Advanced Ab Initio Calculations Research Articles

Electric dipole polarizability of low-lying excited states in atomic nuclei

New equations for the electric dipole polarizability α E1 of low-lying excited states in atomic nuclei—and the related (−2) moment of the total photo-absorption cross section, σ −2—are inferred in terms of electric dipole and quadrupole matrix elements. These equations are valid for arbitrary angular momenta of the initial/ground and final/excited states and have been exploited in fully converged 1ℏ ω shell-model (SM) calculations of selected p- and sd-shell nuclei that consider configuration mixing; advancing previous knowledge from 17O to 36Ar, where thousands of electric dipole matrix elements are computed from isovector excitations which include the giant dipole resonance (GDR) region. Our results are in reasonable agreement with previous SM calculations and follow—except for 6,7Li and 17,18O—Migdal’s global trend provided by the combination of the hydrodynamic model and second-order non-degenerate perturbation theory. Discrepancies in 6,7Li and 17O arise as a result of the presence of α-cluster configurations in odd-mass nuclei, whereas the disagreement in 18O comes from the mixing of intruder states, which is lacking in the SM interactions. More advanced ab initio calculations of the dipole polarizability for low-lying excited states covering all the isovector states within the GDR region are missing and could be very valuable to benchmark the results presented here and shed further light on how atomic nuclei polarize away from the ground state

Solvent Controlled Generation of Spin Active Polarons in Two-Dimensional Material under UV Light Irradiation.

Polarons belong to a class of extensively studied quasiparticles that have found applications spanning diverse fields, including charge transport, colossal magnetoresistance, thermoelectricity, (multi)ferroism, optoelectronics, and photovoltaics. It is notable, though, that their interaction with the local environment has been overlooked so far. We report an unexpected phenomenon of the solvent-induced generation of polaronic spin active states in a two-dimensional (2D) material fluorographene under UV light. Furthermore, we present compelling evidence of the solvent-specific nature of this phenomenon. The generation of spin-active states is robust in acetone, moderate in benzene, and absent in cyclohexane. Continuous wave X-band electron paramagnetic resonance (EPR) spectroscopy experiments revealed a massive increase in the EPR signal for fluorographene dispersed in acetone under UV-light irradiation, while the system did not show any significant signal under dark conditions and without the solvent. The patterns appeared due to the generation of transient magnetic photoexcited states of polaronic character, which encompassed the net 1/2 spin moment detectable by EPR. Advanced ab initio calculations disclosed that polarons are plausibly formed at radical sites in fluorographene which interact strongly with acetone molecules in their vicinity. Additionally, we present a comprehensive scenario for multiplication of polaronic spin active species, highlighting the pivotal role of the photoinduced charge transfer from the solvent to the electrophilic radical centers in fluorographene. We believe that the solvent-tunable polaron formation with the use of UV light and an easily accessible 2D nanomaterial opens up a wide range of future applications, ranging from molecular sensing to magneto-optical devices.

Spectroscopic study of the F[formula omitted] outer well state in H2, HD and D2

Two-photon UV-photolysis of hydrogen sulfide molecules is applied to produce hydrogen molecules in highly excited vibrational levels in the X1Σg+ electronic ground state, up to the dissociation energy and into the quasibound region. Photolysis precursors H2S, HDS and D2S are used to produce vibrationally hot H2, HD and D2. The wave function density at large internuclear separation is excited via two-photon transitions in the F1Σg+ - X1Σg+ system to probe ro-vibrational levels in the first F1Σg+ outer well state of gerade symmetry. Combining with accurate knowledge of the X1Σg+ (v,J) levels from advanced ab initio calculations, energies of rovibrational levels in the F1Σg+ state are determined. For the H2 isotopologue a three-laser scheme is employed yielding level energies at accuracies of 4×10−3 cm−1 for F(v=0,J) up to J=21 and for some low J values of F(v=1). A two-laser scheme was applied to determine level energies in H2 for F(v=0−4) levels as well as for various F levels in HD and D2, also up to large rotational quantum numbers. The latter measurements in the two-laser scheme are performed at lower resolution and the accuracy is strongly limited to 0.5 cm−1 by ac-Stark effects. For H2 a new quasibound resonance X1Σg+ (v=6, J=23) is detected through the Q(23) and O(23) transitions in the F0-X6 band. Also a quasi-bound resonance in D2 is assigned, for the first time in this species: X1Σg+ (v=17 , J=15). The experimental results on F(v,J) level energies are compared with previously reported theoretical results from multi-channel quantum-defect calculations as well as with results from newly performed non-adiabatic quantum calculations.

