Unpolarized Radiation Research Articles

Pretilted and grating alignment of liquid crystals by oblique ultraviolet irradiation of polyimide

We discuss the formation of pretilt by oblique irradiation of photosensitive polymers that homogeneously align liquid crystals in the direction corresponding to the maximum density of unreacted polymer. We show that a tilted polymer distribution is formed by oblique illumination with one beam of unpolarized or partially polarized light. When unpolarized radiation is used, a small azimuthal anisotropy is generated and the polymer tilt angle increases with incident angle. Pretilted liquid crystal alignment is possible up to a threshold angle at which the azimuthal alignment direction changes. Irradiation with partially polarized light gives a higher azimuthal anisotropy but smaller polymer tilt angles. We obtain liquid crystal pretilt angles up to 4° by oblique irradiation of polyimide with light from an excimer laser operating below the threshold for ablation. A simple two step process is used to obtain pretilted alignment for liquid crystals using surface gratings: a grating etched into polyimide by laser ablation is subsequently irradiated at oblique incidence to give planar liquid crystal alignment with a pretilt angle of 3°.

Phase anomalies in fourier-transform spectrometers: an absorbing beam splitter is neither sufficient nor necessary.

Recently it has been a topic of some discussion that the phase associated with part of the self-emission of a Fourier-transform spectrometer may differ neither by 0 nor by pi rad from the phase of an external target if there is absorption in the beam splitter. The conventional interpretation of this has been to separate the self-emission into three terms: instrument emission from the input port, in phase with the external target; emission from the secondary input port, pi rad from the target; and emission from an absorbing beam splitter with an anomalous phase (neither 0 nor pi rad with respect to the target). There is another necessary condition that has not received much attention, that the instrument not be isothermal. For polarized radiation there is an additional condition that suppresses the anomalous phase, and for unpolarized radiation there is a way to produce an anomalous phase without beam-splitter absorption.

The effect of pulse-periodic laser radiation on polyvinylchloride

Changes in the conductivity, permittivity, and dielectric loss tangent are registered for ten polyvinylchloride-based organic dielectrics (polymers), caused by the exposure for 3–5 s to the unpolarized radiation of a pulsed copper-vapor laser with a wavelength of 510.6 nm. The duration of light pulses is 20 ns, and their repetition frequency, 8.5 kHz.

The Effect of Unpolarized Laser Radiation on Organic Dielectrics

Unpolarized radiation from the active species of a copper vapor laser changed the conductivity, dielectric permittivity, and the loss tangent of 14 organic dielectrics (polymers) including PMMA and polyethylene- and poly(vinyl chloride)-based materials. Light pulses of 20 ns duration, mostly at a wavelength of 510.6 nm, illuminated an entire sample by a large spot for 3–5 s, so that the radiation intensity per pulse at the surface was as high as 1.7–8.6 kW/cm2. After irradiation, the conductivity of all materials increased. The electric parameters irreversibly varied in some dielectrics, whereas slow (of many hours) relaxation back to the initial parameters or to new equilibrium values was observed in other materials.

Magnetic dichroism in Co films on Cu(001) using unpolarized light

The thickness and temperature dependence of the angular distribution of the magnetic dichroism in photoemission from Co 2p core levels of Co/Cu(001) ultra-thin films has been investigated, using unpolarized Mg Kα radiation. Photoelectron spectra were collected for emission angles in a range of ±9° around the surface normal in the [001] azimuth. All asymmetry spectra were found to exhibit an identical spectral shape within the experimental error bar, but different scaling. The angular distribution of the Co 2p dichroic signal shows that at emission directions off the sample normal strong variations are present, in contrast to predictions from simple atomic theory. These crystallinity-related modulations were studied for Co films of different thicknesses [5, 3, 2, and 1.5 atomic monolayers (ML)] and different temperatures (100 and 300 K). They are higher for thicker films and lower temperatures. This experimental finding proves a diffraction picture, in which this modulation around the surface normal is explained by diffraction effects in forward scattering along the sample normal. As diffraction effects in magnetic dichroism combine both diffraction and magnetic dichroism, its behavior is influenced by both, structural and magnetic properties of the film.

The topology of the Fermi surface of Bi 2Sr 2CaCu 2O 8−δ from angle resolved photoemission

We present a study of the topology of the normal state Fermi surface (FS) of the high T c superconductor Bi 2Sr 2CaCu 2O 8−δ (Bi2212) using angle-resolved photoemission. We present FS mapping experiments, recorded using unpolarised radiation with high (E, k) resolution, and an extremely dense sampling of k-space. In addition, synchrotron radiation-based ARPES has been used to prove the energy independence of the FS as seen by photoemission. We resolve the current controversy regarding the normal state FS in Bi2212. The true picture is simple, self-consistent and robust: the FS is hole-like, with the form of rounded tubes centred on the corners of the Brillouin zone. Two further types of features are also clearly observed: shadow FSs, and diffraction replicas of the main FS caused by passage of the photoelectrons through the modulated BiO planes.

