Polarization-dependent Spectra Research Articles

X-ray spectroscopy and spectropolarimetry of high energy density plasma complemented by LLNL electron beam ion trap experiments

X-ray spectra of high energy density (HED) Z-pinch plasmas have been analyzed by means of a comprehensive kinetic modeling. A new diagnostic, x-ray spectropolarimetry, was applied to study anisotropy of Z-pinch plasma. This diagnostic is based on theoretical modeling of polarization-dependent spectra measured simultaneously by spectrometers with different sensitivity to polarization. Specifically, K-shell emission from Ti X-pinches was recorded simultaneously with identical LiF crystal spectrometers with the dispersion plane perpendicular and parallel to the discharge axis. Spectroscopic results from seven Ti X-pinch shots have been analyzed. Similar K-shell Ti polarization-dependent spectra generated by a quasi-Maxwellian electron beam at the LLNL EBIT-II electron beam ion trap have been studied. Further, the EBIT-II M-shell W spectra have proved to be important in the development of M-shell diagnostics of HED Z-pinch plasma. The advantages provided by electron beam ion trap data in the interpretation of HED Z-pinch spectra will be presented.

Orientation and charge transfer upon adsorption of ethanethiol on Cu( [formula omitted]) surface at 85 K

Orientation and charge transfer upon adsorption of ethanethiol on Cu(1 1 1) surface at 85 K has been investigated by S K-edge near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) techniques. Exposure-dependent S K-edge NEXAFS identified the monolayer saturation at ∼0.8 L exposure. Polarization-dependent NEXAFS spectra of submonolayer ethanethiol shows that S–C bond is tilted 37±7° from the surface. Temperature-dependent NEXAFS spectra shows that ethyl thiolate starts breaking at 300–350 K and atomic sulfur creates. A significant amount of charge transfer (1.4 electrons) from copper to ethanethiol molecules has measured by S 1s XPS technique.

Monolayer Formation and In-Plane Anisotropy of an Amphiphilic Block Molecule

Abstract Coverage controlled monolayer nano-sheets with homogeneous height were fabricated by casting aqueous solution of amphiphilic rod-coil diblock molecule 1 on mica surface. The monolayer sheet was anisotropic, due to the asymmetric arrangement of the diblock molecule on surface, confirmed by polarization-dependent fluorescence spectra.

Polarization-dependent spectra in the photoassociative ionization of cold atoms in a bright sodium beam

We report measurements of cold photoassociative ionization (PAI) spectra obtained from collisions within a slow, bright Na atomic beam. A high-brightness atom flux, obtained by optical cooling and focusing of the atom beam, permits a high degree of alignment and orientation of binary collisions with respect to the laboratory atom-beam axis. The results reveal features of PAI spectra not accessible in conventional magneto-optical trap studies. We take advantage of this high degree of alignment to selectively excite autoionizing doubly excited states of specific symmetry.

Polarization-Dependent EXAFS Measurements of an α-Molybdenum Trioxide Single Crystal

We observed the polarization-dependent XAFS of an α-MoO3 single crystal. Polarization-dependent XANES spectra change with the polarization direction and the 1s → 4d pre-edge peak is shown to be a good indicator for the Mo=O (double bond oxygen) bond direction. The three Mo–O bond lengths at 0.175, 0.195 and 0.223 nm can clearly be distinguished in the Fourier transforms of polarization-dependent EXAFS spectra. Curve-fitting analysis was carried out on a powder MoO3 spectrum based on the phase shift and amplitude functions derived from the polarization-dependent EXAFS spectra.

Polarization dependence of resonant X-ray emission spectra in early transition metal compounds.

We calculate the polarization dependent spectra of resonant X-ray emission in TiF3, VF3 and Cr2O3, which have one, two and three 3d electron(s) in the ground state, respectively. We study the detailed mechanism of the spectral structures, comparing with a previous result of TiO2. Then we discuss systematically the difference of spectra with the change of the 3d electron number from group theoretical consideration.

