Doppler-broadened Spectra Research Articles

Positron annihilation studies of the AlOx/SiO2/Si interface in solar cell structures

Film and film/substrate interface characteristics of 30 and 60 nm-thick AlOx films grown on Si substrates by thermal atomic layer deposition (ALD), and 30 nm-thick AlOx films by sputtering, have been probed using variable-energy positron annihilation spectroscopy (VEPAS) and Doppler-broadened spectra ratio curves. All samples were found to have an interface which traps positrons, with annealing increasing this trapping response, regardless of growth method. Thermal ALD creates an AlOx/SiOx/Si interface with positron trapping and annihilation occurring in the Si side of the SiOx/Si boundary. An induced positive charge in the Si next to the interface reduces diffusion into the oxides and increases annihilation in the Si. In this region there is a divacancy-type response (20 ± 2%) before annealing which is increased to 47 ± 2% after annealing. Sputtering seems to not produce samples with this same electrostatic shielding; instead, positron trapping occurs directly in the SiOx interface in the as-deposited sample, and the positron response to it increases after annealing as an SiO2 layer is formed. Annealing the film has the effect of lowering the film oxygen response in all film types. Compared to other structural characterization techniques, VEPAS shows larger sensitivity to differences in film preparation method and between as-deposited and annealed samples.

High-resolution laser spectroscopy of a Bose–Einstein condensate using the ultranarrow magnetic quadrupole transition

High-resolution laser spectroscopy of a Bose–Einstein condensate (BEC) was performed by using the ultranarrow magnetic quadrupole (6s2)1S0↔(6s6p)3P2 transition of ytterbium (Yb). The transition from the Doppler-broadened spectrum of thermal atoms to the asymmetric spectrum, reflecting the inhomogeneous density distribution of a BEC in a harmonic trap, was observed. The role of the inter-atomic interaction was highlighted by performing spectroscopy of a BEC loaded in a one-dimensional (1D) optical lattice.

ANALYSIS OF DOPPLER-BROADENED PEAK IN THERMAL NEUTRON INDUCED10B(n,α γ)7Li REACTION USING HYPERGAM

The line shape functions for the Doppler-broadened gamma ray spectrum are considered in the 10 B(n,αγ) 7 Li reaction occurring in a surrounding medium where the excited 7 Li nucleus is slowed down and stopped before decay. The phenomenological form of the stopping power was used for the broadening effect. Convolution with the detailed response of a germanium detector is taken into consideration for the simplest case of solely electronic stopping. A numerical study for the analysis of 10 B by thermal neutron capture is conducted by performing a parametric search and fitting the measured spectrum in a leastsquares approach. In comparison with the previous numerical approach using the same analysis, the computational speed is increased and reliable information concerning the stopping power of the medium is obtained while estimating the uncertainty. Implementation of the routine analysis of 10 B is facilitated on a recent version of the gamma ray spectrum analysis package HyperGam.

Sub-Doppler laser absorption spectroscopy of the A2Π i − X2Σ + (1, 0) band of CN: Measurement of the 14N hyperfine parameters in A2Π CN

Measurements of multiple rotational lines in the (1, 0) band of the A 2Π i − X 2Σ + “red” system of the cyanogen radical (CN) at sub-Doppler resolution are reported. The CN radical was produced by 193 nm photodissociation of NCCN (ethane dinitrile) and detected with a Ti:sapphire ring laser operating near 10 900 cm −1. The sample was exposed to a weak, frequency-modulated probe beam and a strong, counterpropagating bleach laser beam. Demodulated probe laser signals display absorption and dispersion features derived from Doppler-free saturation of the hyperfine components as the laser scans across the central region of a Doppler-broadened rotational line spectrum. Hyperfine-resolved transition frequencies were combined with known ground-state X 2Σ hyperfine term values to determine A 2Π state hyperfine term values, which were analyzed in terms of an effective Hamiltonian for the A 2Π state. All the expected hyperfine and 14N quadrupolar parameters were determined and their values analyzed in terms of a simple molecular orbital picture of the bonding in the radical. Higher sensitivity obtained with 400 kHz amplitude modulation of the bleach laser and additional phase-sensitive detection allowed hyperfine splittings in some rotational lines of 13C 14N to be observed in natural abundance. Excited state hyperfine splittings were determined for a selection of rotational states, but not enough to determine the 13C hyperfine parameters.