A simple quantum model for solving two-electron diatomics approximately

Analytical expressions are obtained for the total energy of two-electron heteronuclear diatomics using a minimal basis set consisting of a 1S-Gaussian orbital centered on each atom and a Heitler-London trial wavefunction. The equations are applied to the recently discovered helium hydride cation (2019, Nature 568, 357–9) to understand its stability. Special attention was given to the fulfillment of the virial theorem and the different limiting cases (homonuclear diatomics, short and long internuclear distances). The correctness of equations and numerical results was confirmed via a brute-force Metropolis Monte Carlo integration. Expressions for the moments of the electron density (including the dipole moments) were obtained. The effect of the spin state on the dipole moment is discussed and the model agrees qualitatively with results from more advanced ab initio calculations. I also applied the model to study the metastable helium dimer dication () and the predicted bond length (0.7359 Å) is in good agreement with the experimental value (0.75 ± 0.02 Å). The advantages and limitations of this simple quantum model are discussed.

Investigating the electronic structure of high explosives with X-ray Raman spectroscopy

We investigate the sensitivity and potential of a synergistic experiment-theory X-ray Raman spectroscopy (XRS) methodology on revealing and following the static and dynamic electronic structure of high explosive molecular materials. We show that advanced ab-initio theoretical calculations accounting for the core-hole effect based on the Bethe-Salpeter Equation (BSE) approximation are critical for accurately predicting the shape and the energy position of the spectral features of C and N core-level spectra. Moreover, the incident X-ray dose typical XRS experiments require can induce, in certain unstable structures, a prominent radiation damage at room temperature. Upon developing a compatible cryostat module for enabling cryogenic temperatures (approx 10 K) we suppress the radiation damage and enable the acquisition of reliable experimental spectra in excellent agreement with the theory. Overall, we demonstrate the high sensitivity of the recently available state-of-the-art X-ray Raman spectroscopy capabilities in characterizing the electronic structure of high explosives. At the same time, the high accuracy of the theoretical approach may enable reliable identification of intermediate structures upon rapid chemical decomposition during detonation. Considering the increasing availability of X-ray free-electron lasers, such a combined experiment-theory approach paves the way for time-resolved dynamic studies of high explosives under detonation conditions.

Saturation spectroscopy of R(0), R(2) and P(2) lines in the (2-0) band of HD

Lamb-dips of R(0), R(2) and P(2) rovibrational transitions in the (2-0) overtone band of the HD molecule are measured at cryogenic temperatures using the technique of NICE-OHMS spectroscopy. Resulting transition frequencies are 214,905,335,240,(100) kHz for R(0), 219,042,856,794,(150) kHz for R(2) and 206 ,898 ,802,150 ,(150) kHz for P(2). Small, but systematic deviations (at 1.7sigma level) are found from advanced ab initio calculations, supporting results from previous studies probing other lines in the (2-0) band. From a combination difference between the R(0) and P(2) lines, an accurate value for rotational-level spacing in the v=0 ground state is determined, of Delta _{(J=2)-(J=0)}= 8,006,533,168,(26) kHz, well in agreement with theory (at 0.6sigma ). Based on the observation of the dispersive line shape of R(0), the status of proposed models for the elusive line shapes of saturated transitions in HD is discussed.

Stacking-dependent deep level emission in boron nitride

We report hyperspectral microscopy in ${sp}^{2}$-bonded boron nitride polytypes with a simultaneous inspection of the photoluminescence signal in the UV-C and UV-B spectral ranges. By comparing two different polytypes extracted from the same polycrystalline sample, we reveal that the well-known ``4 eV defect'' does not emit at the same energy for polytypes in the $AB$ and $A{A}^{\ensuremath{'}}$ stackings. The zero-phonon line is either at 4.14 or 4.16 eV for the noncentrosymmetric $AB$ stacking, instead of the usual 4.09 eV energy in the $A{A}^{\ensuremath{'}}$ stacking. Our results open the way for different characterization methods of the stacking order in ${sp}^{2}$-bonded boron nitride, and bring alternative inputs for the elucidation of the atomistic configuration of points defects by advanced ab initio calculations.

