Angular anisotropy and dichroism in molecular Auger spectroscopy following photo-absorption

This paper shows that angular distribution of spin-unresolved Auger electrons emitted in the decay of a vacancy created by the absorption of a photon in a certain class of free, unpolarized and gaseous non-linear molecules is completely characterized by three, rather than by two well-known parameters. The presence of this additional third parameter gives rise to circular dichroic effect which varies as cosine of the angle made by the departing Auger electron with the direction of incidence of the circularly polarized radiation. Linear dichroic effect varies as the square of sine of the angle made by the direction of emission of Auger electron and the polar axis of the space frame. Linear and circular dichroism in the angular correlation between theE ₁ photoelectron and Auger electron emitted sequentially from a molecule belonging to one of the 32 point groups has also been investigated.     

Spin state transition in LaCoO3 studied using soft x-ray absorption spectroscopy and magnetic circular dichroism

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-L(2,3) edge, we reveal that the spin state transition in LaCoO3 can be well described by a low-spin ground state and a triply degenerate high-spin first excited state. From the temperature dependence of the spectral line shapes, we find that LaCoO3 at finite temperatures is an inhomogeneous mixed-spin state system. It is crucial that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low- or intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance, and inelastic neutron data.     

Laser induced threshold photoemission magnetic circular dichroism and its application to photoelectron microscope

This work enlightens the threshold photoemission magnetic circular dichroism (MCD) and its adaption on photoemission electron microscopy (PEEM) using lasers. MCD is a simple and efficient way to investigate magnetic properties since it does not need any spin analyzers with low efficiency, and thus the MCD related techniques have developed to observe magnetic domains. Usually, MCD in a total yield measurement in the valence band with weak spin–orbit coupling (SOC) excited by low photon energy (hν≤ 6 eV) does not compete with the X-ray magnetic circular dichroism (XMCD) with strong SOC. XMCD PEEM observation of magnetic domains has been successfully established while MCD PEEM derived from valence bands has not been. However, using angle and energy resolved photoelectron, valence band MCD provides large asymmetry similar to that by XMCD. Threshold measurement of photoelectron in a total electron yield procedure can take advantage of the measurement of photoelectrons with a limited angle and energy mode. This restriction of the photoelectron makes the threshold MCD technique an efficient way to get magnetic information and gives more than 10% asymmetry for Ni/Cu(0 0 1), which is comparable to that obtained by angle resolved photoemission. Thus the threshold MCD technique is a suitable method to observe magnetic domains by PEEM. For threshold MCD, incident angle dependence and high sensitivity to out-of-plane magnetized films compared with in-plane ones are discussed. Ultrashort pulse lasers make it feasible to measure two photon photoemission MCD combined with PEEM, where resonant excitation has a possibility to enhance dichroic asymmetry. Recent results for valence band magnetic dichroism PEEM are presented.     

Binding and Fluorescence Resonance Energy Transfer (FRET) of Ruthenium(II)-Bipyridine-Calixarene System with Proteins—Experimental and Docking Studies

The investigation of the interaction of ruthenium(II)-bipyridine-tert-butylcalix[4]arene complexes (Rubc2 and Rubc3) with proteins (BSA and ovalbumin) using absorption, emission, excited state lifetime and circular dichroism techniques and by docking studies show that luminophore-receptor system bind strongly with proteins. An enhancement of absorption as well as emission intensity of Ru(II)-calixarene complexes in the presence of proteins, but the quenching of the emission intensity of proteins in the presence of Ru(II)-calixarene complexes are the interesting observations. The enhancement of emission intensity of Ru(II)-calixarene complex, in the presence of proteins, is due to the fluorescence resonance energy transfer (FRET) from protein to Ru(II)-calixarene complex. Among the two Ru(II)-calixarene complexes synthesized Rubc3 has more efficient binding and energy transfer than Rubc2 and BSA, with a large cavity size, has the advantage for binding over ovalbumin. Docking studies reveal that the presence of tert-butylcalix[4]arene moiety in Ru(II)-calixarene complexes facilitates binding with proteins. After the binding of Rubc2 and Rubc3 with proteins, the nearby fluorophores present in proteins are in optimal distance from the ruthenium centre for efficient FRET process to occur.     