X-ray spectropolarimetry of high-temperature plasmas

X-ray spectropolarimetry is a powerful new tool for investigating the anisotropy of high-temperature plasmas. It is sensitive to the energetic electron distribution function and magnetic field, which, in general, have not been measured adequately. This diagnostic is based on the theoretical modeling of x-ray line polarization-dependent spectra measured simultaneously by spectrometers with different sensitivities to polarization. The increasing sophistication of spectroscopic monitoring of high-temperature plasmas has led to the necessity to account for both non-Maxwellian electron distribution functions and polarization properties of lines. This diagnostic can be applied to a broad range of plasmas, from low-density astrophysical plasmas to dense z-pinch plasmas. This work shows how x-ray spectropolarimetry complements the usual spectroscopic monitoring of hot plasmas and demonstrates the importance of accounting for x-ray line polarization in plasma diagnostics.

NEXAFS investigation of benzaldehyde reductive coupling to form stilbene on reduced surfaces of TiO 2(0 0 1)

Near-edge X-ray absorption fine structure (NEXAFS) was used to investigate the reaction of benzaldehyde on reduced surfaces of TiO 2(0 0 1) to form stilbene by reductive coupling of the carbonyl groups. Reductive coupling of carbonyls (commonly referred to as the “McMurry reaction”) has been extensively studied in both liquid slurries containing reduced metals and as a gas–solid reaction on reduced surfaces of titania. The reactive intermediate proposed for both slurry and surface chemistry is a metal pinacolate. Previous investigations of this chemistry on reduced TiO 2(0 0 1) have yielded circumstantial evidence supporting such an intermediate on the surface. However, spectroscopic evidence has proven difficult to obtain. In the current study, we employ NEXAFS to probe the chemical identity and orientation of reactive intermediates in the reductive coupling reaction further. By heating benzaldehyde-covered surfaces of TiO 2(0 0 1) to progressively higher temperatures and recording polarization-dependent NEXAFS spectra at the Ti L-edge and the O and C K-edges, one can track the progression of the reaction from benzaldehyde to the product, stilbene. These results support an adsorbate structure consistent with a pinacolate intermediate.

Non-linear optically active metal clusters in nanoscaled systems including self-assembled organic films

Linear and non-linear optical spectroscopies are applied to monitor the formation of large, several 10-nm-sized alkali clusters near epitaxially grown Au films on mica substrates. The Au films are covered by ultrathin, self-assembled alkane thiol films. The growth and morphology of these clusters are initially monitored in ultrahigh vacuum by comparison of calculated with measured polarization-dependent extinction spectra. We find that at low surface temperatures (150 K) the cluster growth is very similar to growth directly on insulating substrates. With increasing surface temperature the size distribution of the clusters changes. A quantitative evaluation of ambient-air measurements with scanning force microscopy (SFM) supports the conclusions from optical spectroscopy. Field-enhancement effects at the surface of the clusters facilitate the observation of second harmonic (SH) light. From angular- and polarization-dependent SH-measurements we deduce ratios of χ (2)-tensor components for the cluster-films.

Self-enhanced diffraction properties of a nominally undoped barium titanate crystal with a thermally fixed grating

We describe recent studies on diffraction properties of a photorefractive barium titanate crystal having a thermally fixed grating. Buildup dynamics of diffracted signals are measured to analyze the contributions of ionic and re-created photocarrier (hole) gratings to the temperature-dependent diffraction efficiency in self-enhanced readout. An activation energy of mobile ions as nonphotoactive secondary carriers for fixing volume holograms is determined holographically. This value is compared with that of hydrogen ions estimated from polarization-dependent infrared absorption spectra in order to indentify the fixing species.

Resonance fluorescence spectra and squeezing properties of a bichromatically driven four-level atom

The polarization-dependent resonance-fluorescence spectra of a bichromatically driven four-level atom are studied. It is shown that very narrow spectral lines can occur in the incoherent part of the spectra for polarization directions that are different from that of the driving field. Squeezing properties of the fluorescent light are also investigated. The degree of squeezing depends on the polarization direction and reaches a maximum of remarkably $56%.$

Polarization properties of dielectronic satellite lines in the K-shell x-ray spectra of B-like Fe XXII

We report the results of a combined theoretical and experimental study of B-like dielectronic recombination satellite lines excited by an electron beam. Polarization sensitive spectra were accumulated at the Lawrence Livermore electron beam ion trap facility. Three beam energies were chosen in the region of maximum B-like Fe satellite emission. At each energy, spectra were obtained using two LiF crystals of differing polarization sensitivities. Polarization-dependent spectra have been calculated using the degree of polarization of single lines computed with a photon density matrix formalism. The agreement between the experiment and theory is very good. Two spectral features were observed to have polarizations that were particularly sensitive to the electron beam energy. Spectral features with this property are candidates as diagnostics of energy and direction of electron beams in plasmas.

