Dipole-forbidden Transitions Research Articles

Valence shell electronically excited states of 1-phenylimidazole and 1-benzylimidazole

The valence shell electronically excited states of 1-phenylimidazole and 1-benzylimidazole have been studied by employing synchrotron radiation to measure the absolute photoabsorption cross-section of each molecule, from threshold up to an energy of 10.8 eV. Assignments have been proposed for some of the broad absorption bands using calculated transition energies and oscillator strengths. Natural transition orbital plots have allowed the Rydberg and/or valence character of the electronically excited states to be assessed. Some of the calculated transitions in 1-benzylimidazole and 1-phenylimidazole have initial and final orbitals that are analogous to those of transitions in the isolated constituent rings of imidazole and benzene. Other mixed Rydberg/valence transitions, especially those leading to some of the low energy electronically excited states in 1-phenylimidazole, have an initial orbital located on the imidazole ring while valence character in the final orbital is localised on the phenyl ring. Thus, photoexcitation results in charge transfer from the donor site (imidazole) to the acceptor (the phenyl ring) in the nascent ion core of the Rydberg state. In both 1-benzylimidazole and 1-phenylimidazole the lowest energy excited state arises from a transition analogous to the 1e1g → 1e2u 1B2u electric dipole-forbidden transition in benzene.

Laser spectroscopy of highly charged ions at the storage ring CSRe: Schottky-based spectroscopic investigation and XUV fluorescence detector development

Laser spectroscopy stands out as a powerful tool to investigate atomic and nuclear properties and also test fundamental theories by precisely measuring the energy levels in highly charged ions. In this work, the fine-structure splitting 1s22s2S1/2−1s22p2P1/2 transition in lithium-like 16O5+ has been investigated in a laser spectroscopy experiment at the heavy-ion storage ring CSRe. The excitation resonance of the tunable narrowband UV laser and the ions was observed using a Schottky resonator with very high sensitivity. The experimental uncertainties including the ion beam velocity determination, the space charge effects, laser wavelength calibration, and angular misalignment between the laser and ion beam were analyzed, and the 2S1/2−2P1/2 transition wavelength was determined to be λ0= 103.45(38) nm. The primary source of experimental uncertainty arises from the inaccurate measurement of the high voltage applied on the electron cooler and therewith the ion beam velocity at the CSRe. In order to solve this problem, a high-precision divider to precisely measure the high-voltage of the electron cooler is under construction, which is expected to improve the present experimental accuracy by three orders of magnitude. In addition, an XUV optical detector equipped with an off-axis parabolic mirror has been developed and installed at the CSRe for the investigation of slow dipole-forbidden transitions in highly charged ions, such as hyperfine splitting and M1 transitions. The experimental studies presented here pave the way for future laser spectroscopy experiments at the CSRe as well as at the future larger-scale scientific facility HIAF in China.

Theoretical study of low-lying electronic states spectra and transition properties of BeS molecule

There are only limited theoretical results published on the transition properties of the X1Σ+, A1Π, B1Σ+, C1Δ, D1Σ−, E1Δ, a3Π, b3Σ+, and c3Δ states of BeS, and even some of the states have no theoretical and experimental reports on the relevant aspects of these transitions. Therefore, we derived these transition properties in this study. The potential energy curves of X1Σ+, A1Π, B1Σ+, C1Δ, D1Σ−, E1Δ, a3Π, b3Σ+, c3Δ, d3Σ−, and e3Σ− states of the BeS molecule with their Ω states and the transition dipole moments between them were calculated by the complete active space self-consistent field method, followed by the internally contracted multireference configuration interaction method. For the accurate computation of the transition properties, scalar relativistic corrections and core–valence correlation were considered. The radiative lifetimes were approximately 10−6 s for the A1Π and b3Σ+ states, 10−7 s for the E1Δ state, 10−7–10−8 s for the C1Δ and D1Σ− states, and 10−8 s for the B1Σ+, c3Δ, and e3Σ− states. The transition properties of the seven Ω states generated by the X1Σ+, A1Π, B1Σ+, and a3Π electronic states, including several electric dipole-forbidden transitions were researched. In particular, the B1Σ+ 0+–X1Σ+ 0+, B1Σ+ 0+–A1Π1, and A1Π1–X1Σ+ 0+ systems have strong transitions. In this paper, the radiative lifetimes of A1Π1, B1Σ+ 0+, a3Π0−, a3Π0+, and a3Π1 electronic states were also computed. For the A1Π1, B1Σ+ 0+, and a3Π1 states, the distribution of the radiation lifetime with the rotational angular momentum quantum number J at 0 ≤ υ ≤ 15 and J ≤ 70 was evaluated. It is expected that the outcome of the calculations in this paper can contribute some meaningful guidelines for conducting further theoretical and experimental investigations.

