Circular dichroism in the angle-resolved C 1s photoemission spectra of gas-phase carvone enantiomers

The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvone has been investigated using circularly polarized synchrotron radiation up to 30 eV above threshold. Binding energies of the C=O and CH2= carbon 1s orbitals were determined to be 292.8+/-0.2 and 289.8+/-0.2 eV, respectively. The remaining C-H C 1s levels substantially overlap under an intense central peak centered at 290.5+/-0.2 eV. The angle-resolved photoemission from the carbonyl carbon C=O core orbital in pure carvone enantiomers shows a pronounced circular dichroism of approximately 6% at the magic angle of 54.7 degrees to the light beam propagation direction. This corresponds to an expected 0 degrees -180 degrees forward-backward electron emission asymmetry of approximately 10%. On changing between the R and S enantiomers of carvone the sense or sign of the asymmetry and associated dichroism effectively reverses. The observed circular dichroism, and its energy dependence, is well accounted for by calculations performed in the pure electric dipole approximation.     

Determination of chiral asymmetries in the valence photoionization of camphor enantiomers by photoelectron imaging using tunable circularly polarized light

An electron imaging technique has been used to study the full angular distribution of valence photoelectrons produced from enantiomerically pure molecular beams of camphor when these are photoionized with circularly polarized light. In addition to the familiar beta parameter, this provides a new chiral term, taking the form of an additional cosine function in the angular distribution which consequently displays a forward-backward electron ejection asymmetry. Several ionization channels have been studied using synchrotron radiation in the 8.85-26 eV photon energy range. With alternating left and right circularly polarized radiations the photoelectron circular dichroism (PECD) in the angular distribution can be measured and shows some strong dynamical variations with the photon energy, depending in sign and intensity on the ionized orbital. For all orbitals the measured PECD has a quite perfect antisymmetry when switching between R and S enantiomers, as expected from theory. In the HOMO(-1) channel the PECD chiral asymmetry curves show a double maxima reaching nearly 10% close to threshold, and peaking again at approximately 20% some 11 eV above threshold. This is attributed to a resonance that is also visible in the beta parameter curve. Newly optimized CMS-Xalpha photoionization dynamics calculations are also presented. They are in reasonably good agreement with the experimental data, including in the very challenging threshold regions. These calculations show that PECD in such randomly oriented samples can be understood in the electric dipole approximation and that, unlike the case pertaining in core-shell ionization-where a highly localized achiral initial orbital means that the dichroism arises purely as a final state scattering effect-in valence shell ionization there is a significant additional influence contributed by the initial orbital density.     

Angle-resolved photoelectron spectroscopy of randomly oriented 3-hydroxytetrahydrofuran enantiomers.

Circular dichroism in the angular distribution of valence photoelectrons emitted from randomly oriented 3-hydroxytetrahydrofuran enantiomers (ThS and ThR) has been observed in gas-phase experiments using circularly polarized vacuum ultraviolet (VUV) light. The measured dichroism for both ThS and ThR, acquired at the single magic angle theta=234.73 degrees and at photon energies of 22, 19, 16, and 14 eV, points to an asymmetric forward-backward scattering of the photoelectrons from their highest occupied molecular orbitals (HOMO) HOMO-1 and HOMO-2, of up to 5%, depending on the photon energy. The asymmetry reverses on exchange of either the helicity of the radiation or the configuration of Th. The photoionization dichroic D parameters of ThS and ThR have been measured and their values discussed in the light of LCAO B-spline density functional theory (DFT) predictions. While an acceptable agreement is found between the dichroic parameter measured and calculated at the highest photon energy for the HOMO and HOMO-2 orbitals of Th, a significant discrepancy is observed for the HOMO-1 state which is attributed to the floppiness of Th, in particular to the comparatively large sensitivity of the size and shape of its HOMO-1 on nuclear vibrational motion.     

