Oscillator Strengths Research Articles

Dispersion Analysis of Perpendicular Modes Using a Hybrid Two-Trace Two-Dimensional (2T2D) Smart Error Sum.

Using linear dichroism theory, one would assume that a z-cut of a uniaxial crystal is equivalent to an x-cut to determine the perpendicular component of the dielectric tensor and the corresponding oscillator parameters. However, Fresnel's equations show that the effect of interfaces in the form of the continuity relations of the different components of the electric field must be considered. A consequence of the continuity relations is that perpendicular modes increase less significantly in strength with increasing angle of incidence than expected. This is a consequence of the fact that it is the inverse of the perpendicular component of the dielectric function that increasingly becomes important with a growing angle of incidence. An inverse dielectric function, however, has typically much smaller values than the dielectric function. An additional consequence is that perpendicular modes are blueshifted and coupled in such a way that oscillator strength is transferred to the higher wavenumber mode. Thus, the spectral signatures of perpendicular modes are often weak and masked by the parallel modes when two modes overlap. Accordingly, to enable dispersion analysis, it is suggested to use a hybrid of the conventional residual sum of squares and the two-trace two-dimensional (2T2D) smart error sum, which can correct systematic multiplicable errors in the experimental spectrum. As demonstrated for fresnoite (Ba2TiSi2O8), this is an important step toward determining the perpendicular component of the dielectric tensor and the corresponding oscillator parameters using dispersion analysis, since asynchronous 2T2D correlation spectra are, in particular, sensitive to perpendicular modes.

Relativistic atomic structure calculations of Li-like ions used for plasma diagnostic studies

Abstract We have carried out atomic structure calculations using systematically enlarged multiconfiguration Dirac- Fock wavefunctions of Li-like ions of the most prominent plasma impurities (Ar, Ti, Fe, Ni, Kr and W) found in presently working tokamaks. Relativistic Breit interaction and quantum electrodynamic (QED) corrections such as vacuum polarization and self-energy corrections are also included in the calculations prior to the evaluation of low lying energy levels, transition probabilities, oscillator strengths and line strengths. Selective radiative data for electric dipole and magnetic quadrupole transitions are also reported. Special emphasis is given in the computations of fundamental quantities such as oscillator strengths as they are widely used in atomic data and analysis structure (ADAS) databases to evaluate quantities such as effective collision strengths. Present com- puted values are compared with existing available results on NIST database and few similar earlier computations and a good agreement has been found. We believe that the detailed atomic data with the relativistic and QED corrections will assist in spectroscopic studies such as accurate line identification and plasma modelling work in tokamak plasma, laser induced breakdown spectroscopy (LIBS), highly charged ions clocks and astrophysical observations.

Oscillator strengths, intracenter absorption and photoionization cross sections of optical transitions of shallow donors in silicon

Oscillator strengths, intracenter absorption and photoionization cross sections of optical transitions of shallow donors in silicon

Stellar Population Astrophysics (SPA) with the TNG. Measurement of the He I 10830 angstrom line in the open cluster Stock 2

The precise measurement of stellar abundances plays a pivotal role in providing constraints on the chemical evolution of the Galaxy. However, before spectral lines can be employed as reliable abundance indicators, particularly for challenging elements such as helium, they must undergo thorough scrutiny. Galactic open clusters, representing well-defined single stellar populations, offer an ideal setting for unfolding the information stored in the helium spectral line feature. In this study we characterise the profile and strength of the helium transition at around angstrom in nine giant stars in the Galactic open cluster Stock 2. To remove the influence of weak blending lines near the helium feature, we calibrated their oscillator strengths (loggf ) by employing corresponding abundances obtained from simultaneously observed optical spectra. Our observations reveal that the in all the targets is observed in absorption, with line strengths categorised into two groups. Three stars exhibit strong absorption, including a discernible secondary component, while the remaining stars exhibit weaker absorption. The lines are in symmetry and align with or near their rest wavelengths, suggesting a stable upper chromosphere without a significant systematic mass motion. We find a correlation between the strength and the Ca ii logRpHK index, with a slope similar to that reported in previous studies on dwarf stars. This correlation underscores the necessity of accounting for the stellar chromosphere structure when employing as a probe for the stellar helium abundance. The procedure of measuring the we developed in this study is applicable not only to other Galactic open clusters but also to field stars, and we plan to use it to map the helium abundance across various types of stars in the future.