Circular polarization induced by scintillation in a magnetized medium

A theory is presented for the development of circular polarization as radio waves propagate through the turbulent, birefringent interstellar medium. The fourth order moments of the wave field are calculated, and it is shown that unpolarized incident radiation develops a nonzero variance in circular polarization. A magnetized turbulent medium causes the Stokes parameters to scintillate in a nonidentical manner. A specific model for this effect is developed for the case of density fluctuations in a uniform magnetic field.

Influence of Solid State Environment Effects on Measured Elastic Scattering Cross Sections in the X-ray Regime and the Associated Anomalous Dispersion

Experimental elastic scattering cross sections in the atomic region 21 ≤ Z ≤ 82 are measured at an angle of 90° using monoenergetic unpolarised Mo Kα radiation. The measurements are based on a new method developed to estimate the degree of monochromaticity, the efficiency of the detecting system solid angle correction and some considerations which are necessary in experiments using secondary targets as excitation sources. Unpolarised monoenergetic radiation is produced using an X-ray tube and a secondary target. To reflect the data on the current knowledge, the results are compared with theoretical estimates based on form factor formalisms including the anomalous scattering factors and with the recent S-matrix values. Based on the experimental evidence, the present results indicate the influence of solid state environmental effects, the importance of anomalous corrections near thresholds, superiority of S-matrix values over form factor values, correctness of high energy limit values on measured elastic scattering cross sections. However, the comparison of the experimental results with theoretical estimates indicates the importance of numerical calculations based on the multipole expansion of the S-matrix approach and high energy limit values.

Normal-state Fermi surface of pristine and Pb-doped Bi2Sr2CaCu2O8+δ from angle-resolved photoemission measurements and its photon energy independence

We address the question as to whether the topology of the normal state Fermi surface of Bi2212 - as seen in angle resolved photoemission - depends on the photon energy used to measure it. High resolution photoemission spectra and Fermi surface maps from pristine and Pb-doped Bi2212 are presented, recorded using both polarised and unpolarised radiation of differing energies. The data show clearly that no main band crosses the Fermi surface along the GMZ direction in reciprocal space, even for a photon energy of 32 eV, thus ruling out the existence of a G-centred, electron-like Fermi surface in this archetypal high Tc superconductor. The true topology of the normal state Fermi surface remains that of hole-like barrels centred at the X,Y points of the Brillouin zone.

Application of 2D IR correlation analysis to phase transitions in Langmuir monolayer films

The surface pressure-dependent conformational state of a monolayer film of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) at the air–water (A/W) interface was studied using infrared external-reflection spectroscopy and two-dimensional infrared (2D IR) correlation analysis. When the IR spectra of the DPPC monolayer was collected using polarized IR radiation, a band splitting was observed in both the antisymmetric ( ν a) and symmetric ( ν s) methylene CH 2 stretching modes that was not observed with unpolarized radiation. This band splitting was interpreted as being due to the presence of co-existing ordered and disordered conformational states, however, definitive identification of conformational sub-bands is problematic due to the low signal-to-noise inherent in the polarized IR spectra. To further investigate the spectral changes observed in the CH region, 2D IR correlation analysis was applied to a set of pressure-dependent unpolarized IR spectra of the DPPC monolayer. When these unpolarized spectra were analyzed using 2D IR methods, the 2D asynchronous correlation spectrum of the DPPC monolayer clearly showed that cross peaks attributable to the ν a and ν s CH 2 bands both split into two components, in agreement with the polarized IR monolayer spectra. Since band splitting in 2D IR spectra may be due to several causes, computer simulations were undertaken to help elucidate the exact cause of the observed splitting in the DPPC 2D asynchronous spectrum. Synthetic monolayer IR spectra were calculated for two limiting cases. The first was a ‘frequency shifting’ model in which a single band underwent a simple frequency shift. The second limiting case was an ‘overlapped peaks’ model in which an overall vibrational band was calculated as the sum of two individual sub-bands whose frequencies remained constant, but whose relative intensities changed through the simulated monolayer transition. The results of the computer simulations indicated that a simple frequency shift could be distinguished in the 2D asynchronous spectrum by the presence of a quartet of cross peaks, two with positive correlation intensities, and two with negative. In addition, a curved elongation of these cross peaks along the diagonal was associated with this frequency shift. In contrast, the 2D asynchronous spectrum for two overlapped peaks resulted in a correlation intensity cross peak doublet, one positive and one negative with no elongation along the diagonal. The experimentally measured 2D IR asynchronous correlation spectrum for the DPPC monolayer closely resembled the computer-simulated spectra for the ‘overlapped peaks’ model. Therefore, the origin of the band splitting in the ν a and ν s CH 2 bands in the 2D asynchronous spectrum is due to overlapping sub-bands that represent the ordered and disordered conformational states of the monolayer. Furthermore, these results also support the interpretation that the sub-bands observed in the polarized monolayer IR spectra are correlated with ordered and disordered monolayer states.