Two-dimensional extrapolation procedure for an ab initio study of nuclear size parameters and the properties of the halo nucleus He6

A new two-dimensional procedure for extrapolation of the values of matter, neutron, and proton radii obtained in no-core shell model (NCSM) calculations to infinite size of its basis is proposed. A relationship between the radii is used as an additional test. Together with the JISP16 potential, which is frequently used in NCSM calculations of the radii, the Daejeon16 potential is applied for these purposes for the first time. Halo nucleus 6He is the object of studies. The small spread of radii values, successful testing using relationship between the values of these three radii, and reasonable agreement between the obtained results and experimental data as well as the results of other advanced ab initio calculations demonstrate the high efficiency of the developed approach and, therefore, good prospects for its application. The performed investigations and analysis of the results of other studies indicate that the halo of 6He has a large size 0.7 - 0.8 fm.

Rotational level spacings in HD from vibrational saturation spectroscopy

The $R(1)$, $R(3)$, and $P(3)$ rovibrational transitions in the (2-0) overtone band of the HD molecule are measured in Doppler-free saturation using the technique of NICE-OHMS spectroscopy. For the $P(3)$ line, hitherto not observed in saturation, we report a frequency of $203\phantom{\rule{0.16em}{0ex}}821\phantom{\rule{0.16em}{0ex}}936\phantom{\rule{0.16em}{0ex}}805\phantom{\rule{0.16em}{0ex}}(60)$ kHz. The dispersive line shapes observed in the three spectra show strong correlations, allowing for extraction of accurate information on rotational level spacings. This leads to level spacings of ${\mathrm{\ensuremath{\Delta}}}_{(J=3)\ensuremath{-}(J=1)}=13\phantom{\rule{0.16em}{0ex}}283\phantom{\rule{0.16em}{0ex}}245\phantom{\rule{0.16em}{0ex}}098\phantom{\rule{0.16em}{0ex}}(30)$ kHz in the $v=0$ ground state and ${\mathrm{\ensuremath{\Delta}}}_{(J=4)\ensuremath{-}(J=2)}=16\phantom{\rule{0.16em}{0ex}}882\phantom{\rule{0.16em}{0ex}}368\phantom{\rule{0.16em}{0ex}}179\phantom{\rule{0.16em}{0ex}}(20)$ kHz in the $v=2$ excited vibration in HD. These results show that experimental values for the rotational spacings are consistently larger than those obtained with advanced ab initio theoretical calculations at $1.5\ensuremath{\sigma}$, where the uncertainty is determined by theory. The same holds for the vibrational transitions where systematic deviations of 1.7--1.9$\ensuremath{\sigma}$ are consistently found for the five lines accurately measured in the (2-0) band.

A unified computational view of DNA duplex, triplex, quadruplex and their donor-acceptor interactions.

DNA can assume various structures as a result of interactions at atomic and molecular levels (e.g., hydrogen bonds, π–π stacking interactions, and electrostatic potentials), so understanding of the consequences of these interactions could guide development of ways to produce elaborate programmable DNA for applications in bio- and nanotechnology. We conducted advanced ab initio calculations to investigate nucleobase model structures by componentizing their donor-acceptor interactions. By unifying computational conditions, we compared the independent interactions of DNA duplexes, triplexes, and quadruplexes, which led us to evaluate a stability trend among Watson–Crick and Hoogsteen base pairing, stacking, and even ion binding. For a realistic solution-like environment, the influence of water molecules was carefully considered, and the potassium-ion preference of G-quadruplex was first analyzed at an ab initio level by considering both base-base and ion-water interactions. We devised new structure factors including hydrogen bond length, glycosidic vector angle, and twist angle, which were highly effective for comparison between computationally-predicted and experimentally-determined structures; we clarified the function of phosphate backbone during nucleobase ordering. The simulated tendency of net interaction energies agreed well with that of real world, and this agreement validates the potential of ab initio study to guide programming of complicated DNA constructs.

Ultrafast charge redistribution in small iodine containing molecules

We present studies on intra-molecular charge redistribution in iodine containing molecules upon iodine-4d photoionization. For this, we employed an XUV-pump-XUV-probe scheme based on time-delayed femtosecond pulses delivered by the free-electron laser at DESY in Hamburg (FLASH). The experimental results show delay dependent and molecule-specific iodine charge state distributions that arise upon multiple iodine-4d photoionization. Using the example of CH3I and CH2I2, we compare the delay-dependent yields of I3+. We model the involved processes using advanced ab initio electronic structure calculations which include electron correlations combined with a classical model of the nuclear motion. The qualitative agreement of our model with the experimental results allows us to relate the observed, strongly molecule-specific efficiencies of the intra-molecular charge rearrangement not only to molecule-specific fragmentation timescales but also to molecule-specific electronic structure and molecular environment.

