Doppler-broadened Spectra Research Articles

Computerized Data Reduction and Analysis in Positron Annihilation Coincidence Doppler Broadening Spectroscopy

Positron annihilation spectroscopy is a sensitive probe for studying the electronic structure of defects in solids. The high-momentum part of the Doppler-broadened spectra can be used to distinguish different elements at the annihilation site. This can be achieved by using a two-detector coincidence system, which reduces the peak to background ratio dramatically. The coincident events have to be extracted from a two-dimensional spectrum that is recorded by two high-purity germanium detectors. For this purpose the computer program MePASto was developed, which allows an automated data reduction from such Doppler-coincidence spectra, supplemented by a post-processing unit for data analysis. Additionally a case study of the identification of defect sites in an intermetallic compound is presented.

Trace gas monitor based on difference frequency generation at 4�?m using mass-production laser diodes as pump and signal light sources

New pump and signal laser sources for difference frequency generation (DFG) at 4 μm are described. A laser diode with a 980 nm fiber Bragg grating and a 785 nm Fabry-Perot (FP) laser diode were coupled into an optical fiber and mixed in a periodically poled Mg-doped lithium niobate (PPMgLN ) crystal, resulting in efficient mid-IR DFG. The DFG power was measured to be 0.23 μW for a pump power of 5 mW and a signal power of 50 mW with a slope efficiency of 0.92 mW/W2. A Doppler-broadened absorption spectrum of N2O at 2485.2 cm-1 (3.927 μm) was observed in a 0.1 m-long gas cell at a pressure of 133 Pa. The spectral linewidth of the DFG source was estimated to be 161 MHz (FWHM) for an averaging time of 700 ms. Real-time monitoring of N2O in a multipass cell with an optical path length of 36 m at a concentration level of 1 ppm was demonstrated.

Positron annihilation study of the O–T phase transition for Eu 1+ xBa 2− xCu 3O 7− δ superconductors

The effects of Eu substituting for Ba on structural and superconducting properties were investigated for a series of superconductors Eu 1+ x Ba 2− x Cu 3O 7− δ ( x=0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, and 0.2). With Eu content increasing, the random substitution of excessive Eu for Ba ion forms a Eu–Ba non-stoichiometric distribution on Ba site, which deviates from the ideal 123 phase stoichiometry. The analysis of crystallographic structure shows that the solid solubility x of Eu on Ba site is over 0.2. Eu 1+ x Ba 2− x Cu 3O 7− δ take on an orthorhombic–tetragonal (O–T) phase transition from x=0.06, and end in x=0.18. Corresponding to the O–T phase transition, superconducting critical temperature T c rapidly decreases with Eu content increasing, which is attributed to the decline of carrier density and the proximity effects of non-superconducting clusters. The positron annihilation measurements show anomalous changes for lifetime spectra and Doppler-broadened radiation spectra when the O–T phase transition occurs. By the model of excessive oxygen defects in Cu–O chain regions, we discuss effects of Eu–Ba non-stoichiometric distribution on structural and superconducting properties.

Performance Characteristics of Narrow Linewidth Fiber Laser Pumped Mid-IR Difference Frequency Mixing Light Source for Methane Detection

A high-power, narrow-linewidth Yb fiber laser with a fiber Bragg grating (FBG) pumped difference frequency generation (DFG) in a periodically poled lithium niobate (PPLN) crystal was investigated in detail. A mid-IR power of ∼2.3 µW at 3.3 µm with a slope efficiency of 0.85 mW/W2 was achieved. A Doppler-broadened absorption spectrum of CH4 at 3038.497 cm-1 (3.2911 µm) was obtained with a 0.1-m long-gas cell at a pressure of 133 Pa. The linewidth of the DFG source was evaluated to be less than 96 MHz from the observed spectral linewidth. Real-time monitoring of CH4 (∼1.78 ppm) in ambient air in a multipass cell which has an optical path length of 10 m was also demonstrated.

Optimization of measurement parameters in Doppler broadening spectroscopy

The choice of regions of interest (ROIs) in the annihilation gamma line spectrum, which define the S and W parameters commonly used in the study of vacancy-type defects in Si, has been optimized by simulations using theoretical Doppler-broadened spectra. The figure of merit used is the difference between the S( W) parameter and its value in bulk Si expressed as multiple of the standard deviation in S( W). The optimum ROIs depend on the crystalline orientation of the Si and the energy resolution of the gamma ray detector used. Experimental data on B +-implanted Si were found to be consistent with the simulation results, which can therefore be used reliably to guide further experimental work.