Joys and Pitfalls of Fermi Surface Mapping inBi2Sr2CaCu2O8+δUsing Angle Resolved Photoemission

On the basis of angle-scanned photoemission data recorded using unpolarized radiation, with high (E,k) resolution, and an extremely dense sampling of k space, we resolve the current controversy regarding the normal state Fermi surface (FS) in Bi(2)Sr(2)CaCu(2)O(8+delta). The true picture is simple, self-consistent, and robust: the FS is holelike, with the form of rounded tubes centered on the corners of the Brillouin zone. Two further types of features are also clearly observed: shadow FSs, which are most likely to be due to short range antiferromagnetic spin correlations, and diffraction replicas of the main FS caused by passage of the photoelectrons through the modulated Bi-O planes.

Accretion funnels in AM Herculis systems -- I. Model characteristics

ABSTRA C T We present calculations of the thermal structure of accretion funnels heated by hard X-ray emission from the accretion shock, soft X-ray reprocessed radiation from the white dwarf surface and magnetic heating in the matter‐field interaction region close to the orbital plane. The calculations determine self-consistently the thermal structure of the funnel, allowing for radiative transfer, electron scattering and the trapping of radiation within the funnel, and represent a substantial improvement on previous purely kinematical models. We show that, while models that allow only for X-ray heating can explain the observed intensities of the Balmer lines, they cannot, at the same time, explain the intensities of the He i and He ii lines in the optical spectra. These lines appear to be formed mainly in the magnetically heated transition region near the orbital plane, with this region playing a role similar to the hotspot in accretion discs. We show that, with the inclusion of this region, models can be constructed that are in close agreement with the optical line and continuum emission observed in AM Herculis systems: that is, they exhibit a flat or inverted Balmer decrement, He i lines, a strong He ii l 4686 line and complex emission-line profiles which vary dramatically in velocity and shape over the orbital period of the white dwarf. We also show that the continuum emission from the accretion funnel provides an important source of unpolarized background radiation, which reduces the degree of polarization of the cyclotron radiation from the accretion shocks, and produces the polarization standstills that are a well-known characteristic of these systems.

Quantitation of Secondary Structure in ATR Infrared Spectroscopy

Polarized attenuated total reflection infrared spectroscopy of aligned membranes provides essential information on the secondary structure content and orientation of the associated membrane proteins. Quantitation of the relative content of different secondary structures, however, requires allowance for geometric relations of the electric field components ( E x, E y, E z) of the evanescent wave, and of the components of the infrared transition moments, in combining absorbances ( A //; and A ⊥) measured with radiation polarized parallel with and perpendicular to, respectively, the plane of incidence. This has hitherto not been done. The appropriate combination for exact evaluation of relative integrated absorbances is A // + ( 2E z 2/ E y 2 − E x 2/ E y 2) A ⊥ , where z is the axis of ordering that is normal to the membrane plane, and the x-axis lies in the membrane plane within the plane of incidence. This combination can take values in the range approximately from A // − 0.4A ⊥ to A // + 2.7A ⊥ , depending on experimental conditions and the attenuated total reflection crystal used. With unpolarized radiation, this correction is not possible. Similar considerations apply to the dichroic ratios of multicomponent bands, which are also treated.

Zeeman spectroscopy of the Be acceptor in GaAs to intermediate fields

Magneto-spectroscopy to 17.5 T of the Be acceptor impurity in GaAs in the Faraday configuration using unpolarised radiation is reported. One strong component of the G-line is observed. This moves slowly with field at ∼0.24 cm −1/T . All eight transitions expected for the D-line are observed, allowing some of the splittings of the field-induced components of the ground and the second excited states to be determined.