Optical and ESR studies of an air absorption band in CsI:Na after X-ray irradiation

We have studied the nature of the defect giving rise to a near IR absorption band (717 nm) in X irradiated CsI:Na by measuring its linear dichroism, its magnetic circular dichroism and its change due to the resonance microwave. The defect (g₆ = 1.96, g_⊥ = 2.23 with axis near 〈100〉) involves a Na⁺ ion and an excess electron.     

Three-dimensional noncollinear antiferromagnetic order in single-crystalline FeMn ultrathin films

We present experimental evidence for a three-dimensional noncollinear antiferromagnetic spin structure in ultrathin single-crystalline fcc Fe50Mn50 layers using magnetic circular dichroism photoelectron emission microscopy and x-ray magnetic linear dichroism. Layer-resolved as-grown domain images of epitaxial trilayers grown on Cu(001) in which FeMn is sandwiched between ferromagnetic layers with different easy axes reveal the presence of antiferromagnetic spin components in the film plane and normal to the film plane. An FeMn spin structure with no collinear order in the film plane is consistent with the absence of x-ray magnetic linear dichroism in Fe L3 absorption in FeMn/Co bilayers.     

Observation of a warped helical spin texture in Bi2Se3 from circular dichroism angle-resolved photoemission spectroscopy

A differential coupling of topological surface states to left- versus right-circularly polarized light is the basis of many optospintronics applications of topological insulators. Here we report direct evidence of circular dichroism from the surface states of Bi(2)Se(3) using laser-based time-of-flight angle-resolved photoemission spectroscopy. By employing a novel sample rotational analysis, we resolve unusual modulations in the circular dichroism photoemission pattern as a function of both energy and momentum, which perfectly mimic the predicted but hitherto unobserved three-dimensional warped spin texture of the surface states. By developing a microscopic theory of photoemission from topological surface states, we show that this correlation is a natural consequence of spin-orbit coupling. These results suggest that our technique may be a powerful probe of the spin texture of spin-orbit coupled materials in general.     

Imaging and control of surface magnetization domains in a magnetoelectric antiferromagnet

We report the direct observation of surface magnetization domains of the magnetoelectric Cr(2)O(3) using photoemission electron microscopy with magnetic circular dichroism contrast and magnetic force microscopy. The domain pattern is strongly affected by the applied electric field conditions. Zero-field cooling results in an equal representation of the two domain types, while electric-field cooling selects one dominant domain type. These observations confirm the existence of surface magnetization, required by symmetry in magnetoelectric antiferromagnets.     

Spin texture and circular dichroism in photoelectron spectroscopy from the topological insulator bi_{2}te_{3}: first-principles photoemission calculations

By relativistic first-principles photoemission calculations for the topological insulator Bi_{2}Te_{3}, we study how the spin texture of the Dirac state manifests itself in circular dichroism. On one hand, there are significant modifications of the initial state's spin texture, which are explained by final-state effects and the symmetry of the photoemission setup. On the other hand, a highly symmetric setup allows us to draw conclusions about the detailed Dirac state's spin texture. Our study supports that circular dichroism in angular distribution successfully complements spin- and angle-resolved photoelectron spectroscopy from topological insulators.     

Element-selective mapping of magnetic moments in ultrathin magnetic films using a photoemission microscope

We combine X-ray magnetic circular dichroism (XMCD) and photoelectron emission microscopy to obtain locally resolved magnetic information on a microscopic scale. Scanning the photon energy across elemental absorption edges and recording microscopic images of the local secondary electron intensity for both photon helicities at each photon energy step allows to analyze local XMCD spectra at any position of the imaged area of the sample. With the help of magnetic sum-rules local quantitative information about magnetic moments can be extracted from such microspectroscopic measurements. The full power of XMCD as a spectroscopic tool is so maintained, while microscopic spatial resolution is added.     