Observation of Renner-Teller and predissociation coupled vibronic intensity borrowing in dissociative electron attachment to OCS.

We use a time-of-flight-based velocity map imaging method to look into the dissociative electron attachment to a linear OCS molecule at electron beam energies ranging from 4.5 to 8.5eV. The conical time-gated wedge slice imaging method is utilized to extract fragments' slice images, kinetic energy (KE), and angular distributions, which provide a complete kinematic understanding of this experiment on the dissociative electron attachment process. We observe that the formation of S- is relatively higher than the O- product. Three distinct dissociative KE bands of S-/OCS have been observed for the 5.0 and 6.5eV resonance positions. We notice a prominent rovibrationally coupled bimodality for each KE band in the variation of the most probable KE values. When the electron energy is changed from 5.5 to 6.0eV, we observed vibronic intensity borrowing in the highest momentum band of S- via the Σ → Π symmetric dipole-forbidden transitions within the 1.5eV energy gap. Multiple peaks in the angular distributions of S- and their modeling indicate the presence of Renner-Teller vibronic splitting. Using Q-Chem's implemented complex absorbing potential-equation of motion-electron affinity coupled cluster singles and doubles aug-cc-pVDZ+4s3p level of multireference-based electronic structure theory, we confirm the presence of OCS temporary negative ion bending vibrations and Renner-Teller vibronic splittings for the Π symmetric states. Additionally, we notice the presence of a non-radiative predissociation continuum (bringing down the rotational spectrum) and speed-dependent angular anisotropy in the S- fragmentation. Our findings at the resonance of OCS at 6.5eV closely align with the prediction of vibronic intensity borrowing by Orlandi and Siebrand [J. Chem. Phys. 58, 4513 (1973)].

Highly Monochromatic Ultraviolet LED Based on the SnO2 Microwire Heterojunction Beyond Dipole-Forbidden Band-Gap Transition.

SnO2 has been extensively applied in the fields of optoelectronic devices because of its large band gap, high exciton binding energy, and outstanding optical/electrical properties. However, its applications in ultraviolet light-emitting diodes (LEDs) are still hindered by the dipole-forbidden rule. Herein, the dipole-forbidden rule can be conquered by synthesizing Sb-incorporated SnO2 microwires (SnO2:Sb MWs), which are examined by ultraviolet photoluminescence emitting at 363.2 nm and a line width of 11.3 nm. Subsequently, a highly monochromatic ultraviolet light-emitting diode (LED) based on a SnO2:Sb MW heterojunction was constructed with a p-GaN film serving as the hole supplier. In the LED, the presence of a MgO intermediate layer can modulate carrier transport and recombination path, thus achieving band-edge optical transition in the SnO2:Sb MW. As the LED is modified using Ag nanowires, electrical properties, especially for the hole injection efficiency, were dramatically boosted, contributing significantly to the device high brightness. The LED emits at 365.9 nm and a line width of 12.4 nm. Therefore, we have realized a high-brightness and narrow-band ultraviolet LED with the shortest peak wavelength never seen in previously reported SnO2 LEDs. This work will promote the potential applications of low-dimensional SnO2 optoelectronic devices and provide an effective exemplification to overcome the dipole-forbidden rule in metal-oxide materials with "forbidden" energy gaps.

