Even-electron System Research Articles

The riddle of orange\u2013red luminescence in Bismuth-doped silica glasses

For over the past two decades it has been believed that the intense orange-red photoluminescence in Bismuth-doped materials originates from Bi^{2+} ions. Based on the results from magnetic circular polarization experiments, we demonstrate that this hypothesis fails for Bismuth-doped silica glasses. Our findings contradict the generally accepted statement that the orange-red luminescence arises from ^{2}P_{3/2}(1)rightarrow^{2}P_{1/2} transition in a divalent Bismuth ion. The degree of magnetic circular polarization of this luminescence exhibits non-monotonic temperature and field dependencies, as well as sign reversal. This complex behaviour cannot be explained under the assumption of a single Bi^{2+} ion. The detailed analysis enables us to construct a consistent diagram of energy levels involved in the magneto-optical experiments and propose a new interpretation of the nature of orange-red luminescence in Bismuth-doped silica glass. A centre responsible for this notorious photoluminescence must be an even-electron system with an integer total spin, presumably a dimer of Bismuth ions or a complex consisting of Bi^{2+} and an oxygen vacancy.

On the nature of photoluminescence in Bismuth-doped silica glass

We report on the investigation of Bismuth-doped pure silica glass without other co-dopant by the tech- nique of magnetic circular dichroism (MCD), which allows the direct probing of the ground state of optical centres. Taking into account the results of conventional optical spectroscopy, we show that the observed MCD bands belong to the centre responsible for the red photoluminescence in this material. Measurements of the temperature and field dependences indicate that the MCD effect is caused by the even-electron system. This, however, opposes the widespread opinion that Bi2+ ions are the origin of red photoluminescence in Bismuth-doped silica glasses. On the other hand, the lasing centre responsi- ble for the near infrared photoluminescence does not exhibit any magnetic optical activity connected to its ground state. As a consequence, we conclude that the ground state of lasing centre is a magnetic singlet with the effective spin S = 0.

Finding symmetry breaking Hartree-Fock solutions: The case of triplet instability.

Determining the lowest unrestricted Hartree-Fock (UHF) solution is often difficult in even-electron systems. We have developed a deterministic method for locating approximately the UHF minimum using the restricted Hartree-Fock triplet instability matrix. The current method is truncated to fourth order. The minimum energy solution for this model can be determined by solving a small linear system of equations. This solution gives a suitable starting point to determine the exact UHF solution. This should be useful for the black-box determination of active spaces spanned by the fractionally occupied charge natural orbitals of the ground-state UHF wavefunction. The results can be generalized to higher (6th and 8th) degree expansions (odd expansion orders vanish by symmetry), and to other types of instability, including complex instability. The results are illustrated by calculations on ozone, benzene, nitrobenzene, butadiene, hexatriene, octatetraene, dichromium, and nickel porphine. Further examples are given in the supplementary material.

Magnetic optical activity in Bi-doped Mg–Al–Si glass

We report the results of the experiments on the magnetic optical activity in Bi-doped Mg–Al–Si oxide glass in the temperature range of 1.48–200 K and high magnetic fields—up to 6.5 T. Temperature dependences of the magnetic circular polarization of luminescence and magnetic circular dichroism indicate the spin multiplicity of the ground and excited states. The investigated system has at least three different defect centers. Experimental data are explained in the assumption of an even-electron system with axial and rhombic zero-field splitting components.

Magnetic circular polarization of luminescence in bismuth-doped silica glass

Bismuth-doped-silica-based optical fibers as a laser gain medium covers the spectral regions inaccessible to conventional rare-earth-doped fibers. Unfortunately, the nature of the luminescent centers in these fibers is not well understood, which complicates the development of efficient devices. In this Letter the magnetic-field-induced circular polarization of near-infrared (NIR) photoluminescence (PL) in Bi-doped pure silica glass was studied in the spectral range of 660–1600 nm, covering three excited state levels. The results of variable temperature and magnetic field measurements allow concluding that the NIR PL originates from an isolated non-Kramers doublet of the even-electron system. This result significantly narrows the list of possible origins of NIR PL in Bi-doped silica glasses.

Electronic structure and reactivity of a biradical cluster: Sc3O6−

The Sc(3)O(6)(-) cluster anions were produced by laser ablation and studied by reaction with n-butane in a fast flow reactor and by photoelectron spectroscopy. The reactivity experiments indicated that one Sc(3)O(6)(-) cluster can activate two n-butane molecules consecutively with rate constants on the order of 10(-10) cm(3) molecule(-1) s(-1) under near room-temperature conditions, suggesting that the even-electron system Sc(3)O(6)(-) has a highly reactive electronic structure. The photoelectron spectroscopy determined a high vertical detachment energy (VDE) of 5.63 ± 0.08 eV for the Sc(3)O(6)(-) cluster. Density functional computations indicated that the lowest energy isomer of Sc(3)O(6)(-) is an oxygen-centered biradical with a high VDE and is highly reactive toward n-butane, which is in good agreement with the experiments. The Sc(3)O(6)(-) cluster may serve as an ideal model system to provide insight into the real-life chemistry involved with the coupled O(-)˙···O(-)˙ dimers over the surfaces of metal oxide catalysts.

