Laporte Selection Rule Research Articles

Correlation between the unit cell parameters and electronic transitions in Bi1-xEuxFeO3 thin films

Among AFeO3 (A being divalent or trivalent cation) perovskites, BiFeO3 (BFO) exhibits unique as well as complex electronic structure. In the present study local structure of BFO have been manipulated systematically and correlation with the electronic structure has been presented in detail. An electronic energy-level diagram of polycrystalline Bi1-xEuxFeO3 (0 ≤ x ≤ 0.225) has been presented. Using detailed quantitative structural analysis, diffuse reflectance spectrum and the Kubelka-Munk model, the effect of local structural modification on the electronic structure has been investigated. Systematic structural modification has been achieved by Eu3+ substitution which affects bond length, bond angle, and octahedral rotation, consequently, drives the system towards a higher symmetry configuration. Changes in the multiple electronic transitions in pure BFO thin films as a function of Eu3+ substitution have been investigated in detail. An additional change is the emergence of the 4f levels of the rare earth substituent in the valence and conduction bands, Charge-transfer, and doubly degenerated d-d transitions (6A1g→4T2g and 6A1g→4T1g) have been traced as a function of local changes at the unit cell level. In centrosymmetric configuration, 6A1g→4T1g electronic transition is found to be forbidden by the Laporte Rule.

On-off-on Control of Molecular Inversion Symmetry via Multi-stage Protonation: Elucidating Vibronic Laporte Rule.

The Laporte rule dictates that one- and two-photon absorption spectra of inversion-symmetric molecules should display alternatively forbidden electronic transitions; however, for organic fluorophores, drawing clear distinction between the symmetric- and non-inversion symmetric two-photon spectra is often obscured due to prevalent vibronic interactions. We take advantage of consecutive single- and double-protonation to break and then reconstitute inversion symmetry in a nominally symmetric diketopyrrolopyrrole, causing large changes in two-photon absorption. By performing detailed one- and two-photon titration experiments, with supporting quantum-chemical model calculations, we explain how certain low-frequency vibrational modes may lead to apparent deviations from the strict Laporte rule. As a result, the system may be indeed considered as an on-off-on inversion symmetry switch, opening new avenues for two-photon sensing applications.

On‐off‐on Control of Molecular Inversion Symmetry via Multi‐stage Protonation: Elucidating Vibronic Laporte Rule

AbstractThe Laporte rule dictates that one‐ and two‐photon absorption spectra of inversion‐symmetric molecules should display alternatively forbidden electronic transitions; however, for organic fluorophores, drawing clear distinction between the symmetric‐ and non‐inversion symmetric two‐photon spectra is often obscured due to prevalent vibronic interactions. We take advantage of consecutive single‐ and double‐protonation to break and then reconstitute inversion symmetry in a nominally symmetric diketopyrrolopyrrole, causing large changes in two‐photon absorption. By performing detailed one‐ and two‐photon titration experiments, with supporting quantum‐chemical model calculations, we explain how certain low‐frequency vibrational modes may lead to apparent deviations from the strict Laporte rule. As a result, the system may be indeed considered as an on‐off‐on inversion symmetry switch, opening new avenues for two‐photon sensing applications.

Real- and momentum-space description of the excitons in bulk and monolayer chromium tri-halides

Excitons with large binding energies ~2–3 eV in CrX3 have been characterized as being localized (Frenkel) excitons that emerge from the atomic d − d transitions between the Cr-3d-t2g and eg orbitals. The argument has gathered strength in recent years as the excitons in recently made monolayers are found at almost the same energies as the bulk. The Laporte rule, which restricts such parity forbidden atomic transitions, can relax if a symmetry-breaking mechanism is present. While what can be classified as a purely Frenkel exciton is a matter of definition, we show using an advanced first principles parameter-free approach that these excitons in CrX3, in both its bulk and monolayer variants, have band origin and it is the dp hybridization between Cr and X that primarily acts as the symmetry-breaking mechanism that relaxes the Laporte rule. We show that the character of these excitons is mostly determined by the Cr-d orbital manifold, nevertheless, the fractions of the spectral weight shared with the ligand halogen states increases as the dp hybridization enhances. The hybridization enhances as the halogen atom becomes heavier, bringing the X-p states closer to the Cr-d states in the sequence Cl → Br → I, with an attendant increase in exciton intensity and a decrease in binding energy. By applying a range of different kinds of perturbations that qualitatively mimics the effects originating from the missing vertex in self-energy, we show that moderate changes to the two-particle Hamiltonian that essentially modifies the Cr-d-X-p hybridization, can alter both the intensities and positions of the exciton peaks. A detailed analysis of several deep-lying excitons, with and without strain, elucidates the fact that the exciton is most Frenkel-like in CrCl3 and CrBr3 and acquires mixed Frenkel–Wannier character in CrI3, making the excitons in CrI3 most susceptible to environmental screening and spin–orbit coupling.