Quantitative analysis of angle‐resolved XPS spectra recorded in parallel data acquisition mode

The effects of anisotropy of the photoionization cross‐section and elastic scattering of photoelectrons in solids are investigated for angle‐resolved XPS spectra (ARXPS) recorded from α–Al₂O₃ substrate in parallel data acquisition mode. It is shown that for quantitative analysis of ARXPS spectra recorded in parallel data acquisition mode it is essential to account for the anisotropies of the photoionization cross‐sections of the detected photoelectrons for the concerned elements in the solid due to variation of the angle between the incident x‐rays and the detected photoelectrons. Neglecting the effect of elastic scattering only leads to minor errors in quantitative analysis of the ARXPS spectra. By adopting experimentally determined values for the relative sensitivity factors of the concerned photoelectrons in the solid as a function of the detection angle, cumbersome corrections for the effects of anisotropy of the photoionization cross‐section and elastic scattering can be avoided. Copyright © 2004 John Wiley & Sons, Ltd.     

Photoionization cross section and angular distribution calculations of carbon tetrafluoride

Correlation in the photoionization dynamics of carbon tetrafluoride is studied in the framework of the time-dependent density-functional theory (TDDFT) approach by employing a multicentric basis set expansion of the scattering wave function linear combination of atomic orbitals (LCAO) TDDFT. Results obtained with the statistical average of orbital potentials and LB94 exchange-correlation (xc) potentials are compared with photoabsorption, photoionization, and electron-scattering experiments as well as with past theoretical calculations. Inadequacies in both the V(xc) parametrizations employed have been suggested from the analysis of the intensity plots for the D2A1 ionization. The formation of resonant scattering states in selected continuum channels has been studied through the analysis of the dipole-prepared scattering wave function; our findings are then compared with results of electron-scattering calculations. Overall, the LCAO-TDDFT results highlight the effectiveness of the approach for the calculation of the unbound spectrum of fairly large molecules.     

Chiral signatures in angle-resolved valence photoelectron spectroscopy of pure glycidol enantiomers

Photoionization of the chiral molecule glycidol has been investigated in the valence region. Photoelectron circular dichroism (PECD) curves have been obtained at various photon energies by using circularly polarized VUV synchrotron radiation and a velocity map imaging technique to record angle-resolved photoelectron spectra (PES). The measured chiral asymmetries vary dramatically with the photon energy as well as with the ionized orbital, improving the effective orbital resolution of the PECD spectrum with respect to the PES. Typical asymmetry factors of 5% are observed, but the peak values measured range up to 15%. The experimental results are interpreted by continuum multiple scattering (CMS-Xalpha) calculations for several thermally accessible glycidol conformers. We find that a nearly quantitative agreement between theory and experiments can be achieved for the ionization of several molecular orbitals. Owing to the sensitivity of PECD to molecular conformation this allows us to identify the dominant conformer. The influence of intramolecular hydrogen bond orbital polarization is found to play a small yet significant role in determining the chiral asymmetry in the electron angular distributions.     

Small angle neutron scattering study of demixing in micellar solutions containing CTAC and a partially fluorinated cationic surfactant

Demixing of fluorocarbon and hydrocarbon surfactants to form coexisting fluorocarbon-rich and hydrocarbon-rich micelles has been studied by small angle neutron scattering in aqueous solution, using an equimolar mixture of cetyltrimethylammonium chloride and the partially fluorinated cationic surfactant N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride, with a deuterated pyridinium headgroup. Measurements have been performed under varying experimental conditions: in both pure aqueous solutions and with salt (0.10 M NaCl), at several contrasts for neutrons obtained by varying the H(2)O/D(2)O ratio, mainly at 25 degrees C but also at 60 degrees C to promote mixing of the surfactants. The experiments show that a substantial residual scattering is retained at the solvent composition where the average scattering length density of mixed micelles would match that of the solvent. It is moreover observed that, in solutions without added salt, a prominent correlation peak observed in 100% D(2)O disappears at the match point. These observations are in accordance with a very broad composition distribution, possibly composed of two populations of mixed micelles of similar sizes but different compositions, but would not result from micelles with merely a highly inhomogeneous internal structure. Increasing the temperature from 25 to 60 degrees C reduces substantially the scattered intensity at zero angle at the match point, as expected for a less broad population of mixed micelles. In the numerical analysis, the scattering data for scattering vector q > or = 0.02 A(-1) were analyzed by the indirect Fourier transform method to give the scattering at zero angle. From these data, the average micelle aggregation number was obtained as 76 at 25 degrees C and 54 at 60 degrees C. The contrast variation results for the intensity at zero angle give a measure of the width of the micelle distribution, which is obtained as sigma = 0.33 at the lower temperature and sigma = 0.20 at 60 degrees C. The result at the low temperature is compatible with the formation of two populations that are polydisperse (sigma = 0.07) and centered around 18 and 82%; other broad distributions cannot be excluded.     