Yb Fiber Laser Pumped Mid-IR Source Based on Difference Frequency Generation and Its Application to Ammonia Detection

A Yb fiber laser pumped cw narrow-linewidth tunable mid-IR source based on a difference frequency generation (DFG) in a periodically poled LiNbO3 (PPLN) crystal for trace gas detection was demonstrated. A high power Yb fiber laser and a distributed feedback (DFB) laser diode were used as DFG pump sources. This source generates mid-IR at 3 µm with a power of ∼2.5 µW and a spectral linewidth of less than 30 MHz. A frequency tuning range of 300 GHz (10 cm-1) was obtained by varying the current and temperature of the DFB laser diode. A high-resolution NH3 absorption Doppler-broadened spectrum at 3295.4 cm-1 (3.0345 µm) was obtained at a cell pressure of 27 Pa from which a detection sensitivity of 24 ppm·m was estimated.

State Correlations in the Unimolecular Dissociation of Ketene

Doppler-broadened absorption spectra of selected rotational levels of singlet CH2 fragments from the 308-nm photolysis of ketene have been observed by frequency-modulated transient absorption spectroscopy in a slit jet expansion. Nascent Doppler profiles of CH2 transitions have been analyzed to yield the internal energy distributions of coincident CO fragments at a total energy 2333 cm-1 above the singlet dissociation threshold. The vibrational branching ratio of CO (υ = 1):(υ = 0) is determined in coincidence with selected rotational levels of CH2. The coincident rotational distributions of CO (υ = 0) are also estimated and found to differ from simple statistical predictions, revealing a relative deficiency of those pairs in which both fragments have a low internal energy.

Degradation of Polymer Coating Systems Studied by Positron Annihilation Spectroscopy. 3. Wavelength Dependence of UV Irradiation Effect

Photodegradation of a polyurethane coating induced by accelerated UV irradiation is studied using three different light sources: 340 nm UVA, 313 nm UVB, and Xe lamps. Doppler-broadened energy spectra (DBES) and positron annihilation lifetime (PAL) are measured as a function of incident positron energy (0−30 keV) and of irradiation time (0−800 h). The photodegradation of the coating is characterized in terms of subnanometer defect changes. Significant variations of S parameter, orthopositronium lifetime, intensity, and lifetime distribution are observed as a function of depth and of exposure time. These results indicate a decrease of free volume and holes at the atomic level during the degradation process. The UV irradiation at 313 nm shows a significant effect compared with that at 340 nm on the photodegradation: (1) the magnitude of S reduction increases a factor of 2 near the surface; (2) the photodegradation rate increases ∼25 times near the surface; (3) the irradiation effect attenuates with depth, and a larger extinction coefficient is observed with shorter wavelength UV irradiation. The observed effect by Xe light irradiation is the strongest among these three light sources. Enhanced degradation at shorter wavelengths is also indicated by increased free radical production, as observed by electron spin resonance spectroscopy.

Positron annihilation study on the stress-field pinning mechanism in (Eu,Y)-123 superconductors

The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra have been performed on (Y 1− x Eu x )Ba 2Cu 3O 7− y ( x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1) superconductors. The mean lifetime and S-parameter, respectively show a characteristic significantly symmetric to Eu-doping concentration. Since lifetime and Doppler broadening spectroscopies are rather sensitive to the variation of the lattice distortion and defects induced by the stress-field of lattice mismatch, such symmetric behaviors of the S-parameter and the mean lifetime suggest a striking characteristic of stress in Y 1− x Eu x -123 superconductor. When the doping concentration x reaches 30%, the density of defects corresponding to stress-field appears the maximum, and there is an optimum distribution of the stress-field so as to increase critical current J c in Eu-doped Y-123 superconductors. Our results of the critical current density obtained by means of measurements of susceptibility χ′( T, H AC) and χ″( T, H AC) also suggest that the enhancement of the critical current density to reach an optimum when Eu doping x is 30%. We also qualitatively discuss experimental results in the framework of the model of stress-field pinning. The model calculating results also show the stress-field induced by lattice mismatch is an effective pinning center, and further conform that the stress pinning mechanism is dominant in RE-doped Y-123 superconductors.

Positronium in a layered-structure material: Montmorillonite

Positron-lifetime and Doppler-broadened spectra were measured for ion-exchanged (Na+, Zn2+, Al3+, Cu2+ and alkylammonium) montmorillonites (mont). The line-shape parameters of the spectra for alkylammonium-intercalated monts were almost equal to those observed for the corresponding alkylammonium salts used for the intercalation. Upon exchange of diamagnetic ions in the interlayer spaces of mont for paramagnetic Cu2+ ions, the longest-lived component ascribed to ortho-positronium (Ps) was shortened from ca 1 to 0.62 ns, and the Doppler-broadened spectrum was narrowed. These results demonstrate that Ps atoms are predominantly distributed in the interlayer spaces of mont, rather than in the aluminosilicate layers.