Effect of substituting on the transition dipole moment of the double perovskite Cs2AgInCl6

The all-inorganic double perovskite Cs2AgInCl6 with three dimensional structure has attracted much attention due to its direct bandgap property and particular luminescence mechanism, which is self-trapped exciton emission. However, it is a pity that Cs2AgInCl6 exhibits low photoluminescence quantum yield, which affects its application for light-emitting devices. In this paper, the band structure and transition dipole moment of Cs2AgIn(1−x)Sb x Cl6 (x = 0, 0.25, 0.5, 0.75) are calculated using first principle calculation. The calculated results shows that the pure material Cs2AgInCl6 not only has a large band gap but also has the dipole forbidden transition, which means that the electrons cannot be excited from the valence band maximum to the conduction band minimum. However, the substituted Cs2AgIn0.75Sb0.25Cl6 have a good property for the band gap about 3.066 eV and break forbidden transition at point X. The reason for its change is due to the overlap of electron and hole for charge density. Our work provides theoretical guidance for the design of more efficient light-emitting devices.

Controlling magnon-magnon entanglement and steering by atomic coherence.

Here we show that it is possible to control magnon-magnon entanglement in a hybrid magnon-atom-cavity system based on atomic coherence. In a four-level V-type atomic system, two strong fields are applied to drive two dipole-allowed transitions and two microwave cavity modes are coupled with two dipole forbidden transitions as well as two magnon modes simultaneously. It is found that the stable magnon-magnon entanglement, one-way steering and two-way EPR steering can be generated and controlled by atomic coherence according to the following two points: (i) the coherent coupling between magnon and atoms is established via exchange of virtual photons; (ii) the dissipation of magnon mode is dominant over amplification since one of the atomic states mediated one-channel interaction always keeps empty. The coherent control of magnon-magnon correlations provides an effective approach to modify macroscopic quantum effects using the laser-driven atomic systems.

Satellites of the Dipole-Forbidden Transitions to the Low-Lying 2S1/2 and 2D3/2,5/2 Excited States of K, Rb, and Cs Atoms in the Spectra of Gas-Phase Mixtures with CF4

Satellites of the Dipole-Forbidden Transitions to the Low-Lying 2S1/2 and 2D3/2,5/2 Excited States of K, Rb, and Cs Atoms in the Spectra of Gas-Phase Mixtures with CF4

Enhancement of specular behavior in atom mirrors with quadrupole-active transitions

Modern studies have confirmed the possibility of making atomic mirrors using quadrupole-active atomic transition evanescent waves. The reflection process has been shown to be enhanced to approach specular behavior in several ways. In this study, we considered the atomic mirror due to the optical quadrupole potential generated by two counter-propagating optical Gaussian beams. This arrangement provides good enhancement for the evanescent field in the vacuum above the surface, which can be used as an atomic mirror. A high-quality atomic mirror should reflect the atoms specularly while maintaining the coherence of the de Broglie wave function. We focused on a particular atomic transition in Cs, which is a dipole-forbidden but quadrupole-allowed transition. We illustrate the optical quadrupole potential and suggest that quadrupolar transitions generally ignored in atom optics exist for exploitation. This study investigated the conditions facilitating atomic mirrors using typical experimentally accessible parameters.

Plasma-relevant fast electron impact study of difluoromethane

Difluoromethane () is a commonly used etching gas in manufacturing very-large-scale integrated circuits as the fluorine source. Electron impact excitation cross sections for are important input data for modeling the reactive etching plasmas. In this work, we present the experimental generalized oscillator strengths for the , , and transitions on the absolute scale. The measurement was realized by employing the crossed-beam based relative flow technique at an angle-resolved electron energy loss spectrometer, operating at an incident energy of 1500 eV and a resolution of 80 meV. The dipole-forbidden transition at around 9.7 eV is identified for the first time. The measured data are fitted with the well-known Lassettre formula to yield analytical expressions and optical oscillator strengths at for dipole-allowed transitions. The analytical expressions enable integrations over momentum transfer to obtain the Born integral cross sections for electron impact excitations from the threshold to 5000 eV. The integral cross sections for dipole-allowed transitions are scaled to a more reliable level by using the BE-scaling method. The experimental data can benchmark theoretical calculations and supplement the molecular database for plasma modelers.