Second-harmonic generation circular dichroism spectroscopy from tripod-like chiral molecular films

The second-harmonic generation (SHG) circular dichroism in the light of reflection from chiral films of tripod-like chiral molecules is investigated.The expressions of the second-harmonic generation circular dichroism are derived from our presented three-coupled-oscillator model for the tripod-like chiral molecules.Spectral dependence of the circular dichroism of SHG from film surface composed of tripod-like chiral molecules is simulated numerically and analysed.Influence of chiral parameters on the second-harmonic generation circular dichroism spectrum in chiral films is studied.The result shows that the second-harmonic generation circular dichroism is a sensitive method of detecting chirality compared with the ordinary circular dichroism in linear optics.All of our work indicates that the classical molecular models are very effective to explain the second-harmonic generation circular dichroism of chiral molecular system.The classical molecular model theory can give us a clear physical picture and brings us very instructive information about the link between the molecular configuration and the nonlinear processes.     

Elliptic and circular dichroism effects in two-photon double ionization of atoms

An ab initio parametrization of the two-photon double ionization amplitude from an s2 subshell of an atom in a 1S state is presented and used to predict two light polarization effects on photoelectron angular distributions that do not exist in single-photon double ionization: (i) elliptic dichroism and (ii) circular dichroism at equal energy sharing. Estimates for He show large magnitudes for these effects, which provide a means for polarization control of double ionization by vacuum ultraviolet light.     

Polarization dependence of the permanent deformation of silver nanoparticles in glass by ultrashort laser pulses

Glass containing spherical silver nanoparticles has been irradiated with single, intense, ultrashort laser pulses, with a wavelength corresponding to the surface plasmon resonance of the particles. After irradiation with linear polarization, dichroism is observed. Transmission electron microscopy studies reveal that these spectral changes are caused by deformation of the particles to anisotropic (in the TEM projection approximately ellipsoidal) shapes with an additional halo of small silver particles around the central one. The deformed particles are uniformly oriented with their longer axes perpendicular to the laser polarization. Using laser pulses with circular polarization, again a halo is formed around the particles, but the central particles remain spherical, and no dichroism was observed in the optical spectra. Received 30 November 2000     

Asymmetry in photoelectron emission from chiral molecules induced by circularly polarized light

In photoionization of free, unoriented chiral molecules with circularly polarized radiation, a significant circular dichroism, i.e., an asymmetry in the forward-backward electron emission, has been observed in the photoelectron angular distribution. This leads also to an asymmetry in the momentum transfer to the photoions. The spectra for the left- and right-handed enantiomers of bromocamphor exhibit asymmetries up to several percent which vary as a function of orbital binding energy. This enantioselective effect can similarly occur for biomolecules with handedness, like amino acids, and may thus be a contributing factor related to the origin of the terrestrial biomolecular homochirality.     

Spin resonance in EuTiO3 probed by time-domain gigahertz ellipsometry

We show an example of a purely magnetic spin resonance in EuTiO(3) and the resulting new record high Faraday rotation of 590°/mm at 1.6 T for 1 cm wavelengths probed by a novel technique of magneto-optical gigahertz time-domain ellipsometry. From our transmission measurements of linear polarized light, we map out the complex index of refraction n=√?μ in the gigahertz to terahertz range. We observe a strong resonant absorption by magnetic dipole transitions involving the Zeeman split S=7/2 magnetic energy levels of the Eu(2+) ions, which causes a very large dichroism for circular polarized radiation.     