Dynamics of the Rydberg electron in H*+D2-->D*+HD reactive collisions

Experimental crossed-beam studies carried out previously have indicated that the dynamics of the Rydberg-atom-molecule reaction H*+D2-->D*+HD are very similar to those of the corresponding ion-molecule reaction H++D2-->D++HD. The equivalence of the cross sections for these related systems would open up a new approach to the experimental study of ion-molecule reactions. However, a recent experimental and theoretical study has brought to light some important qualitative differences between the Rydberg-atom reaction and the ion-molecule reaction; in particular, the experimental cross section for the Rydberg-atom reaction exhibits a higher degree of forward-backward scattering asymmetry than predicted by a quasiclassical trajectory study of the ion-molecule reaction. In this paper, the authors consider the dynamics of the Rydberg-electron over the course of a reactive collision and the implications of these dynamics for the Rydberg-atom-molecule crossed-beam experiment. Using an approach based on perturbation theory, they estimate the attenuation of the experimental signal due to the Rydberg-electron dynamics as a function of the scattering angle. They show that at least part of the experimental asymmetry can be ascribed to this angle dependent attenuation. Their results offer general insight into the practical aspects of the experimental study of ion-molecule reactions by means of their Rydberg-atom counterparts.     

Optimization of complex slater‐type functions with analytic derivative methods for describing photoionization differential cross sections

The complex basis function (CBF) method applied to various atomic and molecular photoionization problems can be interpreted as an method to solve the driven‐type (inhomogeneous) Schrödinger equation, whose driven term being dipole operator times the initial state wave function. However, efficient basis functions for representing the solution have not fully been studied. Moreover, the relation between their solution and that of the ordinary Schrödinger equation has been unclear. For these reasons, most previous applications have been limited to total cross sections. To examine the applicability of the CBF method to differential cross sections and asymmetry parameters, we show that the complex valued solution to the driven‐type Schrödinger equation can be variationally obtained by optimizing the complex trial functions for the frequency dependent polarizability. In the test calculations made for the hydrogen photoionization problem with five or six complex Slater‐type orbitals (cSTOs), their complex valued expansion coefficients and the orbital exponents have been optimized with the analytic derivative method. Both the real and imaginary parts of the solution have been obtained accurately in a wide region covering typical molecular regions. Their phase shifts and asymmetry parameters are successfully obtained by extrapolating the CBF solution from the inner matching region to the asymptotic region using WKB method. The distribution of the optimized orbital exponents in the complex plane is explained based on the close connection between the CBF method and the driven‐type equation method. The obtained information is essential to constructing the appropriate basis sets in future molecular applications. © 2017 Wiley Periodicals, Inc.     

Angular-ratiometric plasmon-resonance based light scattering for bioaffinity sensing

We describe an exciting opportunity for affinity biosensing using a ratiometric approach to the angular-dependent light scattering from bioactivated and subsequently aggregated noble metal colloids. This new model sensing platform utilizes the changes in particle scattering from very small colloids, which scatter light according to traditional Rayleigh theory, as compared to the changes in scattering observed by much larger colloidal aggregates, formed due to a bioaffinity reaction. These larger aggregates no longer scatter incident light in a Cos(2) theta dependence, as is the case for Rayleigh scattering, but instead scatter light in an increased forward direction as compared to the incident geometry. By subsequently taking the ratio of the scattered intensity at two angles, namely 90 degrees and 140 degrees , relative to the incident light, we can follow the association of biotinylated bovine serum albumin-coated 20 nm gold colloids, cross-linked by additions of streptavidin. This new model system can be potentially applied to many other nanoparticle assays and has many advantages over traditional fluorescence sensing and indeed light-scattering approaches. For example, a single nanoparticle can have the equivalent scattered intensity as 10(5) fluorescing fluorescein molecules substantially increasing detection; the angular distribution of scattered light from noble metal colloids is substantially easier to predict as compared to fluorescence; the scattered light is not quenched by biospecies; the ratiometric measurements described here are not dependent on colloid concentration as are other scattering techniques; and finally, the noble metal colloids are not prone to photodestruction, as is the case with organic fluorophores.     