A Diabatic Representation Including Both Valence Nonadiabatic Interactions and Spin−Orbit Effects for Reaction Dynamics

A diabatic representation is convenient in the study of electronically nonadiabatic chemical reactions because the diabatic energies and couplings are smooth functions of the nuclear coordinates and the couplings are scalar quantities. A method called the fourfold way was devised in our group to generate diabatic representations for spin-free electronic states. One drawback of diabatic states computed from the spin-free Hamiltonian, called a valence diabatic representation, for systems in which spin-orbit coupling cannot be ignored is that the couplings between the states are not zero in asymptotic regions, leading to difficulties in the calculation of reaction probabilities and other properties by semiclassical dynamics methods. Here we report an extension of the fourfold way to construct diabatic representations suitable for spin-coupled systems. In this article we formulate the method for the case of even-electron systems that yield pairs of fragments with doublet spin multiplicity. For this type of system, we introduce the further simplification of calculating the triplet diabatic energies in terms of the singlet diabatic energies via Slater's rules and assuming constant ratios of Coulomb to exchange integrals. Furthermore, the valence diabatic couplings in the triplet manifold are taken equal to the singlet ones. An important feature of the method is the introduction of scaling functions, as they allow one to deal with multibond reactions without having to include high-energy diabatic states. The global transformation matrix to the new diabatic representation, called the spin-valence diabatic representation, is constructed as the product of channel-specific transformation matrices, each one taken as the product of an asymptotic transformation matrix and a scaling function that depends on ratios of the spin-orbit splitting and the valence splittings. Thus the underlying basis functions are recoupled into suitable diabatic basis functions in a manner that provides a multibond generalization of the switch between Hund's cases in diatomic spectroscopy. The spin-orbit matrix elements in this representation are taken equal to their atomic values times a scaling function that depends on the internuclear distances. The spin-valence diabatic potential energy matrix is suitable for semiclassical dynamics simulations. Diagonalization of this matrix produces the spin-coupled adiabatic energies. For the sake of illustration, diabatic potential energy matrices are constructed along bond-fission coordinates for the HBr and the BrCH(2)Cl molecules. Comparison of the spin-coupled adiabatic energies obtained from the spin-valence diabatics with those obtained by ab initio calculations with geometry-dependent spin-orbit matrix elements shows that the new method is sufficiently accurate for practical purposes. The method formulated here should be most useful for systems with a large number of atoms, especially heavy atoms, and/or a large number of spin-coupled electronic states.

Antisymmetric resonant vibrational transition polarizabilities for even-electron molecules

Under the adiabatic approximation, antisymmetric resonant vibrational transition polarizabilities for even-electron systems with the ground and excited vibronic states belonging to the real representations have been deduced to be zero by the time-reversal symmetry and group theory. Beyond the adiabatic approximation the non-adiabatic corrections to them are non-zero, particularly in the degenerate or near degenerate electronic states, such as Jahn–Teller and pseudo Jahn–Teller cases where adiabatic approximation breaks down. The results verify Buckingham’s conjecture of non-zero antisymmetric transition polarizabilities beyond the adiabatic approximation in 1988. The conclusions of this letter agree with the experiments of antisymmetric vibrational Raman scattering. The compact expressions for the non-resonant and resonant antisymmetric transition polarizabilities are given in the Appendix.

Antisymmetric Transition Polarizability Induced by Intermolecular Charge-Transfer Interactions

Antisymmetric vibrational transition polarizability induced by intermolecular charge-transfer interactions is proposed and studied by using group theory, especially time-reversal symmetry, and Milliken's theory of charge-transfer complexes. The results show that under the Born−Oppenheimer (B−O) approximation, the antisymmetric transition polarizability of even-electron systems with real represention ground states is zero. But when a donor−acceptor complex is formed by the partial or complete transfer of an electron from donor to acceptor, the acceptor (donor) can have nonzero antisymmetric transition polarizability. Thus charge-transfer-induced antisymmetric light scattering from the interaction even-electron systems, especially charge-transfer complexes, can exist, which is characteristic of charge-transfer interaction and processes. The theoretical results are associated qualitatively with anomalous Raman polarization experiment of anthracene anion−Na+ ion complex and with the complexation enhancements of depolarization ratios of some substituted pyridine−Br2 complexes. The connections of the charge-transfer-induced antisymmetric Raman tensors to sum-frequency vibrational spectroscopy (SFVS) in chiral solutions and in photoionization processes have been discussed.