DFT and TD‐DFT Study of Many‐Body and Hydrogen Bonding Interactions: Nitrosamine Oligomers

AbstractThis study focuses on the properties and intermolecular interactions of nitrosamine oligomers, which are important in both semiconductor manufacturing and nuclear waste management. The study explores various configurations of nitrosamine oligomers, ranging from monomers to pentamers in linear, ladder, and cyclic clusters, with the finding that dimer cyclic forms and hexamers and above are unstable. Many‐body analysis revealed that two‐body interaction energies made remarkable contributions to the binding energy, while the sum of 3‐, 4‐, and 5‐body energy contributed less, indicating the decrease in stability of nitrosamine from dimer to pentamer clusters. Using advanced computational techniques, such as dispersion‐corrected B3LYP‐D3 and TD‐DFT, the study investigates the vibrational modes and electronic absorption spectra of nitrosamine oligomers. The results reveal that different configurations can significantly affect the vibrational modes and electronic properties of nitrosamine oligomers. The NH2 symmetric and asymmetric modes were particularly intense in various forms of nitrosamine oligomers. The wavelengths of electronic transition, oscillator strength, and HOMO to LUMO gap were reported. Additionally, the study reports on nitrosamine oligomers’ hydrogen‐bonded cooperativity and binding energy contributions. Overall, these findings offer valuable insights into the stability and properties of nitrosamine oligomers, with potential applications in materials science and chemical engineering.

Nearly zero Poisson’s ratio and spin-state-controlled magnetoelastic response in an RKKY-mediated two-dimensional magnetic metal–organic framework

By means of first-principles calculations, the two-dimensional magnetic metal–organic framework, manganese phthalocyanine (MnPc), is proposed to exhibit nearly zero Poisson’s ratio within the elastic regime. Negative Poisson’s ratio can be reached by further increasing the uniaxial tensile strain beyond the yield point. Such an unusual mechanical property is found to rely on the spin-state of Mn atoms. Without strain, the ground state is ferrimagnetic and half-metallic. Interestingly, as the interatomic distance between Mn atoms increases under biaxial tensile strain, the alignment of magnetic moments of neighboring Mn atoms changes from antiparallel to parallel, associated with oscillatory exchange coupling strength. This not only reveals the strain-induced magnetic phase transition, but also indicates the magnetic interaction is mediated by the Ruderman–Kittel–Kasuya–Yosida exchange coupling and thus Kondo effect is likely to exist. Rather than the reported RKKY exchange coupling between supramolecular MnPc adsorbed on substrates, the predicted RKKY exchange coupling here exists in the pristine two-dimensional MnPc lattice. It would be more suitable for device applications because of the reduced size. Besides, it is noted that neighboring Mn atoms with antiparallel magnetic moments are responsible for the conducting behaviors, while those with parallel magnetic moments obstruct the flow of electrons. This property provides possibility of molding the flow of fully spin-polarized electrons through the manipulation of local magnetization by an external magnetic field, which is an attractive magnetoelectric response: the magnetic-field-controlled electron flow.

Relativistic configuration-interaction calculations on K α X-ray satellites of medium-charge Be-like ions

This study puts forward a calculation of the transitions of 1s2s22p 1P1−1s22s2 1S0 and 1s2s22p 3P1−1s22s2 1S0 for Be-like ions (Z = 6–30) using the multiconfiguration Dirac-Hartree–Fock (MCDHF) and relativistic configuration interaction methods. The computational results include transition wavelengths, transition probabilities, line strengths, and weighted oscillator strengths. The calculations also consider the contributions of the Breit interaction and quantum electrodynamics corrections (QED) to these levels. The results show that the contribution of the Breit interaction, self-energy, and vacuum polarization increase rapidly with increasing nuclear charge for certain configurations. The present calculated values are in good agreement with previous theoretical and experimental results, with errors better than 0.3%. A V/A L comparisons demonstrate the reliability of the data.

