Rydberg excitations in X-ray absorption spectra

It is demonstrated that the Rydberg term values observed in the spectra of polyatomic molecules at 5–15 eV also apply at 100–500 eV, and can be profitably used to assign Rydberg spectra in the X-ray region. The constancy of the term value follows from the one-electron nature of the optical electron in the Rydberg state.     

The absorption and circular dichroism spectra of chiral olefins

Absorption and CD spectra over the quartz and vacuum UV region down to 165 nm are reported for a range of chiral alkenes in the vapour phase and in solution from +70° to ?185°C. A major couplet of oppositely-signed CD bands with comparable band areas, near 48 and 55 kK, is observed in a number of dissymmetric olefins and in some cases a weaker Rydberg CD absorption is found at lower frequency. The Rydberg CD band is characterised by its sharp vibronic structure in the vapour phase and by large blue-shifts produced on passing to the condensed phase and by a reduction in temperature. The olefin couplet of major CD bands with opposite sign is assigned to a near-complete mixing of the electric-dipole π_x→π_x^* and the magnetic-dipole π_x→π_y^* excitations, producing a pair of isotiopic absorption bands with the same polarisation and comparable dipole strengths associated with the CD couplet. Three mixing mechanisms are discussed; sterically-induced π-bond torsion, a first-order static field model, and a second-order dynamic-coupling model dependent, respectively, upon the effective charge and upon the mean polarisability of dissymmetrically-located substituent. The latter two models give the octant rule previously proposed empirically connecting the sign of the rotational strength of the lower- and higher-frequency member of the olefin CD couplet with the position of the substituent in the chromophore coordinate frame.     

Vibronic structure of the circular dichroism and absorption spectra of bianthryls

It will be shown that factors of virtually no influence on the vibronic structure of the absorption spectra in biaryls may profoundly change the correspoding vibronic structure in the CD spectra. This is illustrated by the theoretical analysis of the experimental vibronic absorption and CD spectra of some bianthryl derivatives.     

Linear dichroism spectra of cubic sodium uranyl acetate single crystal

The anisotropy in the absorption spectra of cubic sodium uranyl acetate single crystal has been established by measurement of linear dichroism. This represents the first observation of linear dichroism in the absence of external perturbations in a cubic system. The optical anisotropy is attributed to the partial electric-quadrupolar nature of the transition. The ratio of the electricquadrupole to the electric-dipole contributions to the intensity of the ⁵∑_g⁺ → Δ_g transitions is estimated to be of the order of 10⁻⁶.     

Magnetic circular dichroism and absorption spectra of phosphinidene in noble-gas matrices

Electronic magnetic circular dichroism and absorption spectra are reported for the A 3pi(i) <-- X 3sigma- transitions of phosphinidene (PH) isolated in Ar, Kr, and Xe matrices at cryogenic temperatures (approximately 1.4-20 K) and over a range of magnetic field strengths (0-5 T). The results are analyzed by the method of moments, and parameters are extracted by fitting the experimental data to a model in which the A 3pi(i) term is split by spin-orbit (SO) coupling interactions, while the X 3sigma- term is split by spin-spin and higher-order SO coupling. The analysis indicates that, unlike the equivalent imidogen (NH) systems, ground-state PH radicals isolated in noble-gas matrices do not behave as free rotors. Trends in excited-state SO coupling constants are attributed to the external heavy-atom effect and guest-host orbital mixing. It is tentatively concluded that librational amplitudes of the guest radical decrease in the order Ar > Xe > Kr, probably as a consequence of competition between stronger guest-host interactions and larger matrix sites in heavier hosts.     

Theory of the absorption and circular dichroism spectra of helical molecular aggregates

A theory of the electronic circular dichroism (CD) and optical rotatory dispersion (ORD) of infinite aggregates exhibiting cylindrical symmetry is presented in which, to the authors' knowledge, for the first time vibrational structure is included explicitly. It is shown that, with the coherent exciton scattering approximation in the Green function approach, the detailed vibrational structure of the aggregate absorption. CD and ORD bands can be calculated from a knowledge of the electronic coupling and the monomer absorption line shape alone. Detailed model calculations for a single helix are made and the results are used to expose the origin of different spectral features. A good reproduction of experimental J-aggregate spectra is obtained, using the same electronic interaction to fit both absorption and CD spectral line shapes. The theory allows some prediction of aggregate geometry to be made, but it is shown that an unambiguous geometrical assignment can only be made where experimental spectra for light of different propagation directions with respect to the cylinder axis are available.     

X-ray absorption and natural circular dichroism spectra of C84: A theoretical study using the complex polarization propagator approach

The x-ray absorption and circular dichroism K-edge spectra for the D(2)-isomer of C(84) have been determined using the complex polarization propagator method in conjunction with Kohn-Sham density functional theory. The circular dichroism spectrum is rich in details and, in comparison to the absorption spectrum, it provides a superior resolution of the electronic transitions below the ionization threshold.     