Including vibrational effects in magnetic circular dichroism spectrum calculations in the framework of excited state dynamics.

In this work, we present a computational approach that is able to incorporate vibrational effects in the computations of magnetic circular dichroism (MCD) spectra. The method combines our previous implementations to model absorption as well as fluorescence and phosphorescence spectra in the framework of excited state dynamics with a new technique to calculate MCD intensities, where molecular orientational averages are treated via semi-numerical quadrature. The implementation relies on a path integral approach that is employed to compute nuclear dynamics under the harmonic oscillator approximation (accounting for the nuclear potential energy surface) together with quasi-degenerate perturbative theory (to include the perturbation of an external magnetic field). We evaluate our implementation with a selected molecular set consisting of five aromatic organic molecules, namely, 1,4-benzoquinone, naphthalene, 2-naphthylamine, 2-naphthaldehyde, and benzene; we also included the MnO4- and the [Co(NH3)6]3+ transition metal complexes. This set is used to validate the ability of the approach to compute MCD A- and B-terms in conjunction with time-dependent density functional theory. The computed intensities are discussed in terms of the overall quality of the electronic structure treatments, vibrational modes, and the quality of the nuclear Hessians. It is shown that in the cases in which the potential energy surface is accurately represented, electric dipole-forbidden transitions are vibrationally activated, producing intensities relative to the dipole-allowed transitions in the same order of magnitude as the experimental measurements.

Exciton dynamics of a fused ring π-conjugated nonfullerene molecule based on dithienonaphthobisthiadiazole

Herein, we have studied the exciton dynamics of a novel fused ring π-conjugated molecule (YS3) in solution and film states by spectroscopic measurements. This molecule incorporates dithienonaphthobisthiadiazole as a core unit that is a two-dimensionally π-extended fused ring. As a result, we found a long exciton lifetime in YS3 films originating from reduced radiative and nonradiative transitions. This is partly because radiative deactivation is effectively suppressed because of the dipole-forbidden transition in H-aggregates and partly because rotational deactivation is effectively suppressed in the crystalline film state.

Understanding the electronic structure of Y2Ti2O5S2 for green hydrogen production: a hybrid-DFT and GW study.

Utilising photocatalytic water splitting to produce green hydrogen is the key to reducing the carbon footprint of this crucial chemical feedstock. In this study, density functional theory (DFT) is employed to gain insights into the photocatalytic performance of an up-and-coming photocatalyst Y2Ti2O5S2 from first principles. Eleven non-polar clean surfaces are evaluated at the generalised gradient approximation level to obtain a plate-like Wulff shape that agrees well with the experimental data. The (001), (101) and (211) surfaces are considered further at hybrid-DFT level to determine their band alignments with respect to vacuum. The large band offset between the basal (001) and side (101) and (211) surfaces confirms experimentally observed spatial separation of hydrogen and oxygen evolution facets. Furthermore, relevant optoelectronic bulk properties were established using a combination of hybrid-DFT and many-body perturbation theory. The optical absorption of Y2Ti2O5S2 weakly onsets due to dipole-forbidden transitions, and hybrid Wannier-Mott/Frenkel excitonic behaviour is predicted to occur due to the two-dimensional electronic structure, with an exciton binding energy of 0.4 eV.

Electronic structures and optical properties of uniform ordered hexagonal Ge0.5Si0.5 alloys from first principles

The electronic structures and optical properties of uniform ordered hexagonal Ge0.5Si0.5 alloys were investigated by the first-principle calculations. We first validated our approach for hexagonal Ge and Si, obtaining atomic geometries and band structures in excellent agreement with available experimental data. Then, the same approach was applied to predict electronic and optical properties of uniform ordered hexagonal Ge0.5Si0.5 alloys. The hex-Ge0.5Si0.5 alloy is a direct bandgap semiconductor. The dipole-forbidden or dipole-allowed transitions from the three highest valence bands (Γ9v+, Γ7v++, Γ7v-+) to the first (Γ8c-) or second (Γ7c-) lowest conduction band at Γ point are obtained. The maximum absorption coefficient is close to 2 × 106 cm−1. The three small absorption peaks probably originates from the transition from the Γ9v+, Γ7v++ or Γ7v-+ to Γ7c- band around the Γ point, respectively.