Heterojunctions utilizing CuInxGa1−xTe2 thin films

Polycrystalline CuInxGa1−xTe2 thin films are prepared by pulsed laser evaporation. The room-temperature hole densities and mobilities of the films are determined. It is established that direct optical contact of the postgrowth surface of such films with the surface of a cleaved InSe wafer exhibits the photovoltaic effect. The spectra of the relative quantum efficiency of photoconversion of the heterojunctions are investigated as a function of the composition of the CuInxGa1−xTe2 films and the photodetection geometry. It is concluded that the fabricated heterojunctions have potential applications in photodetectors of unpolarized radiation.

Diffuse radiation intensity propagating through a particulate slab

A multiple-scattering approach is used to solve the radiative transfer equation in order to describe the angular and optical depth dependence of diffuse intensity propagating through an unsupported particulate slab perpendicularly illuminated with unpolarized electromagnetic radiation. Boundary reflections are taken into account for the unscattered radiation, as is each of the successive reflection and scattering orders, which are characterized by corresponding average path-length parameters and forward-scattering ratios. The optical depth dependence of the reflection- and scattering-order coefficients is displayed for given values of the particle size parameter, particle volume fraction, relative refractive index, and optical thickness of the slab. Diffuse intensity patterns at a given optical depth, and the angular dependence of the reflected and transmitted intensities, are also considered.

Magnetic dichroism in valence-band photoemission spectroscopy of disordered bccFexCo1−xalloys

A fully relativistic calculation of spin-resolved valence-band photoemission spectra for bcc ${\mathrm{Fe}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}$ alloys is presented that is based on the Korringa-Kohn-Rostoker coherent-potential approximation method of band-structure calculation. The result for the spin-integrated spectra for unpolarized radiation was found to be in very satisfying agreement with corresponding experimental data. For the spin-resolved spectra a pronounced magnetic circular dichroism is predicted that should be observable in a corresponding experiment.

A new method to estimate elastic scattering cross sections in the X-ray region and the associated anomalous dispersion

Using nearly monoenergetic unpolarised Kα radiation, we have measured the elastic scattering cross sections for Pt, Au and Pb in the energy region 5.41 ≤ E ≤ 9.23 keV. The experimental results for Pt, Au and Pb are new. A new X-ray scattering system was assembled using an X-ray tube and secondary targets. In order to improve the peak-to-background ratio and the efficiency of the detection system, particularly in the low-energy region, excitation source, sample and detector assembly were placed in a vacuum chamber at a pressure of 10 −2 mbar. The present system helped considerably in reducing the scattering and background effects, and in removing the interference of Ar K X-rays which fall in the investigated regions. A new method was developed to estimate the degree of monochromaticity, geometrical effects of the measuring system, solid angle correction and some considerations which are necessary in experiments using X-ray tube with secondary target arrangement. Experimental elastic scattering cross sections are compared with theoretical estimates, a combination of relativistic modified form factors with angle independent anomalous scattering factors (RMF + ASFs) and recent theoretical estimates based on the multipole expansion of the second order S-matrix approach. Deviations are observed around the edges (high energy side) indicating the importance of the dispersion correction terms. The present experimental results are useful taking into account that the available calculated values describe the free atom and deviations for a solid have to be expected. This is due to the difference of the electronic structure of the solid in comparison to the free atom as a base for theoretical estimations. However, the comparison of experimental results with theoretical estimates indicates the importance of numerical calculations based on the multipole expansion of the second order S-matrix approach and high energy limit values (RMF + ASFs).

A search for optical polarization from the narrow-line Seyfert 1 galaxy RE J1034+396

We present spectropolarimetry data on RE J1034+396, an ultra-soft X-ray narrow-line Seyfert 1 galaxy, and find an upper limit of similar to 0.4 to the linear polarization in the optical band. This suggests that there is no synchrotron emission in this AGN, and thus it is unlikely that RE J1034+396 is related to BL Lac objects. Furthermore, any other polarization arising from transmission through dust, or reflection from dust and electrons, must be cancelled out by geometrical effects or diluted to a high degree by unpolarized radiation.

Determination of directional optical properties of an electrochromic device

In this paper we discuss the methods for measuring directional optical properties of coated glass samples. Spectral data and reflectance and transmittance coefficients for visible daylight and solar radiation are presented, obtained for unpolarised radiation at (near-)normal and oblique (30° and 60°) incidence on an electrochromic device in the bleached state as well as in the coloured state. At low angles, the reflectance in the bleached state is larger as compared with the reflectance in the coloured state. The opposite is the case at larger angles. Furthermore, the results demonstrate a stronger angular dependency of the optical properties in the coloured state as compared with those in the bleached state. Comparing visible light transmittance with solar transmittance, coloration had a greater effect on the solar transmittance.