Polyaniline films electrodeposited on iron from oxalic acid solution: linear dichroism of X-ray absorption and molecular arrangement

Journal of Solid State Electrochemistry - The linear dichroism in the near-edge X-ray absorption fine structure (NEXAFS) spectra measured at the K edges of carbon and nitrogen was used to analyze...     

Magnetic circular dichroism and absorption spectra of methylidyne in a krypton matrix

Electronic absorption and magnetic circular dichroism spectra are reported for the A(2)Δ, B(2)Σ(-), and C(2)Σ(+) ← X(2)Π transitions of methylidyne radicals isolated in a Kr matrix at cryogenic temperatures. The results are interpreted in the framework of a model in which the X(2)Π term is split by combination of spin-orbit and crystal-field interactions with the atoms of the host matrix. Analysis of the zeroth moments of the spectra yields an empirical spin-orbit coupling constant A(Π) = 11 ± 2 cm(-1) and orbital reduction factor κ = 0.26 ± 0.05, corresponding to a crystal-field splitting of V(Π) = 43 ± 10 cm(-1) for the X(2)Π term. For the A(2)Δ excited-state term, analysis of the first MCD moments gives a spin-orbit coupling constant of A(Δ) = 4.4 ± 0.9 cm(-1).     

Simulations of X-ray absorption spectra: the effect of the solvent

We perform quantum mechanics/molecular mechanics molecular dynamics simulations on the [Pt(2)(P(2)O(5)H(2))(4)](4-) (abbreviated PtPOP) complex; in water, dimethylformamide and ethanol. These are used to calculate the ground state X-ray absorption spectrum of the complex. The structural parameters from X-ray spectra are usually extracted using a fit of the experimental data. In such simulations the solvent is neglected meaning that any effect of the local environment will be compensated for by structural changes of the solute, leading to possible discrepancies in the extracted structural parameters. Our simulations show a significant solvent effect on the spectra, which has important implications for the structural analysis of molecules in solution.     

Many-electron effects in the X-ray absorption spectra and the one-particle Green's function

Some aspects of influence of many-electron processes on X-ray absorption spectra by the Green's function formalism are investigated. The discrete and continuous near edge and extended fine structures are analyzed in detail, each taken separately. A connection is established between the photoelectron and X-ray absorption spectroscopy data.     

Exciton absorption spectra of optically excited linear molecular aggregates

Exciton absorption spectrum of optically excited linear molecular aggregate is theoretically investigated. The sum rules for the integral intensity of the absorption spectrum are derived. The dipole moments of the optical transitions from the one-exciton states to the two-exciton states are presented. The results obtained indicate an energy increase of the exciton transition after a single excitation of the aggregate. It accounts for the observed short-wavelength shift of the J-band of the pseudoisocyanine (PIC) J-aggregates after their optical excitation. The comparison of the experimental energy of the shift with its theoretical evaluation allows to estimate the number of monomers forming a typical PIC J-aggregate in the solutionN ?20–30.     

Theoretical insight into the magnetic circular dichroism of uranium N6,7-edge X-ray absorption

We use ligand-field density functional theory to determine the electronic structure and to model magnetic circular dichroism in the X-ray absorption spectroscopy (XAS) of uranium compounds. This study extends earlier work on tetravalent uranium ion, in which a model Hamiltonian was set up in order to study electronic structure with three nonequivalent 4f, 5f, and 6d electrons. In the earlier work, the model Hamiltonian took into consideration the interelectron repulsion, spin-orbit coupling interaction, and ligand-field splitting. Uranium N_6,7-edge XAS spectra were calculated on the basis of the 5f² → 4f¹³5f²6d¹ electron transition, showing spectral profiles that were mainly dominated by 4f electron spin-orbit coupling, as well as 6d ligand-field splitting. Fine structures were also observed due to the interelectronic repulsion between 4f-5f, 4f-6d, and 5f-6d electrons. Here, the theoretical study is extended to take into consideration the presence of an external magnetic field, incorporating into the model Hamiltonian for three-open-shell electron configuration a term for Zeeman interaction. Therefore, we are able to model spectra with a left-circularly and right-circularly polarized X-ray, demonstrating evidence of X-ray magnetic circular dichroism (XMCD) for a tetravalent U⁴⁺ ion in the molecular (U(η⁸-C₈H₈)₂) complex. The XMCD originates from a ground-state electronic structure with open-shell 5f electrons. Furthermore, the present calculation of uranium N_6,7-edge XAS and XMCD spectra also enables the ligand-field bonding analysis of the coordination compound.     

The linear dichroism spectra of free base and manganese(III) porphyrins

Linear dichroism of tetraphenylporphyrin and mesoporphyrin IX dimethyl ester are reported together with their manganese (III) derivatives, using both stretched polythene films and liquid crystals as the orienting solvents. All observed electronic transitions in the range 350–900 nm are found to be polarised in the plane of the porphyrin ring.