O1s photoionization dynamics in oriented NO2

We have performed extensive density functional theory (DFT) calculations, partial cross sections, dipole prepared continuum orbitals, dipole amplitudes and phase shifts, asymmetry parameters β, and molecular frame photoelectron angular distributions, to elucidate the O1s photoionization dynamics of NO(2) molecule with emphasis on the shape resonances in the O1s ionization continuum. In the shape resonance region, the β parameters and photoelectron angular distributions have been compared with our experimental results. Fairly good agreement between the theory and experiment has confirmed that the DFT level calculations can well describe the photoionization dynamics of the simple molecule such as NO(2). Interference due to equivalent atom photoionization is theoretically considered, and the possibility of detection of the effect in the two degenerate channels with different combinations of light polarization and photoemission direction is discussed.     

N 1s photoelectron angular distributions from fixed-in-space NO2 molecules: stereodynamics and symmetry considerations

Angular distributions of N 1s photoelectrons from fixed-in-space NO(2) molecules have been measured over the energy region of shape resonance and above. A multiple-coincidence velocity-map imaging technique for observation of molecular frame photoelectron angular distributions (MF-PADs) has been extended to nonlinear molecular targets. Density functional theory calculations have also been conducted to elucidate the photoionization dynamics and shape resonance in the N 1s photoionization of NO(2). Results show that the N 1s MF-PADs exhibit strong shape variation as a function of both photoelectron kinetic energy and symmetries of final states, whereas asymmetry parameters of laboratory frame PADs show a local minimum around the shape resonance region and increase monotonically as the photon energy increases. Over the shape resonance, the spatial shape of the photoelectron wave function with b(2)-symmetry closely resembles that of 5b(2)(?) unoccupied molecular orbital of NO(2), although the MF-PAD pattern for b(2)-symmetry does not correspond directly to the 5b(2)(?) orbital shape. At higher kinetic energy of 90 eV, MF-PADs become less structured, but still show a significant dependence on the symmetry of final states.     

The impact of ultraviolet light on bacterial adhesion to glass and metal oxide-coated surface

Biofouling of glass and quartz surfaces can be reduced when the surface is coated with photocatalytically active metal oxides, such as TiO2 (anatase form) or SnO2. We measured the attachment of eight strains of bacteria to these two metal oxides (TiO2 and SnO2), and to an uncoated glass (control; designated Si-m) before and after exposure to UV light at wavelengths of 254 nm (UVC) or 340 nm UV (UVA). TiO2-coated surfaces were photocatalytically active at both 254 and 340 nm as evidenced by a decrease in the water contact angle of the surface from 59 degrees +/-2 to <5 degrees. The water contact angle of the SnO2 surface was reduced only at 254 nm, while contact angle of the Si-m glass surface was not altered by light of either wavelength. Bacterial adhesion decreased by 10-50% to photocatalyzed glass surfaces. In all cases, bacteria exposed to the UV light were completely killed due to a combination of exposure to UV light and the photocatalytic activity of the glass surfaces. These results show that UV light irradiation of TiO2-coated surfaces can be an effective method of reducing bacterial adhesion.     