Growth and electronic structure of 2D hexagonal nanosheets on a corrugated rectangular substrate

Graphene and h-BN are grown by chemical vapor deposition in ultra high vacuum conditions on the Pt(110) surface. Scanning tunneling microscopy measurements and low-energy electron diffraction data indicate that graphene forms a variety of differently oriented incommensurate domains although with a strong preference to align its direction with the direction of Pt. Meanwhile, h-BN exhibits a c(8 × 10) commensurate superstructure, which presents a high level of defectivity that implies local variation of the periodicity (i.e. mixed c(8 × 10) and c(8 × 12) patches) and the introduction of local defects. The combination of advanced photoemission spectroscopy data (angle-resolved photoemission spectroscopy from the valence band) and ab initio calculations indicates that both 2D materials interact weakly with the substrate: graphene exhibits neutral doping and is morphologically flat, even if it nucleates on the relatively highly corrugated rectangular (110) surface. In the case of h-BN, the interaction is slightly stronger and is characterized by a small electron transfer from surface Pt atoms to nitrogen atoms. The (110) termination of Pt is therefore a quite interesting surface for the growth of 2D materials because given its low symmetry, it may favor the growth of selectively oriented domains but does not affect their pristine electronic properties.

Charge doping and large lattice expansion in oxygen-deficient heteroepitaxial WO3

Tungsten trioxide is a versatile material with widespread applications ranging from electrochromic and optoelectronic devices to water splitting and catalysis of chemical reactions. For technological applications, thin films of WO3 are particularly appealing, taking advantage from high surface-to-volume ratio and tunable physical properties. However, the growth of stoichiometric, crystalline thin films is challenging because the deposition conditions are very sensitive to the formation of oxygen vacancies. In this work, we show how background oxygen pressure during pulsed laser deposition can be used to tune the structural and electronic properties of WO3 thin films. By performing X-ray diffraction and low-temperature transport measurements, we find changes in WO3 lattice volume up to 10%, concomitantly with an insulator-to-metal transition as a function of increased level of electron doping. We use advanced ab initio calculations to describe in detail the properties of the oxygen vacancy defect states, and their evolution in terms of excess charge concentration. Our results depict an intriguing scenario where structural, electronic, optical, and transport properties of WO3 single-crystal thin films can all be purposely tuned by a suited control of oxygen vacancies formation during growth.

Understanding in-line probing experiments by modeling cleavage of nonreactive RNA nucleotides.

Ribonucleic acid (RNA) is involved in many regulatory and catalytic processes in the cell. The function of any RNA molecule is intimately related with its structure. In-line probing experiments provide valuable structural data sets for a variety of RNAs and are used to characterize conformational changes in riboswitches. However, the structural determinants that lead to differential reactivities in unpaired nucleotides have not been investigated yet. In this work, we used a combination of theoretical approaches, i.e., classical molecular dynamics simulations, multiscale quantum mechanical/molecular mechanical calculations, and enhanced sampling techniques in order to compute and interpret the differential reactivity of individual residues in several RNA motifs, including members of the most important GNRA and UNCG tetraloop families. Simulations on the multinanosecond timescale are required to converge the related free-energy landscapes. The results for uGAAAg and cUUCGg tetraloops and double helices are compared with available data from in-line probing experiments and show that the introduced technique is able to distinguish between nucleotides of the uGAAAg tetraloop based on their structural predispositions toward phosphodiester backbone cleavage. For the cUUCGg tetraloop, more advanced ab initio calculations would be required. This study is the first attempt to computationally classify chemical probing experiments and paves the way for an identification of tertiary structures based on the measured reactivity of nonreactive nucleotides.

Stoner versus Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization

The excitation of a ferromagnetic film by a femtosecond laser pulse causes an unexpectedly fast quenching of the film's magnetization on subpicosecond time scales. The microscopic physical mechanisms responsible for this remain a scientific puzzle. The authors employ femtosecond extreme ultraviolet pulses produced by high harmonic generation to follow how the magnetization of a thin cobalt film evolves after the excitation by a 40-fs laser pulse. By measuring the time-, energy-, and angle-resolved magneto-optical response of the Co films across the ${M}_{2,3}$ absorption edge, they obtain a set of time-lapsed magnetic asymmetry spectra, which contain a wealth of information about the different mechanisms at work. When combined with advanced ab initio magneto-optical calculations, they identify two dominant contributions: first, a transient reduction of exchange splitting, and second, magnon excitation. This work thus distinguishes between two fundamental models of magnetism, the Stoner and Heisenberg models, which ascribe magnetization dynamics to an exchange splitting reduction and spin wave excitations, respectively.