Extension of nature's NIR-I chromophore into the NIR-II region.

The development of chromophores that absorb in the near-infrared (NIR) region beyond 1000 nm underpins numerous applications in medical and energy sciences, yet also presents substantial challenges to molecular design and chemical synthesis. Here, the core bacteriochlorin chromophore of nature's NIR absorbers, bacteriochlorophylls, has been adapted and tailored by annulation in an effort to achieve absorption in the NIR-II region. The resulting bacteriochlorin, Phen2,1-BC, contains two annulated naphthalene groups spanning meso,β-positions of the bacteriochlorin and the 1,2-positions of the naphthalene. Phen2,1-BC was prepared via a new synthetic route. Phen2,1-BC is an isomer of previously examined Phen-BC, which differs only in attachment via the 1,8-positions of the naphthalene. Despite identical π-systems, the two bacteriochlorins have distinct spectroscopic and photophysical features. Phen-BC has long-wavelength absorption maximum (912 nm), oscillator strength (1.0), and S1 excited-state lifetime (150 ps) much different than Phen2,1-BC (1292 nm, 0.23, and 0.4 ps, respectively). These two molecules and an analogue with intermediate characteristics bearing annulated phenyl rings have unexpected properties relative to those of non-annulated counterparts. Understanding the distinctions requires extending concepts beyond the four-orbital-model description of tetrapyrrole spectroscopic features. In particular, a reduction in symmetry resulting from annulation results in electronic mixing of x- and y-polarized transitions/states, as well as vibronic coupling that together reduce oscillator strength of the long-wavelength absorption manifold and shorten the S1 excited-state lifetime. Collectively, the results suggest a heuristic for the molecular design of tetrapyrrole chromophores for deep penetration into the relatively unutilized NIR-II region.

On a Spectral Method for β-particle Bound Excitation Collisions in Kilonovae

The interaction of β-particles with the weakly ionized plasma background is an important mechanism for powering the kilonova (KN) transient signal from neutron star mergers. For this purpose, we present an implementation of the approximate fast-particle collision kernel, described by Inokuti following the seminal formulation of Bethe, in a spectral solver of the Vlasov–Maxwell–Boltzmann equation. In particular, we expand the fast-particle plane-wave atomic excitation kernel into coefficients of the Hermite basis, and derive the relevant discrete spectral system. In this fast-particle limit, the approach permits the direct use of atomic data, including optical oscillator strengths, normally applied to photon–matter interaction. The resulting spectral matrix is implemented in the MASS-APP spectral solver framework, in a way that avoids full matrix storage per spatial zone. We numerically verify aspects of the matrix construction, and present a proof-of-principle 3D simulation of a 2D axisymmetric KN ejecta snapshot. Our preliminary numerical results indicate that a reasonable choice of Hermite basis parameters for β-particles in the KN is a bulk velocity parameter u = 0, a thermal velocity parameter α = 0.5c, and a 9 × 9 × 9 mode velocity basis set (Hermite orders of 0–8 in each dimension). For interior-ejecta sample zones, we estimate that the ratio of thermalization from large-angle (≳2.°5) bound excitation scattering to total thermalization is ∼0.002–0.003.

Fluctuations in the Photoluminescence Excitation Spectra of Individual Semiconductor Nanocrystals.

Most single quantum emitters display non-steady emission properties. Models that explain this effect have primarily relied on photoluminescence measurements that reveal variations in intensity, wavelength, and excited-state lifetime. While photoluminescence excitation spectroscopy could provide complementary information, existing experimental methods cannot collect spectra before individual emitters change in intensity (blink) or wavelength (spectrally diffuse). Here, we present an experimental approach that circumvents such issues, allowing the collection of excitation spectra from individual emitters. Using rapid modulation of the excitation wavelength, we collect and classify excitation spectra from individual CdSe/CdS/ZnS core/shell/shell quantum dots. The spectra, along with simultaneous time-correlated single-photon counting, reveal two separate emission-reduction mechanisms caused by charging and trapping, respectively. During bright emission periods, we also observe a correlation between emission red-shifts and the increased oscillator strength of higher excited states. Quantum-mechanical modeling indicates that diffusion of charges in the vicinity of an emitter polarizes the exciton and transfers the oscillator strength to higher-energy transitions.