Dipole-forbidden transitions induced by the gradient of optical near fields

The electric-field spatial gradient is a very important property of an optical near-field (ONF). In this study, its effect on the single-photon ionization is theoretically investigated via the three-dimensional time-dependent Schr\"odinger equation and perturbation theory. The results show that the field gradient can lead to dipole-forbidden transitions, which competes with the usual dipole transition and results in changes of the photoelectron momentum distribution. Additionally, we establish a relationship between the field gradient with the angular distribution of the photoelectrons. Through this mapping, the field gradient can be accurately retrieved. Furthermore, a way for extracting the phase difference between the partial waves with odd and even parity from the photoelectron momentum distribution is proved to be feasible. We find that the Cooper minimum can enhance the effect of the field gradient and thus improves the accuracy of our proposed retrieval procedure. Our results pave the way to exploit atomic structural properties as a probe of the gradient of the ONF.

Mean-field Floquet theory for a three-level cold-atom laser

We present a theoretical description for a lasing scheme for atoms with three internal levels in a $V$ configuration and interacting with an optical cavity. The use of a $V$-level system allows for an efficient closed lasing cycle to be sustained on a dipole-forbidden transition without the need for incoherent repumping. This is made possible by utilizing an additional dipole-allowed transition. We determine the lasing threshold and emission frequency by performing a stability analysis of the nonlasing solution. In the lasing regime, we use a mean-field Floquet method (MFFM) to calculate the lasing intensity and emission frequency. This MFFM predicts the lasing transition to be accompanied by the breaking of a continuous U(1) symmetry in a single Fourier component of the total field. In addition, we use the MFFM to derive bistable lasing and nonlasing solutions that highlight the nonlinear nature of this system. We then test the bistability by studying hysteresis when slowly ramping external parameters across the threshold and back. Furthermore, we also compare our mean-field results to a second-order cumulant approach. The work provides simple methods for understanding complex physics that occur in cold atom lasers with narrow line transitions.

Excited States of Metal-Adsorbed Dimethyl Disulfide: A TDDFT Study with Cluster Model.

The optical near field refers to a localized light field near a surface that can induce photochemical phenomena such as dipole-forbidden transitions. Recently, the photodissociation of the S–S bond of dimethyl disulfide (DMDS) was investigated using a scanning tunneling microscope with far- and near-field light. This reaction is thought to be initiated by the lowest-energy highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition of the DMDS molecule under far-field light. In near-field light, photodissociation proceeds at lower photon energies than in far-field light. To gain insight into the underlying mechanism, we theoretically investigated the excited states of DMDS adsorbed on Cu and Ag surfaces modeled by a tetrahedral 20-atom cluster. The frontier orbitals of the molecule were delocalized by the interaction with the metal, resulting in narrowing of the HOMO–LUMO gap energy. The excited-state distribution was analyzed using the Mulliken population analysis, decomposing molecular orbitals into metal and DMDS fragments. The excited states of the intra-DMDS transitions were found over a wider energy range, but at low energies, their oscillator strengths were negligible, which is consistent with the experimental results. Sparse modeling analysis showed that typical electronic transitions differed between the higher and lower excited states. If these low-lying excited states are efficiently excited by near-field light with different selection rules, the S–S bond dissociation reaction can proceed.