Excited-State Symmetry Breaking and the Laporte Rule.

Excited-state symmetry breaking (ES-SB) is common to a large number of multibranched electron donor-acceptor (DA) molecules in polar environments. During this process, the electronic excitation, originally evenly distributed over the molecule, localizes, at least partially, on one branch. Due to the absence of an unambiguous spectroscopic signature in the UV-vis region, electronic transient absorption (TA) has not been the method of choice for real-time observation of this phenomenon. Herein, we demonstrate that the Laporte rule, which states that one-photon transitions conserving parity are forbidden in centrosymmetric molecules, provides such clear signature of ES-SB in electronic TA spectroscopy. Using a dicyanoanthracene-based D-A-D dye, we show that transitions from the S1 state of this molecule, which are initially Laporte forbidden, become allowed upon ES-SB. This leads to the rise of new TA bands, whose intensity provides a direct measure of the extent of asymmetry in the excited state.

Upconversion properties in lanthanide doped layered-perovskite, CsBiNb2O7

Despite advances of lanthanide-doped upconversion (UC) materials, the applications such as light-emitting diode and biological imaging are limited by low quantum efficiency. For this context, the understanding of unique interactions between the doped-lanthanides and the host crystals has attracted a huge amount of the researcher's interest. In particular, it was revealed that doping lanthanide ions in a non-centrosymmetric site of host lattice is the cause of relaxation of the Laporte selection rule in the 4f-4f transition of lanthanide ions. One of the layered perovskites CsBiNb2O7 is known to have non-centrosymmetric sites, which would lead to highly bright UC emission. Nevertheless, to our knowledge, there has been no research on the UC comparison between host materials of CsBiNb2O7 with other hosts. In this article, we present the UC intensity comparison of Yb3+-Er3+ ion doped CsBiNb2O7, NaYF4, BaTiO3, and SrTiO3 hosts (the UC in CsBiNb2O7:Er3+,Yb3+ was 2.4 times that of NaYF4:Er3+,Yb3+ and ∼70 times that of SrTiO3:Er3+,Yb3+). After that, we dig into UC, downshifting, and double beam system UC properties. The activator concentration was optimized by varying the doping ratio of Yb3+ and Er3+, and we found out the main reason for the concentration quenching behavior in Er3+ ion doped CsBiNb2O7 is dipole-dipole interaction. In addition, the double excitation experiment indicates that the absorption (4I15/2 → 4I13/2) factor is stronger than the stimulated emission (4I13/2 → 4I15/2) factor in CsBiNb2O7 under 1540 nm laser irradiation.

Crystal Structure of Regularly Th‐Symmetric [U(NO3)6]2− Salts with Hydrogen Bond Polymers of Diamide Building Blocks

AbstractHexanitratouranate(IV), [U(NO3)6]2−, has been crystallized with anhydrous H+‐involving hydrogen bond polymers connected by selected diamide building blocks. Thanks to the significant moderation of electrostatic interactions between the anions and cations, the molecular structure of [U(NO3)6]2− in these compounds is regularly Th‐symmetric. The f–f transitions stemming from 5f2 configuration of U4+ are strictly forbidden by the Laporte selection rule in such a centrosymmetric system, so that the obtained compounds are nearly colourless in contrast to other UIV species usually coloured in green.

Crystal Structure of Regularly Th -Symmetric [U(NO3 )6 ]2- Salts with Hydrogen Bond Polymers of Diamide Building Blocks.