Multireference Correlated Oscillator Strengths from Adiabatic Connection Approaches Based on Extended Random Phase Approximation.

We show that accurate oscillator strengths can be obtained from adiabatic connection (AC) approaches based on the extended random phase approximation (ERPA) combined with multireference (complete active space, CAS) wave functions. The oscillator strengths calculated using the perturbation-corrected ERPA transition density matrices, proposed in this work, and the excitation energies calculated with recently introduced AC correlation energy methods, AC0 and AC0D, compete with accuracy in the perturbational CASPT2 approach and require less computational effort. AC0 and AC0D methods scale more favorably with the number of active orbitals than multiconfigurational perturbation approaches like CASPT2 and NEVPT2 thanks to their dependence on reduced density matrices up to the order of 2. Importantly, the newly developed approach for computing correlated transition dipole moments does not entail any additional costs, as all intermediate quantities become available when AC0 energies are being computed. We also test the performance of the recently proposed AC method corrected for the negative-transition contributions to the correlation energy, AC0D, for triplet excitation energies. Similarly, as for the singlet excitations, the correction improves the performance of the AC0 method, particularly for the low-lying excited states.

Generalized Transition Moment and Oscillator Strength for Optimal Control of Excited States Using Near-Field Light.

The near field is a highly localized light field that is present in the vicinity of materials. It holds considerable potential for manipulating molecular excitation through the design of nanostructures. To that end, a simple yet applicable computational method is required. This letter presents an optimization strategy to achieve targeted electronic excitation in molecules using near-field light. The generalized transition moment and oscillator strength based on the multipolar Hamiltonian can incorporate key parameters affecting the near field, including shape, size, and material composition. This calculation helps optimize electronic transitions through manipulation of the aforementioned parameters. The feasibility of the proposed scheme was demonstrated using a scanning tunneling microscopic setup with light, wherein the near field is controlled by varying the tip position in two dimensions. This approach paves the way for further innovations, which may ultimately enable precise control of molecular behavior by using tailored near-field interactions.

PyExoCross: a Python program for generating spectra and cross sections from molecular line lists

Abstract PyExoCross is a Python adaptation of the ExoCross Fortran application (Yurchenko, A&A, 614, A131 (2018)). PyExoCross is designed for postprocessing the huge molecular line lists generated by the ExoMol project and other similar initiatives such as the HITRAN and HITEMP databases. PyExoCross generates absorption and emission stick spectra, cross sections and other properties (partition functions, specific heats, cooling functions, lifetimes and oscillator strengths) based on molecular line lists. PyExoCross calculates cross sections with four line profiles: Doppler, Gaussian, Lorentzian and Voigt profiles in both sampling and binned methods; a number of options are available for computing Voigt profiles which we test for speed and accuracy. PyExoCross supports importing and exporting line lists in the ExoMol and HITRAN/HITEMP formats. PyExoCross also provides conversion between the ExoMol and HITRAN data format. In addition, PyExoCross has extra code for users to automate the batch download of line list files from the ExoMol database.

Effect of thermal annealing on the surface plasmon properties of gold and silver nanodisk structures fabricated by electron beam lithography

This study investigates the effect of thermal annealing on the surface plasmon resonance properties of gold and silver nanodisk structures fabricated by electron beam lithography. Despite no significant size change due to a nickel adhesion layer, thermal annealing at 500 °C notably altered surface plasmon resonance properties, especially in gold nanodisks, which showed peak blue-shifts and narrowing, indicating improved crystallinity. We fitted the peak shift by using an electromagnetic field analysis and discussed the changes in the dielectric function. The dielectric function of the simple Drude model was adopted, which was fitted to the values of the Lorentz Drude model in a limited wavelength region. While both silver and gold exhibited increased oscillation strength, only gold nanodisk structures showed a decrease in damping frequency. Increased oscillation strength indicated that these structures should be useful for selective enhancement of the light at specific wavelengths by a very simple heat treatment.