Characterization of polymer materials exposed at the Mir orbital space station

The results of studies of polyimide and fluoropolymer films after a prolonged exposure at the Mir orbital space station are reported. The weight loss of external polyimide films was ～40–60%; the weight of FEP-100A fluoropolymer films remained unchanged, and the weight of F-4MB films increased by ～50%. The external and internal surfaces of polyimide films were hydrophilized. The external surface of a polyimide film acquired anisotropic properties, which manifested themselves as the shape anisotropy of liquid drops, the surface tension and the work of adhesion, the light scattering circular diagrams, and in the formation of spatially oriented fractal structures. The appearance of anisotropic properties is a consequence of the arrangement of a film on the station surface at an angle to the flow of atomic oxygen. The possible reactions of fast and scattered oxygen atoms, which are responsible for the chemical and structural transformations of polyimide, are discussed.     

Optical studies of the stress-induced crystallization of rubber

The low-angle light scattering by films of stretched natural and synthetic rubbers was investigated. Intense V_v scattering is found under conditions when crystallization occurs which is characteristic of the scattering from aggregates of dimensions comparable with the wavelength of visible light. These were identified with the γ fibrils described by Andrews. The dependence of scattering was studied as a function of light polarization direction, orientation direction, elongation, temperature, degree of swelling, type of swelling liquid, and degree of crosslinking. It was concluded that the scattering unit consists of an assembly of crystals with their chain axes parallel to the stretching direction, but (in the case of natural rubber at high elongations) with the fibril axis at a slight angle to the stretching direction. The scattering is not affected much by swelling but is decreased upon increasing the temperature. Upon recooling the scattering returns, but does so over several hours, indicating that much of the scattering arises from secondary crystallization.     

Electronic structure and photoionization and dissociation processes of bis(trifluoromethoxy)disulfurylperoxide, CF3OS(O)2OOS(O)2OCF3

In this work, we present a complete study of the photoionization and dissociation processes for bis(trifluoromethoxy)disulfurylperoxide, CF3OS(O)2OOS(O)2OCF3, which was generated by UV photolysis of a mixture of (CF3CO)2O, SO2, and O2 at a low temperature. The reaction product was detected and characterized by the photoelectron (PE) and photoionization mass spectroscopy (PIMS). For comparison, the geometric and electronic structures of CF3S(O)2OS(O)2CF3 (a), CF3OS(O)2OS(O)2OCF3 (b), and CF3OS(O)2OOS(O)2OCF3 (c) were investigated by the combination of experiments and theoretical studies. The PES results show that the outer electrons residing in nO(S=O) of b and c are more tightly bound than those of a. It is worthwhile mentioning that drastic changes occur in the geometry of c after one-electron ionization. The neutral molecule exhibits a gauche structure with the SOOS dihedral angle of 124.4 degrees . The first ionization process happens on the O-O antibonding orbital. The remarkable geometric changes between the ground-state molecule and cation are computed to be the gauche-to-trans rotation of deltaSOOS and the prolongation of the S1-O1 single bond length. According to the calculated bond dissociation energies, the dissociation process was discussed. The calculated results indicate that once the parent ion is formed, the dissociation of the S1-O1 bond to form CF3OSO2+ is inevitable.     

THE SPATIAL DISTRIBUTION OF LIGHT EMISSION FROM LIQUID PHASE BIO- AND CHEMILUMINESCENCE: VARIATIONS WITH CONTAINER TYPES, TURBIDITY AND CONTAINER FROSTING

Abstract— The spatial distribution of emitted light from liquid phase bioluminescence and chemiluminescent sources in three common types of containers has been evaluated. These sources, while theoretically isotropic, exhibit considerable anisotropy due to reflection and refraction effects at the interfaces between the solution, the container and the surrounding air. This anisotropy represents a considerable systematic error (>25%) in some values of quantum yields reported in the literature. The degree of asymmetry in spatial distribution depends on the solution volume, the refractive index, and the degree of light scattering in the solution. Quantitative evaluation of the projected image of these sources using a video camera system indicates that a major contribution to this asymmetry is due to reflection at the meniscus. Since container frosting removes the variability due to scattering, volume changes, refractive index differences and even, to some extent, use of different types of containers, it is recommended that quantum yields and comparative measurements be determined using frosted containers and point source geometry. The container of choice is a frosted, 1 ± 7.5 cm test tube.