Hexanitratouranate(IV), [U(NO3 )6 ]2- , has been crystallized with anhydrous H+ -involving hydrogen bond polymers connected by selected diamide building blocks. Thanks to the significant moderation of electrostatic interactions between the anions and cations, the molecular structure of [U(NO3 )6 ]2- in these compounds is regularly Th -symmetric. The f-f transitions stemming from 5f2 configuration of U4+ are strictly forbidden by the Laporte selection rule in such a centrosymmetric system, so that the obtained compounds are nearly colourless in contrast to other UIV species usually coloured in green.

Solar-driven H2 evolution over CuNb2O6: Effect of two polymorphs (monoclinic and orthorhombic) on optical property and photocatalytic activity

We synthesized two different polymorphs (monoclinic and orthorhombic phases) of CuNb2O6 as a d0–d9 photocatalyst by a solid-state reaction route and performed photocatalytic H2 evolution from water under AM 1.5G solar light irradiation. The monoclinic phase (P21/c) was transformed to the orthorhombic phase (Pcna) at ambient temperature (>700 °C), and XRD data revealed that the direct environment of octahedral sites was occupied by Cu2+ cations. The difference in the local crystal structure of the CuO6 octahedral in the two polymorphs of CuNb2O6 was closely related to the optical absorption property. For monoclinic CuNb2O6, the optical band gap transition was a metal-to-metal charge transfer between Cu2+ and Nb5+, and d-d absorption of Cu2+ was a forbidden transition by the Laporte selection rule due to its symmetric CuO6 octahedral. For orthorhombic CuNb2O6, d-d absorption of Cu2+ was partially allowed because of its distorted CuO6 octahedral. We experimentally revealed that d-d absorption of Cu2+ contributed a significant effect on the photocatalytic activity of both polymorphs of CuNb2O6; the photocatalytic activity of the monoclinic phase was insensitive to d-d transition in Cu2+ ions, while the photocatalytic activity of the orthorhombic phase was strongly influenced by d-d transition in Cu2+ ions because an electron configuration as d9 readily induces structural distortion by Jahn-Teller effects in an octahedral crystal field. The distortion was avoided by trapping photo-generated electrons, leading to a decrease in photocatalytic activity. We discuss the photocatalytic activities of both polymorphs of CuNb2O6 in conjunction with structural, optical and electrochemical properties. We also show that a deposition of platinum nanoparticles on CuNb2O6 further improved the photocatalytic activity.

High-pressure optical spectroscopy study of natural siderite

Optical absorption spectra of siderite were taken across the high-spin (HS)-to-low-spin (LS) transition up to a pressure of 70 GPa in the spectral range between 28,500 and 10,000 cm−1. Up to a pressure of 44.5 GPa, a pair of two overlapping broad bands was observed that are caused by the electronic spin-allowed 5 T 2g → 5 E g transition of the octahedrally coordinated Fe2+. Furthermore, eight spin-forbidden bands are observable at high pressures up to 44.5 GPa, but they are gradually overlapped by the increasing high-energy absorption edge to be tracked down over the whole pressure range. Due to the HS-to-LS-spin-state transition of Fe2+ between 44.5 and 47.6 GPa, a new broad intense absorption band appears on the steep background of the edge, which is assigned to the electronic spin-allowed 1 A 1g → 1 T 1g transition of octahedral Fe2+ in LS configuration. We estimated a mean octahedral module $$\text{K}_{\text{oct}}^{\text{spectr}}$$ of Fe2+ in the LS state for pressure range 47.6–65.5 GPa as 263 (17) GPa. Especially, a strong intensification of the spin-allowed and spin-forbidden bands with increasing pressure is observed in the HS state. This is assumed to be caused by the borrowing of intensity from the UV absorption bands, which are allowed by the Laporte selection rule and are caused by electronic ligand-to-metal charge-transfer transitions.