Dirac-Coulomb-Breit Molecular Mean-Field Exact-Two-Component Relativistic Equation-of-Motion Coupled-Cluster Theory.

We present a relativistic equation-of-motion coupled-cluster with single and double excitation formalism within the exact two-component framework (X2C-EOM-CCSD), where both scalar relativistic effects and spin-orbit coupling are variationally included at the reference level. Three different molecular mean-field treatments of relativistic corrections, including the one-electron, Dirac-Coulomb, and Dirac-Coulomb-Breit Hamiltonian, are considered in this work. Benchmark calculations include atomic excitations and fine-structure splittings arising from spin-orbit coupling. Comparison with experimental values and relativistic time-dependent density functional theory is also carried out. The computation of the oscillator strength using the relativistic X2C-EOM-CCSD approach allows for studies of spin-orbit-driven processes, such as the spontaneous phosphorescence lifetime.

R-Matrix calculations for opacities: I. Methodology and computations

Abstract An extended version of the R-matrix methodology is presented for calculation of radiative parameters for improved plasma opacities. Contrast and comparisons with existing methods primarily relying on the Distorted Wave (DW) approximation are discussed to verify accuracy and resolve outstanding issues, particularly with reference to the Opacity Project (OP). Among the improvements incorporated are: (i) large-scale Breit-Pauli R-matrix (BPRM) calculations for complex atomic systems including fine structure, (ii) convergent close coupling wave function expansions for the (e + ion) system to compute oscillator strengths and photoionization cross sections, (iii) open and closed shell iron ions of interest in astrophysics and experiments, (iv) a treatment for plasma broadening of autoionizing resonances as function of energy-temperature-density dependent cross sections, (v) a ”top-up” procedure to compare convergence with R-matrix calculations for highly excited levels, and (vi) spectroscopic identification of resonances and bound (e + ion) levels. The present R-matrix monochromatic opacity spectra are fundamentally different from OP and lead to enhanced Rosseland and Planck mean opacities. An outline of the work reported in other papers in this series and those in progress is presented. Based on the present re-examination of the OP work, it is evident that opacities of heavy elements require revisions in high temperature-density plasma sources.

R-Matrix calculations for opacities: II. Photoionization and oscillator strengths of iron ion FeXVII, FeXVIII and FeXIX

Abstract Iron is the dominant heavy element that plays an important role in radiation transport in stellar interiors. Owing to its abundance and large number of bound levels and transitions, iron ions determine the opacity more than any other astrophysically abundant element. A few iron ions constitute the abundance and opacity of iron at the base of the convection zone (BCZ) at the boundary between the solar convection and radiative zones, and are the focus of the present study. Together, Fe xvii , Fe xviii and Fe xix contribute 85% of iron ion fractions 20%, 39% and 26% respectively, at the BCZ physical conditions of temperature T ∼ 2.11 × 106K and electron density Ne = 3.1 × 1022cc. We report heretofore the most extensive R-matrix atomic calculations for these ions for bound-bound and bound-free transitions, the two main processes of radiation absorption. We consider wavefunction expansions with 218 target or core ion fine structure levels of Fe xviii for Fe xvii , 276 levels of Fe xix for Fe xviii , in the Breit-Pauli R-matrix (BPRM) approximation, and 180 LS terms (equivalent to 415 fine structure levels) of Fe xx for Fe xix calculations. These large target expansions which includes core ion excitations to n=2,3,4 complexes enable accuracy and convergence of photoionization cross sections, as well as inclusion of high lying resonances. The resulting R-matrix datasets include 454 bound levels for Fe xvii , 1,174 levels for Fe xviii , and 1,626 for Fe xix up to n ≤ 10 and l=0 - 9. Corresponding datasets of oscillator strengths for photoabsorption are: 20,951 transitions for Fe xvii , 141,869 for Fe xviii , and 289,291 for Fe xix . Photoionization cross sections have obtained for all bound fine structure levels of Fe xvii and Fe xviii , and for 900 bound LS states of Fe xix . Selected results demonstrating prominent characteristic features of photoionization are presented, particularly the strong Seaton PEC (photoexcitation-of-core) resonances formed via high-lying core excitations with ∆n = 1 that significantly impact bound-free opacity.