Antisymmetric resonant vibrational Raman scattering

Antisymmetric resonant vibrational Raman scattering is studied using time-reversal symmetry. Under the Born-Oppenheimer approximation, for even-electron systems with the ground and the excited vibronic states belonging to real representations, the antisymmetric resonant Raman transition polarizabilities are strictly zero due to time-reversal symmetry. Only if the ground or the resonant excited vibronic states of an even-electron system belong to E representations of groups C n , C nh or S n ( n ⩾ 3), or for an odd-electron system, can the antisymmetric vibrational resonant Raman transition polarizability be nonzero. The results explain the inconsistency between the space symmetry prediction and the experimental facts of antisymmetric resonant Raman scattering.

Comparative investigation of reactivities in even- and odd-electronic cations: McLafferty rearrangements

The feasibility of applying mechanistic concepts about the “McLafferty reaction” in radical cations to even-electron systems is investigated. Aliphatic ketones, aliphatic and aromatic diesters and aromatic diketones are used as potentially promising model compounds. High resolution measurements, specific deuterium labelling, isotope effects and metastabls ion information are used as means of scrutiny. Surprising similarities of the electron impact mass spectra of meta- and para-disubstituted aromatic compounds were found. Difficulties involved in applying experimental metastable ion information to the solution of such problems are discussed.

A paramagnetic intermediate in the reduction of oxygen by reduced laccase

1. Introduction The four-electron reduction of molecular oxygen in the catalytic reaction of lactase, like that of cyto- chrome c oxidase, poses great mechanistic problems [ 1,2] . With lactase, in particular, a mechanism involv- ing four consecutive one-electron steps would appear unlikely, since the high oxidation-reduction poten- tials of the redox sites of the enzymes [3] would lead to a very high, positive free-energy change for the formation of the superoxide radical. This fact, together with the presence of a co-operative two- electron acceptor in the enzyme, has led to the sug- gestion [2] that the reaction of reduced lactase with oxygen comprises consecutive two-electron steps. Spectroscopic evidence for an intermediate in the reaction of reduced lactase with oxygen has been presented earlier [4]. It was tentatively proposed that this species is HZ02 (or one of its ions) bound to Type 2 Cu2’. However, recent experiments [5]with tree as well as fungal lactase indicate, that when the fully reduced enzymes react with oxygen, both the Type 1 copper and the two-electron acceptor become rapidly re-oxidized, while the Type 2 copper remains reduced. The intermediate so formed has optical properties similar to that described previously [4]. The e.p.r. spectrum at 77 K showed only the presence of Type 1 Cu”. As 02 is an even-electron system and three electrons had been transferred from reduced enzyme-sites, the absence of another e.p.r. signal appeared inconsistent with the kinetic results. This prompted an e.p.r. study at lower temperatures, leading to the discovery of the paramagnetic inter-

Zeeman and uniaxial stress studies of some zero-phonon lines in luminescent CaO

A sharp emission line at 682.8 nm has been found in luminescent CaO containing F-centres. Zeeman data were similar to those obtained on the well-documented 3P to 1S emission line at 574.1 nm from the F-centre. The effect of uniaxial stress on the 682.8 nm line was qualitatively similar to that on the 574.1 nm line. The 682.8 nm emission line may derive from H- ions, although the presence of F-centres appears to be necessary for the existence of the emission line. Further evidence is presented that the adsorption lines at 571 and 1731 nm, previously reported in neutron-irradiated CaO, both derive from an even-electron system with orthorhombic I symmetry. F-centres also appear to be required for this centre to exist in CaO and the centre may be a nearest-neighbour pair of F-centres.

Spectroscopic Study of Magnetic Behavior and Jahn-Teller Distortions in the Even-Electron System TbPO4

The high-resolution optical spectra of TbP${\mathrm{O}}_{4}$ show that there is antiferromagnetic ordering in the vicinity of 2.25 K, but that there must also be a Jahn-Teller distortion. Evidence that the distortion behavior couples strongly to the magnetic behavior is provided by optical experiments which show that (i) the distortion behavior must occur simultaneously with either magnetic ordering or the application of a large magnetic field and that (ii) the distortion is rotatable by an external magnetic field. It is possible to deduce from the distortion behavior the possible directions for the easy axis. A model is presented to explain the magnetic and distortion behavior. A variety of spectral features, including metamagnetic behavior, are observed but are not fully understood.

Suppression of McLafferty rearrangement in the mass spectrometric fragmentation of even-electron systems

Suppression of McLafferty rearrangement in the mass spectrometric fragmentation of even-electron systems

Suppression of McLafferty rearrangements in the mass-spectrometric fragmentation of even-electron systems

Suppression of McLafferty rearrangements in the mass-spectrometric fragmentation of even-electron systems

Spin Hamiltonian for Even-Electron Systems Having Even Multiplicity

The form of spin Hamiltonian necessary to represent the most general zero-field splitting and lowest order magnetic and quadrupole interactions is investigated for systems having an even number of electrons and an even multiplicity. It is shown that, when referred to principal axes, two components of the $g$ tensor are necessarily identically zero. Various other peculiar features emerge. In particular, it is shown that in interpretations of electron spin resonance measurements using a spin Hamiltonian for a non-Kramers doublet, considerable misapprehension and error exist in the literature.