Reduced radiative conductivity of low spin FeO6-octahedra in FeCO3 at high pressure and temperature

The ability of Earth's mantle to conduct heat by radiation is determined by optical properties of mantle phases. Optical properties of mantle minerals at high pressure are accessible through diamond anvil cell experiments, but because of the intense thermal radiation at T>1000 K such studies are limited to lower temperatures. Accordingly, radiative thermal conductivity at mantle conditions has been evaluated with the assumption of the temperature-independent optical properties. Particularly uncertain is the temperature-dependence of optical properties of lower mantle minerals across the spin transition, as the spin state itself is a strong function of temperature. Here we use laser-heated diamond anvil cells combined with a pulsed ultra-bright supercontinuum laser probe and a synchronized time-gated detector to examine optical properties of high and low spin ferrous iron at 45–73 GPa up to 1600 K in an octahedral crystallographic unit (FeO6), one of the most abundant building blocks in the mantle. Siderite (FeCO3) is used as a model for FeO6-octahedra as it contains no ferric iron and exhibits a sharp optically apparent pressure-induced spin transition at 44 GPa, simplifying data interpretation. We find that the optical absorbance of low spin FeO6 increases with temperature due to the partially lifted Laporte selection rule. The temperature-induced low-to-high spin transition, however, results in a dramatic drop in absorbance of the FeO6 unit in siderite. The absorption edge (Fe–O charge transfer) red-shifts (∼1 cm−1/K) with increasing temperature and at T>1600 K and P>70 GPa becomes the dominant absorption mechanism in the visible range, suggesting its superior role in reducing the ability of mantle minerals to conduct heat by radiation. This implies that the radiative thermal conductivity of analogous FeO6-bearing minerals such as ferropericlase, the second most abundant mineral in the Earth's lower mantle, is substantially reduced approaching the core–mantle boundary conditions.

Formation, structure, and optical properties of PuO22 + complexes with N,N,N′,N′-tetramethyl-3-oxa-glutaramide

SynopsisPu atom is at the inversion center of the PuO2L2(ClO4)2 complex where L stands for N,N,N’,N’-tetrtametryl-3-oxa-glutaramide (TMOGA). Due to the forbiddance of the f-f transitions, PuO2L2(ClO4)2 in solution is “silent” in near IR optical absorption and PuO2L2(ClO4)2 in solid does not have significant diffuse reflectance bands.The variation of the absorption spectra of Pu(VI) in the presence of a tridentate organic ligand, N,N,N′,N′-tetramethyl-3-oxa-glutaramide (TMOGA denoted as L), was interpreted with the assumption that the 1:2 complex, PuO2L2(ClO4)2, is “silent” in optical absorption of the near IR region because the f–f transitions of Pu(VI) are forbidden due to the high symmetry of the complex. To test the assumption and demonstrate the validity of the Laporte's rule that governs the probability of f–f transitions, crystals of the PuO2L2(ClO4)2 complex were synthesized from the aqueous solution and characterized with single-crystal X-ray diffraction and diffuse reflectance spectra. The structural data, showing that the square-based prism salt crystallized in a highly symmetrical tetragonal space group, I4/mcm, and the plutonium atom is on a center of inversion at the intersection of three perpendicular mirror planes, support the above assumption. The diffuse reflectance spectra of the solid PuO2L2(ClO4)2 also showed no significant bands in the near IR region, consistent with the “silent” feature in the absorption spectra of this complex in solution.

Hetero-Polymetallic Complexes Incorporating Luminescent Lanthanide Ions

The long-lived f-centred luminescence of lanthanide chelates is of considerable interest for the development of luminescent molecular probes in cellular imaging and bioassay and as dopants in light emitting devices. The intermetallic communication in molecular hetero-polymetallic compounds containing f and s or p block ions, d and f block ions and different f-block ions opens up new avenues in FRET assays, directional light conversion, colour tuning and mixing, and the combination of multiple molecular imaging techniques (e.g. optical-MRI) that, particularly when coupled with time-gating protocols, can eliminate competitive fluorescence from endogenous molecules. In all these applications, the focus remains on optimising the luminescence properties of the assemblies by directing the energy transfer processes involved. In this short review, an account of recent findings in the area of luminescent hetero-polymetallic lanthanide complexes is presented, focussing on the different synthetic strategies employed to construct s-f, p-f, d-f and f-f arrays. An evaluation of the energy transfer processes and photophysical properties of this genre of compounds is provided, with specific emphasis on potential applications. Keywords: Energy transfer, hetero-polymetallics, lanthanide complexes, luminescence, FRET assays, optical-MRI, Polymetallic MRI, emitting diodes (OLED's), Laporte's rule, tris-thionapththolate