Theory of bound-to-continuum infrared absorption inp-type quantum wells based on a mapping of the continuum spectrum

Theoretical treatment of the infrared absorption in $p$-type quantum wells (QW's) is important because of the possibility of selection rule breaking for the in-plane polarized light. Here, we present the theory of the bound-to-continuum $(\stackrel{\ensuremath{\rightarrow}}{B}C)$ linear absorption in $p$-type QW's. The anisotropy of the hole spectra, fully incorporated in the calculations, is responsible for the in-plane optical anisotropy which varies with the layer orientation. In order to simplify the problem of $\stackrel{\ensuremath{\rightarrow}}{B}C$ absorption it is accepted to put the QW in an artificial additional enclosure. In this way one can reduce the calculational problem to the bound-to-bound one, but the results depend on the artificial parameter, i.e., the size of this enclosure. This problem is known as the ``artificial quantization.'' The rate of optical excitations is calculated here after mapping the continuum states at the asymptotic limit of an infinitely wide external enclosure. An advantage of this approach is the avoidance of any artificial quantization. Also we eschew computational complexity associated with processes related to the fine energy mesh in a finite enclosure. These processes are replaced by a proper phase parametrization followed by a statistical averaging. As an example we present the polarization-dependent absorption spectra of GaAs/AlGaAs QW's for both the $〈001〉$ and $〈011〉$ orientations.

A single crystal, linearly polarized Fe 2p X-ray absorption study of gillespite

AbstractThe Fe 2p X-ray absorption spectra of single crystal gillespite, BaFeSi4O10, show a strong linear dichroism, i.e. a large difference in the absorption when measured with the polarization of the X-rays either parallel or perpendicular to the plane of the FeO4 group. The isotropic spectrum, obtained from measurement at the ‘magic angle’, and the polarization dependent spectra have been compared to atomic multiplet calculations and show an excellent agreement with theory. Analysis of the branching ratio, the linear dichroism, and the detailed peak structure confirms that the 5A1 level is the ground state at room temperature and pressure. The 5B2 level is sufficiently low in energy that a distortion of the electronic charge density, induced by increased pressure, may result in a 5B2 ground state.

Polarization-dependent spectra of x-ray dielectronic satellite lines of Be-like Fe

We have studied the polarization properties of dielectronic satellite lines in Be-like Fe ions excited through resonant electron capture by an electron beam. Using the photon density-matrix formalism, we have calculated the degree of polarization and polarization-dependent spectra of dielectronic satellite lines, i.e., the spectral intensity distribution of lines associated with a given polarization state. Theoretical results have been compared with experiments performed at the Lawrence Livermore National Laboratory electron-beam ion trap where dielectronic satellite line emission from Fe ions produced at different energies of the electron beam was simultaneously recorded with two crystal spectrometers. These spectrometers had different polarization sensitivities. The experimental spectra recorded by the two spectrometers are reproduced by the theory. This ability to model the polarization dependence of x-ray line spectra is important for the diagnosis of electron beams in plasmas.

Theoretical study of polarization properties of dielectronic satellite lines of Be-like Mg and Fe ions

We have calculated the polarization properties of dielectronic satellite lines in Be-like Mg and Fe ions excited by an electron beam. Polarization-dependent synthetic spectra have been computed associated with two polarization states: parallel and perpendicular to the electron beam. We will discuss the dependence of the polarization-dependent spectra on the characteristics of the electron beam and on the nuclear charge of the ion. These results are relevant to diagnostics applications of x-ray line polarization spectroscopy in non-equilibrium plasmas whose electron distribution functions can show anisotropy.

Theoretical investigation of two-color, two-photon, 6s 2S1/2-->5d 2Dj-->11p 2P3/2 excitation and depolarization spectra in atomic Cs.

Results of a theoretical investigation of two-photon, two-color excitation and linear depolarization spectra in atomic cesium are presented. The case of two-beam excitation, where one photon is absorbed through a quadrupole amplitude and the other through a dipole amplitude, is considered. When excitation is through a spin multiplet, the final-state population produced depends sensitively on the relative transition amplitudes through the nondengenerate spin-multiplet terms. In addition, the transition rate contains interference terms depending on the relative directions of the two beams. These terms are manifested strongly when the intermediate states are nonresonant and when the two beams have approximately the same frequency. Application of the results to the 6s${\mathrm{}}^{2}$${\mathit{S}}_{1/2}$\ensuremath{\rightarrow}5d${\mathrm{}}^{2}$${\mathit{D}}_{\mathit{j}}$\ensuremath{\rightarrow}11p${\mathrm{}}^{2}$${\mathit{P}}_{3/2}$, quadrupole-dipole transition in atomic Cs is made. Polarization-dependent excitation spectra are presented for this case. Effects of hyperfine interactions are also considered.