Precision measurements and test of molecular theory in highly excited vibrational states of H2 (v=11).

Accurate EF^1varSigma ^+_g{-}X^1varSigma ^+_g transition energies in molecular hydrogen were determined for transitions originating from levels with highly excited vibrational quantum number, v = 11, in the ground electronic state. Doppler-free two-photon spectroscopy was applied on vibrationally excited hbox {H}_2^*, produced via the photodissociation of H2S, yielding transition frequencies with accuracies of 45 MHz or 0.0015 cm−1. An important improvement is the enhanced detection efficiency by resonant excitation to autoionizing 7ppi electronic Rydberg states, resulting in narrow transitions due to reduced ac-Stark effects. Using known EF level energies, the level energies of X(v = 11, J = 1, 3–5) states are derived with accuracies of typically 0.002 cm−1. These experimental values are in excellent agreement with and are more accurate than the results obtained from the most advanced ab initio molecular theory calculations including relativistic and QED contributions.

Hartree–Fock many-body perturbation theory for nuclear ground-states

We investigate the order-by-order convergence behavior of many-body perturbation theory (MBPT) as a simple and efficient tool to approximate the ground-state energy of closed-shell nuclei. To address the convergence properties directly, we explore perturbative corrections up to 30th order and highlight the role of the partitioning for convergence. The use of a simple Hartree-Fock solution to construct the unperturbed basis leads to a convergent MBPT series for soft interactions, in contrast to, e.g., a harmonic oscillator basis. For larger model spaces and heavier nuclei, where a direct high-order MBPT calculation in not feasible, we perform third-order calculation and compare to advanced ab initio coupled-cluster calculations for the same interactions and model spaces. We demonstrate that third-order MBPT provides ground-state energies for nuclei up into tin isotopic chain that are in excellent agreement with the best available coupled-cluster results at a fraction of the computational cost.

Modeling of molecular nitrogen collisions and dissociation processes for direct simulation Monte Carlo.

The Direct Simulation Monte Carlo (DSMC) method typically used for simulating hypersonic Earth re-entry flows requires accurate total collision cross sections and reaction probabilities. However, total cross sections are often determined from extrapolations of relatively low-temperature viscosity data, so their reliability is unknown for the high temperatures observed in hypersonic flows. Existing DSMC reaction models accurately reproduce experimental equilibrium reaction rates, but the applicability of these rates to the strong thermal nonequilibrium observed in hypersonic shocks is unknown. For hypersonic flows, these modeling issues are particularly relevant for nitrogen, the dominant species of air. To rectify this deficiency, the Molecular Dynamics/Quasi-Classical Trajectories (MD/QCT) method is used to accurately compute collision and reaction cross sections for the N2(Σg+1)-N2(Σg+1) collision pair for conditions expected in hypersonic shocks using a new potential energy surface developed using a ReaxFF fit to recent advanced ab initio calculations. The MD/QCT-computed reaction probabilities were found to exhibit better physical behavior and predict less dissociation than the baseline total collision energy reaction model for strong nonequilibrium conditions expected in a shock. The MD/QCT reaction model compared well with computed equilibrium reaction rates and shock-tube data. In addition, the MD/QCT-computed total cross sections were found to agree well with established variable hard sphere total cross sections.

ChemInform Abstract: Solid‐State NMR as a Spectroscopic Tool for Characterizing Phosphane‐Borane Frustrated Lewis Pairs

AbstractReview: 65 refs.

Advanced ab initio relativistic calculations of transition probabilities for some O I and O III emission lines

This work presents new ab initio relativistic calculations using the multiconfiguration Dirac–Hartree–Fock method of some O i and O iii transition lines detected in B-type and Wolf–Rayet stars. Our results are the first able to be presented in both the length and velocity gauges, with excellent gauge convergence. Compared to previous experimental and theoretical uncertainties of up to 50 per cent, our accuracies appear to be in the range of 0.33–5.60 per cent, with gauge convergence up to 0.6 per cent. Similar impressive convergence of the calculated energies is also shown. Two sets of theoretical computations are compared with earlier tabulated measurements. Excellent agreement is obtained with one set of transitions but an interesting and consistent discrepancy exists between the current work and the prior literature, deserving of future experimental studies.