Self-assembled crown ether (15CR5) with alkaline earth metals to stimulate nonlinear optical properties: The comprehensive study by silico technique

Crown Ether (15CR5) molecule is a worthwhile organic material exhibiting excellent NLO properties and characteristics. In this research, density functional theory (DFT) was extensively employed to study the nonlinear optical (NLO) properties of alkaline earth metal doped 15CR5 by using WB97XD/6-31G+(d, p). Binding energies, interaction energies, vertical ionization energies and reactivity parameters unveiled the thermodynamic stability of these systems. These molecules show a lower value of HUMO-LUMO energy gap indicating their superior electronic properties. Moreover, these complexes possess high oscillator strength and lower excitation energies displaying their better electrical and optical characteristics than pure 15CR5. Non-covalent analysis was carried out for all the designed systems forecasted the van der wall interactions between 15CR5 and dopants. These complexes in the visible region have excess electrons that are highly absorbed in the range of 405.73 to 608.27 compared to 15CR5. The Hyperpolarizability (βtot) of these designed materials was in the range of 9097.21–21627.85 au which is significantly higher than its pure counterpart. Results based on the density of state (DOS), natural bond orbital analysis (NBO) and transition density matrix (TDM) were also strongly supported the enhancement of the NLO properties of these systems. These newly designed alkaline earth metal doped 15CR5 systems with promising NLO characteristics can be used for optoelectronic applications.

Relativistic calculations of energy levels, lifetimes, transition parameters, and electron impact excitation cross sections for Kr XIX

In this study, we present a detailed analysis of atomic properties for Kr XIX, specifically focusing on the lowest 128 fine-structure levels originating from the 3s23p6, 3s23p53d, 3s3p63d and 3s23p43d2 configurations. We report energy levels, lifetimes, wavelengths, weighted oscillator strengths, and transition probabilities for multipole transition types (E1, E2, M1, and M2). To achieve this, we employed the GRASP2018 code, which implements the fully relativistic multiconfigurational Dirac–Hartree–Fock method and accounts for Breit interaction and quantum electrodynamic effects. We performed another set of calculations using the many-body perturbation theory implemented in the flexible atomic code (FAC). By comparing the results obtained from GRASP2018 and FAC, we established the accuracy and consistency of our calculations. Furthermore, using the relativistic distorted wave theory, we studied electron impact excitation of all transitions to upper levels from the ground and metastable levels and reported excitation cross sections for incident electron energies up to 5 keV. To enhance the practical utility of our findings, we also provided analytical fittings of these cross sections and excitation rate coefficients for their applications in plasma modeling. This work contributes to the atomic properties and excitation cross sections of Kr XIX, addressing a marked gap in the available atomic data.

Ab Initio Study on Electronic Excited States of Monochlorosilylene.

We perform a high-level ab initio study on 20 electronic states of monochlorosilylene (HSiCl) using an internally contracted multireference configuration interaction method including Davidson correction (icMRCI+Q). The spin-orbit coupling (SOC) effect is investigated, leading to splitting of the 20 spin-orbit-free states into 50 spin-orbit-coupled states. Vertical transition energies, oscillator strengths, and potential energy curves are presented with and without considering the SOC effect. Analysis indicates that the SOC effect plays an important role, especially for the high-lying excited states of HSiCl. The state interaction and the dynamics of the electronic states of HSiCl in the ultraviolet region are discussed based on our calculation results. Our study paves the way to understanding the behavior of electronic excited states of monochlorosilylene.