Hetero-Polymetallic Complexes Incorporating Luminescent Lanthanide Ions

The long-lived f-centred luminescence of lanthanide chelates is of considerable interest for the development of luminescent molecular probes in cellular imaging and bioassay and as dopants in light emitting devices. The intermetallic communication in molecular hetero-polymetallic compounds containing f and s or p block ions, d and f block ions and different f-block ions opens up new avenues in FRET assays, directional light conversion, colour tuning and mixing, and the combination of multiple molecular imaging techniques (e.g. optical-MRI) that, particularly when coupled with time-gating protocols, can eliminate competitive fluorescence from endogenous molecules. In all these applications, the focus remains on optimising the luminescence properties of the assemblies by directing the energy transfer processes involved. In this short review, an account of recent findings in the area of luminescent hetero-polymetallic lanthanide complexes is presented, focussing on the different synthetic strategies employed to construct s-f, p-f, d-f and f-f arrays. An evaluation of the energy transfer processes and photophysical properties of this genre of compounds is provided, with specific emphasis on potential applications. Keywords: Energy transfer, hetero-polymetallics, lanthanide complexes, luminescence, FRET assays, optical-MRI, Polymetallic MRI, emitting diodes (OLED's), Laporte's rule, tris-thionapththolate

Analysis of Photoinduced Magnetization in a (Co, Fe) Prussian Blue Model

The electronic properties of a modified (Co, Fe) Prussian blue compound, which exhibits interesting photoinduced magnetic properties, are analyzed. For band structure calculations we set realistic models (for the ground and excited states) that involve interstitial potassium ions, crystal water, and lattice defects. The top of the “t2g” block consists mainly of Fe 3d orbitals, the low-lying “eg(1)” block Co 3d, and the high-lying “eg(2)” block Fe 3d. Because of the influence of crystal water coordinating to the Co ion, the “eg(1)”-block bands split into two parts; one is the “eg(1)(N)” block originating from the Co−NC fragment and another the “eg(1)(O)” block from the Co−OH2 fragment. The energy of visible light causing the magnetization enhancement is close to the computed “t2g(Fe-based)−eg(1)(N)” and “t2g(Fe-based)−eg(1)(O)” separations in the ground-state model. It is reasonable from DOS (density of states) analyses that irradiation with visible light causes an electron transfer from the Fe(II) to the Co(III) ions, through which the magnetization is effectively enhanced. Such an electron transfer is not a d−d transition on the same metal ion so that the Laporte rule may not be applied, or if applied, this rule may be relaxed in the (Co, Fe) Prussian blue.

Study of closed-shell molecular complexes as possible laser media

We have evaluated a series of blue-green emitting molecular-complex materials (d/sup 0/ electronic system) from the point of view of their suitability as possible tunable laser media. All of them reveal a very strong, broad-band excited-state absorption (ESA), occurring between states of different configurations and positioned at the energy related to the crystal field parameter 10 Dq. The ESA strongly overlaps the luminescence spectra. Because of the nature of the terminal state of ESA, the high rate of ESA transitions can be ascribed to the efficient breaking of the Laporte selection rule, because of considerable admixture of % orbitals of ligands and/or p-orbitals of the central ion into the d-orbitals of the central ion in the terminal configuration, and to the existence of triplet terms in this configuration. We survey the ESA characteristics and compare its cross sections to those of stimulated emission. We interpret the obtained results by means of a configuration-coordinate model. Some possible avenues of future work are indicated. >

Optical absorption investigation of Cr3+ ion-bearing minerals in the temperature range 77?797 K