Serpentine superlattice nanowire-array lasers

We report characterization and modeling of serpentine superlattice nanowire-array lasers. These samples were grown by molecular beam epitaxy on (100) n/sup +/-GaAs vicinal substrates. In-plane ridge-waveguide lasers with ridge stripes either parallel or perpendicular to the nanowire arrays have been characterized at low temperatures. The measured net gain spectra at 1.4 K showed strong optical gain anisotropy such that the TM mode gain became greater than the TE mode gain when the optical cavity was placed along the nanowire direction. This provides strong evidence that the lateral quantum confinement in the serpentine superlattice is stronger than the vertical quantum confinement. Optical gain spectra in the serpentine superlattice are calculated with consideration of coupling between wires and homogeneous line broadening. A good fit to the measured polarization-dependent gain spectra is achieved when the lateral Al segregation and the homogeneous line broadening are chosen to be 8% and 7 meV, respectively. This small but finite Al segregation in the serpentine superlattice provides lateral quantum confinement for holes, which results in significant anisotropy in the relation between net gain and injection current density,.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reduction of etching damage in the fabrication of quantum wires

Quantum wire structures were fabricated by patterning quantum well samples with electron beam lithography and various wet chemical etching procedures. Wire structures with 800 Å wire width were achieved by wet etching in NH 4OHH 2O 2H 2O (20:7:973). These samples were characterized by scanning electron microscopy, photoluminescence (PL), and polarization-dependent photoluminescence excitation (PLE) measurements. The PL spectra show significantly stronger peaks than that taken from an unetched quantum well sample. A wire width of 400 Å was estimated from the blue shift of PL peaks. A 22% anisotropy was observed from polarization-dependent PLE spectra, further indicating the existence of two-dimensional quantum confinement.

Polarization spectra of excited-state-Mg(3p)-rare-gas-atom optical collisions.

Experimental, polarization-dependent excitation spectra for excited-state-Mg--rare-gas-atom optical collisions are reported. In these first studies of the process, polarized Mg atoms in the 3p $^{1}$${\mathit{P}}_{1}$ level are produced by absorption of linearly polarized light tuned to the 3s $^{1}$${\mathit{S}}_{0}$\ensuremath{\rightarrow}3p $^{1}$${\mathit{P}}_{1}$ resonance transition at 285.2 nm. Detuning-dependent, collision-induced polarization spectra are measured in a \ifmmode\pm\else\textpm\fi{}200-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range around the Mg 3p $^{1}$${\mathit{P}}_{1}$\ensuremath{\rightarrow}5s $^{1}$${\mathit{S}}_{0}$ transition at 571.2 nm. The spectra correspond to probing transient Mg-Ne and Mg-Ar molecules on 3p $^{1}\mathrm{\ensuremath{\Pi}}_{1}$\ensuremath{\rightarrow}5s $^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}$ and 3p $^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}$\ensuremath{\rightarrow}5s $^{1}\mathrm{\ensuremath{\Sigma}}_{0}^{+}$ electronic transitions. Measurements of these excited-state polarization spectra for Mg-Ne optical collisions reveal that for detunings to the red of the atomic Mg 3p $^{1}$${\mathit{P}}_{1}$\ensuremath{\rightarrow}5s $^{1}$${\mathit{S}}_{0}$ transition, electronic linear polarization greater than 50% survives far into the molecular regime. This represents a direct measure of the polarization important to alignment-dependent inelastic processes in alkaline-earth-metal--rare-gas-atom collisions. The polarization spectra are discussed in terms of existing information on the interatomic potentials and through an axial recoil limit for the polarization degree for parallel and perpendicular molecular transitions. Rate coefficients k for disalignment of Mg 3p $^{1}$${\mathit{P}}_{1}$ atoms by collisions with Ar [k=9.4(5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$/s] and with Ne [k=6.5(7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ ${\mathrm{cm}}^{3}$/s] are also extracted from the data.