Self-Assembly of Chiral Nematic Liquid Crystalline Phases of AgNR@SiO2@Cysteine@CsPbBr3 Hybrid Nanorods with Plasmon-Dependent Photoluminescence

The direct synthesis of a chiral nematic liquid crystalline phase of AgNR@SiO₂@cysteine@CsPbBr₃ hybrid nanorods (HNRs) is reported. The circular dichroism spectra can be divided into three components: (1) the interband absorption–enhanced optical activity of structural arrangement of cysteine (cys) molecules, 200–320 nm, (2) the chiral nematic liquid crystalline arrangement of the Ag nanorods (AgNRs), 350–450 nm, and (3) the exciton adsorption edge of the perovskite, 500–550 nm. The polarizing optical microscope images indicate that the chiroptical response of perovskite arises from chiral nematic crystalline arrangement rather than cys-induced electronic coupling between a chiral ligand and otherwise achiral perovskite quantum dots (QDs). The luminescent intensity of CsPbBr₃ QDs in AgNR@SiO₂@cys@CsPbBr₃ HNRs is boosted 87-fold due to the local surface plasmon resonance field enhancement effect. Furthermore, the high-performance green light emitting diode is constructed employing AgNR@SiO₂@cys@CsPbBr₃ complexes, which exhibit excellent luminescent properties. This work contributes insights into structure–property relationships and this strategy promisingly provides guidance for the other inorganic chiral semiconductor suprastructures.     

Theoretical studies on magnetic atom imaging by use of photoelectron diffraction

We theoretically study possible methods how to obtain imaging of magnetic atoms by use of photoelectron diffraction (PD). We propose a novel method to apply Daimon effect where PD peaks are rotated around forward focusing peaks. In usual circular dichroism, we simply use the difference of the PD intensities for different X-ray circular polarization. In contrast to this dichroism, we rather use the difference of the PD intensities for different circular polarizations but - is used for the – circular polarization only in the data handling. This technique allows us to obtain clear atomic image only of spin polarized atoms, and to distinguish magnetic atoms with up-spins from those with down spins. Some illustrative calculations demonstrate the potential use and also the limitation of this technique.     

Molecular frame photoelectron emission in the presence of autoionizing resonances

We have measured the angular distribution of valence-shell photoelectrons excited by circularly polarized light from fixed-in-space N2O molecules, near to and on top of resonances due to Rydberg states embedded in the ionization continuum. The sign of the circular dichroism for ionization into the N2O+ (B2Pi, (1pi)-1) state is reversed on top of the lowest dominant resonances. Measured angular distributions are well predicted by state-of-the-art multichannel configuration interaction calculations. The change in sign of the circular dichroism at the peak of the resonance is the result of a rapid change in the phases of resonant dipole matrix elements by a factor of 2pi as the energy is scanned across the resonance.     

Circular dichroism in laser-assisted short-pulse photoionization

A remarkable effect of circular dichroism, i.e., a difference in photoelectron spectra produced by right and left circularly polarized light in two-color multiphoton ionization of atoms, is predicted for the case when the atom is ionized by an extreme ultraviolet or x-ray femtosecond pulse in the field of a strong infrared laser pulse, both pulses being circularly polarized. We show that the sidebands formed in the spectra exhibit different circular dichroism often of different signs both in angle-resolved and angle-integrated experimental conditions. The effect can be used for detecting and measuring circular polarization of x rays in a spectral range where other methods are not effective.     

Characterize the interaction between polyethylenimine and serum albumin using surface plasmon resonance and fluorescence method

Polyethylenimine (PEI) is a type of cationic polymer which is efficient in DNA transfer. The characters of PEI binding to bovine serum albumin and human serum albumin (HSA) were described by fluorescence quenching, surface plasmon resonance (SPR) and circular dichroism (CD) spectroscopy. The fluorescence quenching results showed that the binding processes occurred on the surface of the protein molecules. The accurate binding constants between PEI and the two proteins were obtained by SPR spectroscopy. The CD spectra results showed that the confirmations of the two proteins were affected with the addition of PEI.