The effect of raising temperature on spin-allowed dd-transitions of octahedral Cr3+ was studied for various point symmetries of the Cr3+-bearing structural sites, i.e. 3 m and 3 with inversion center in spinel and garnets, respectively, or 32, 3, 2 and 1, lacking the inversion centre, in beryl, corundum, diopside and topaz, respectively. For this purpose, crystals of Cr3+-bearing spinel, pyrope, andradite, grossular, uvarovite, emerald, ruby, diopside and topaz were analyzed by microprobe, oriented, and measured in polarized radiation (except for the cubic minerals) in the spectral range 30 000 to 11 100 cm-1 and at temperatures between 77 and 797 K. The evaluation of the intensities, half widths, and energy positions of bands due to Cr3+-transitions derived from 4 A 2g → 4 T 2g (F) and → 4 T 1g (F) as well as of Dq- and B-values derived, had the following results: In all cases, red shift of the above bands and, hence, independent on the site symmetry of Cr3+, decreases in the Dq-values were obtained. The dependcies of Dq on T are nearly linear above room temperature and amount between -1.6% in topaz and -5.1% in pyrope in the temperature range studied. From this, values for the local thermal expansion of the Cr3+-centered octahedra, α loc, were derived on the basis of the R M-0 -5 -proportionality of 10Dq. Such values are consistently higher than those obtained from X-ray refinements, a method averaging rm-o for all the respective octahedral positions. On increasing temperature, the half band widths increase. The evaluation for spinel, pyrope and uvarovite showed $$\Delta \tilde v_{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}$$ to increase by about 60%, for both types of Cr3+ transitions, in the temperature range studied. The temperature dependence of band intensities is complex, because it depends on the site symmetry and related symmetry selection rules: symmetry forbidden transitions increase strongly on heating. This was observed for Cr3+ in centosymmetric sites $$\bar 3$$ m of spinel and $$\bar 3$$ of garnets as well as for the symmetry-forbidden Cr3+-transitions in the acentric sites 32 in beryl and 3 in ruby. Where no symmetry-related selection rules exist, as in point symmetry 1 in topaz, almost no temperature dependence of band intensities was observed. This holds also for diopside M 1-octahedra with point symmetry 2, although here the U-band should be forbidden at least in one direction. As for the intensity of the U-bands in spinel, pyrope and uvarovite, the observed intensity increase with T, agreed with that calculated on the basis of vibronic coupling with an odd vibration near ω= 450 cm-1. In order to differentiate between the influences of static and dynamic relaxation of the Laporte rule on the intensities, the relative band intensities, subject to static effects only, were calculated for Cr3+ in ruby and emerald. This proved the temperature dependence of the U π and Y π -band, respectively, to be caused by vibronic coupling only.

Excited-state absorption in ZnWO 4 crystal

We report the first measurement of Excited-State Absorption (ESA) in a zinc tungstate (ZnWO 4) crystal. The ESA is assigned to the 2e→4t 2 transitions and, in the simplest picture, can be related to the 10 Dq energy in T d symmetry. The breadth and height of the ESA spectrum, compared with stimulated emission spectrum, precludes any possibility of laser action in this material. The very high strength of ESA transitions suggests a triplet character of the terminal ESA term, originating in (t 5 1,4t 2) configuration, and breaking the Laporte selection rule. The latter is consistent with considerable contribution of p-orbitals of the ligands and the tungsten ion in the terminal ESA state.

Experimental Test of Laporte's Rule In Atomic Hydrogen

An experiment has been performed with atomic hydrogen which yields an upper limit on an intrinsic violation of Laporte's rule. This limit, expressed in terms of mixing of the $2{S}_{\frac{1}{2}}\ensuremath{-}2{P}_{\frac{1}{2}}$ parity doublet, is 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$.

Parity conservation in atoms: Testing Laporte's rule

There are two independent reasons to expect the existence of a new type of weak interaction, involving elastic scattering of electrons and nucleons, e+N→e+N. One is the successful development of unified theories of weak and electromagnetic interactions, based on isospin multiplets of leptons, hadrons and vector mesons. The other is the experimental discovery of the elastic scattering of high energy neutrinos from nucleons, ν+N→ν+N. Together, these developments provide strong motivation to the search for weak electron-nucleon interactions in ordinary atoms, through a breakdown of Laporte's rule. I will present a review of recent developments in this field, including a qualitative discussion of this type of weak interaction and a short summary of the experiments in progress. I will also give a more detailed discussion of one particular experiment at Michigan, involving microwave transitions in a metastable hydrogen beam.