Differential and integral cross sections for the valence-shell excitations in D2O studied by fast electron impact

The experimental generalized oscillator strengths (GOSs) for the valence-shell excitations of heavy water in 6.50--12.17 eV are determined by fast electron scattering with an incident electron energy of 1500 eV and an energy resolution of 80 meV. The experimental data of the $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{A}{\phantom{\rule{0.16em}{0ex}}}^{1}{B}_{1}\ensuremath{\leftarrow}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{0.16em}{0ex}}}^{1}{A}_{1}$ transition between ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{D}}_{2}\mathrm{O}$ show significant discrepancies around the minimum, and further theoretical calculations with nuclear effects accounted are recommended. The comparisons of the present GOSs of $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{A}{\phantom{\rule{0.16em}{0ex}}}^{1}{B}_{1}\ensuremath{\leftarrow}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{0.16em}{0ex}}}^{1}{A}_{1}$ and $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}{\phantom{\rule{0.16em}{0ex}}}^{1}{A}_{1}\ensuremath{\leftarrow}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{0.16em}{0ex}}}^{1}{A}_{1}$ transitions in ${\mathrm{D}}_{2}\mathrm{O}$ with theoretical calculations within the first Born approximation and the second Born approximation indicate that the higher Born terms become prominent at large momentum transfers. In particular, the GOSs of the dipole-forbidden transition ${}^{1}{A}_{2}\ensuremath{\leftarrow}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{0.16em}{0ex}}}^{1}{A}_{1}$ are extracted, benefiting from the present high-energy electron scattering, where most of electronic dipole-forbidden transitions are not involved. By extrapolating the GOSs to the zero momentum transfer, the optical oscillator strengths for the corresponding dipole-allowed transitions are determined, which provide an independent cross-check for previous optical measurements. Moreover, the integral Born cross sections are scaled to an accurate level with the aid of the binding energy--excitation energy--scaling method. The relevant experimental data in this work can supplement the molecular database with electron collision data involving heavy water.

Resonant Auger decay induced by the symmetry-forbidden 1a1g → 6a1g transition of the SF6 molecule

Resonant Auger electron spectroscopic study at the symmetry-forbidden 1a1g→6a1g excitation below the S K-shell threshold of SF6 is reported. Partial electron yield and resonant KLL Auger spectra have been measured by using monochromatized undulator synchrotron radiation. By changing the photon energy in small steps, a so-called 2D map is produced. In this map, the dipole-forbidden transition exhibits spectral features (e.g., an S-shaped dispersion relation), which are well known and understood for dipole-allowed transitions. We validate by a theory that for the case of dipole-forbidden transitions, these spectral features can be analyzed in the same way as previously established for the dipole-allowed ones. This approach grants information on the nuclear dynamics in the K-shell core-excited states of SF6 on the femtosecond (fs) timescale. In particular, for the potential-energy curves of the states S 1s−16a1g and S 2p−26a1g, the slopes at the equilibrium distance of the ground state are derived. Symmetry breaking as a result of ultrafast vibronic coupling is revealed by the population of the electronically forbidden excited state.

Probing the delocalized core-hole via inner-shell excitation in N2

The dispute about whether the 1s core-hole is localized on one atom or delocalized over both in a homonuclear diatomic molecule has continued for decades, which has been extensively studied by the photoelectron and electron–ion coincidence spectroscopies. For N2, if the 1s core-hole is delocalized, the K-shell excitation into the 1π g orbital should split into two components, i.e., the dipole-allowed transition from the ungerade 1σ u state and the dipole-forbidden transition from the gerade 1σ g state. However, only the dipole-allowed transition has been observed up to now. Here, we report the inner-shell electron energy loss spectra of N2 at different scattering angles with an incident electron energy of 1500 eV and an energy resolution of 65 meV. The vibrational structures of both the dipole-allowed and dipole-forbidden states of N2 have been identified at different momentum transfers. The splitting between the and states with the reverse symmetry is determined to be 67 ± 7 meV. Moreover, the momentum transfer dependence behavior of the transition intensity ratio agrees with the theoretical predictions, as increasing to a maximum and then decreasing. The experimental observations clearly show that the inner most electrons can be described by 1σ g and 1σ u , which indicates that the inner-shell 1s core-hole of N2 is delocalized over two N atoms based on the excitation process.