Strong saturation of Doppler-broadened spectra in the presence of weak collisions

Strong saturation of Doppler-broadened spectra in the presence of weak collisions

A simple frequency lock of green YAG laser to Doppler-broadened spectrum of iodine molecules utilizing an acousto-optic frequency shifter

A simple frequency stabilization method for frequency-doubled output (532 nm) of an Nd:YAG laser is described. The frequency shift caused by an acousto-optic modulator is effectively applied to stabilize the laser frequency. For the frequency stabilization, there are two kinds of error signal: (1) the first-derivative of absorption of iodine molecules and (2) the first-derivative of dispersion were extracted to examine which signal is suitable for the actual stabilization of the laser frequency. Based on this examination, the stabilization was executed utilizing the error signal (1), resulting in a frequency instability of .

Chemical Identity of Atoms Using Core Electron Annihilations

Positron annihilation spectroscopy is a sensitive probe for studying the electronic structure of defects in solids. The high momentum part of the Doppler-broadened annihilation spectra can be used to distinguish different elements. This is achieved by using a new two-detector coincidence system and by imposing appropriate kinematic cuts to exclude background events. The new setup improves the peak to background ratio in the annihilation spectrum to {approximately}10{sup 5}. As a result, the line shape variations arising from different core electrons can be studied. The new approach adds elemental specificity to the Doppler broadening technique, and is useful in studying elemental variations around a defect site. Results from several case studies are reviewed.

PAT study on stress-field pinning in high-T c superconductors

The simultaneous measurements of Doppler-broadened radiation spectra and lifetime spectra were performed by positron annihilation analysis on Eu-doped Y-123 superconductors. When the doping concentration reaches 30% and 70%, the density of defeat and its stress-field reach the maximum for (Y1-x Eu x )Ba2CU3O7-y The pinning mechanism of such superconductors is discussed, and the results support the stress pinning mechanism.

Positron beam analysis of semiconductor materials using a two-detector Doppler broadening coincidence system

Doppler-broadening measurements can be improved by using a second Ge-detector for the coincidence detection of the second annihilation photon. The coincidence condition in combination with an energy relation results in a reduction of the background by a factor of 100 compared to a single detector system. This background reduction opens up the possibility of performing accurate measurements of the high momentum part of Doppler-broadened annihilation spectra. We have used this technique for the analysis of a metal oxide semiconductor system and electron-irradiated, As- and Sb-doped silicon.

Increased Elemental Specificity of Positron Annihilation Spectra.

Positron annihilation spectroscopy (PAS) is a sensitive probe for studying the electronic structure of defects in solids. We show that the high-momentum part of the Doppler-broadened annihilation spectra can be used to distinguish different elements. This is achieved by using a new two-detector coincidence system to examine the line shape variations originating from high-momentum core electrons. Because the core electrons retain their atomic character even when atoms form a solid, these results can be directly compared to simple theoretical predictions. The new approach adds increased elemental specificity to the PAS technique, and is useful in studying the elemental variations around a defect site.

Positron-annihilation study on the Tl-1223 superconductor

We have measured the Doppler-broadened radiation spectra at the various temperatures from 77K to room temperature for the sample of Tl-system 1223 phase. The temperature dependence of the line-shape parameters ( S, W, S W ) show the anomalous changes in the temperature range 10–20K above Tc, which indicates that electron momentum distribution alters anomalously due to the localization of the electrons the result is discussed.

Diamond-like carbon films investigated with positrons

Abstract Normal and silicated diamond-like carbon films on steel and silicon substrates have been investigated with a slow positron beam. A diffusion model, making allowance for different densities, was applied to Doppler-broadened spectra. Measurements reveal the nature of the films and the importance of the substrate in the production processes.

Positron annihilation lifetime studies and doppler broadening measurement on polyaniline

Polyaniline is studied by the PALS (positron annihilation lifetime spectra) as a function of temperature which varies from 145 to 315 K and by the DBARS (Doppler-broadened annihilation γ-radiation spectra) as a function of protonation level. The variation of positron lifetime parameter shows three stages in our temperature range. This can be explained by two kinds of thermal effects, thermally generated lattice defects and thermal expansion. The S parameter shows almost the same tendency with the conductivity of the sample as a function of pH value, and can supply useful information on the conduction mechanism in polyaniline.

Positron annihilation in porous silicon

Three lifetime components, one of which is extremely long (25±2 ns), have been observed in experimental studies of positron annihilation in porous silicon, made by anodization in hydrofluoric acid. The Doppler-broadened spectrum of the porous silicon is sharp compared with that of crystal silicon and becomes even narrower in an applied magnetic field. The positronium yield in the porous silicon therefore is concluded from the long lifetime, narrow Doppler spectrum and its narrowing in a magnetic field. The porous structure is the cause of positronium formation.