related to the strength function phenomenon and giant resonances; and arise when two systems interact: one with a high‐density eigenvalue spectrum and the other with a comparatively lower density. They are important because they constitute a very efficient mechanism for energy absorption when a system has such a state. These concepts, studied first in nuclear physics in the 1940s, are analyzed here from a theoretical point of view in some particular graphene structures, obtained after applying appropriate voltages to a graphene sheet. The influence of the DWSs on the electronic transport in these systems is also studied. To analyze these effects, a graphene sheet is considered to which three electrodes are applied to produce two potential barriers separated by a well. It is shown that, for the thin barrier, the transmission coefficient as a function of the energy has well‐localized maxima, each maximum occurring at the DWS energies. This transmission clearly shows the strength function phenomenon. This function is an envelope for the resonances of the composite system. The results suggest the possibility of building graphene electronic devices, such as filters matching the DWSs.

Enhanced ionization of counter-rotating electrons via doorway states in ultrashort circularly polarized laser pulses

We have performed single-active-electron numerical calculations of neon- and argonlike atoms interacting with short, intense, circularly polarized laser pulses at wavelengths between 10 nm and 1000 nm. Our results reveal a surprisingly large change by a factor of about 100 in the ionization ratio of electrons in initial states counter-rotating with respect to the field over corotating electrons in the previously unexplored intermediate few-photon ionization regime. The physical mechanism behind this observation is related to resonant enhanced ionization via states close in energy to the initial states. These doorway states are accessible exclusively from the initial state for counter-rotating electrons within the respective wavelength regime. The results may open a new route for controlling the generation of spin-polarized electrons by ultrashort laser pulses.

The Role of Triplet States in the Photodissociation of a Platinum Azide Complex by a Density Matrix Renormalization Group Method.

Platinum azide complexes are appealing anticancer photochemotherapy drug candidates because they release cytotoxic azide radicals upon light irradiation. Here we present a density matrix renormalization group self-consistent field (DMRG-SCF) study of the azide photodissociation mechanism of trans,trans,trans-[Pt(N3)2(OH)2(NH3)2], including spin–orbit coupling. We find a complex interplay of singlet and triplet electronic excited states that falls into three different dissociation channels at well-separated energies. These channels can be accessed either via direct excitation into barrierless dissociative states or via intermediate doorway states from which the system undergoes non-radiative internal conversion and intersystem crossing. The high density of states, particularly of spin-mixed states, is key to aid non-radiative population transfer and enhance photodissociation along the lowest electronic excited states.

The identification of {{varvec{alpha }}}-clustered doorway states in ^{44,48,52}Ti using machine learning

A novel experimental analysis method has been developed, making use of the continuous wavelet transform and machine learning to rapidly identify alpha -clustering in nuclei in regions of high nuclear state density. This technique was applied to resonant scattering measurements of the ^text {4}He(^text {40,44,48}Ca,alpha ) resonant reactions, allowing the alpha -cluster structure of ^text {44,48,52}Ti to be investigated. Fragmented alpha -clustering was identified in ^text {44}Ti and ^text {52}Ti, while the results for ^text {48}Ti were less conclusive, but suggest no such clustering.