Doppler-broadened boron peak analysis by using a modified spectral decomposition algorithm

Doppler broadening of a 477.6 keV line combined with a recoil of an excited Li nucleus is the characteristic of PGAA, which leads to complicated gamma-ray spectrum which is difficult to analyze. For this solution, a modified algorithm for an automated analysis of the Doppler-broadened peak spectrum is presented. The modified algorithm maintained the consistency of a Doppler-broadened peak with the finest analysis algorithm used for a Gaussian gamma-ray peak analysis in the HYPERMET code.

Spectroscopy of the [formula omitted] transition of formaldehyde in the 30140–30790 cm −1 range: The [formula omitted] and [formula omitted] rovibrational bands

Room-temperature absorption spectroscopy of the A ∼ 1 A 2 ← X ∼ 1 A 1 transition of formaldehyde has been performed in the 30140–30790 cm −1 range allowing the identification of individual lines of the 2 0 1 4 0 3 and 2 0 2 4 0 1 rovibrational bands. Using tunable ultraviolet continuous-wave laser light, individual rotational lines are well resolved in the Doppler-broadened spectrum. Making use of genetic algorithms, the main features of the spectrum are reproduced. Spectral data is made available as Supporting Information.

Temperature-insensitive laser frequency stabilization with magnetic tuning

We have implemented a tunable laser frequency lock with a wide recapture range and low sensitivity to temperature fluctuation, based on electronically power-normalized Doppler-broadened absorption spectra. The method requires no frequency modulation. A distributed-feedback diode laser locked to this system exhibits submegahertz stability over many hours. It has been used to magneto-optically trap rubidium atoms for a full day.

Effect of optical pumping on alkali-atom Doppler-limited spectra

Using a three-level model and semiclassical density matrix formalism, we show that optical pumping influences both line positions and amplitudes in Doppler-broadened alkali-atom D1 and D2 spectra for intensities much lower than the two-level atom saturation intensity. The influence on the D2 spectrum is particularly interesting due to the presence of closed hyperfine transitions unaffected by optical pumping. This effect is of importance for example when lasers are frequency stabilized using linear absorption or when the alkali-atom vapour pressure is determined using absorption measurements.

Effect of Doppler broadening on Autler-Townes splitting in the molecular cascade excitation scheme

Theoretical analysis of the Autler-Townes splitting pattern in a three-level cascade excitation of a Doppler-broadened sample reveals that the observed Autler-Townes splitting is not only a function of the coupling-laser Rabi frequency, as in the homogeneously broadened case, but can also strongly depend on the wavenumber ratio of the coupling and probe lasers. Utilizing a steady-state density matrix formalism, we derive exact expressions for the populations of the intermediate and upper levels for a homogeneously broadened system. In the limit of a weak probe field, analytical expressions for Doppler-broadened excitation spectra from the intermediate and upper levels are derived. Using these expressions, we investigate the critical role of Doppler broadening on the observation of Autler-Townes splitting and its asymptotic behavior as a function of the coupling Rabi frequency for counterpropagating and copropagating configurations for different wavenumber ratios of the lasers.

Magneto Optical Rotation in the Doppler- broadened Spectrum of Rb atoms in Intermediate fields

Doppler-broadened absorption spectrum of rubidium (87Rb and 85Rb) D2 line has been studied in the presence of a magnetic field of intermediate strength i.e. up to 5mT. Resonant non - linear magneto optical rotation is measured in the Doppler broadened spectrum of rubidium atomic vapour. Low light intensity approximation in the intermediate field region has been made to calculate the magneto optical rotation and to compare with experimental data. The enhancement of non-linear magneto optical rotation is discussed taking into account hole burning and circular birefringence in the intermediate field region.

Variations of intensity in Rb D2 line at weak/intermediate fields

Zeeman splitting in the D2 line of rubidium atoms (87Rb and 85Rb) has been studied using ‘Doppler broadened’ as well as ‘saturation absorption spectroscopy’. While a linearly polarized beam was used for the former experiment, in the latter case a (π, σ±) polarization configuration was employed for both pump and probe beams. Zeeman lines have been observed by applying a field up to 5 mT. The field variation of relative line intensities in Doppler-broadened spectrum was determined following Tremblay et al and Nakayama's four-level model. For the saturation spectrum, a four-level model was used. Because the enhancement of absorption at the field is as low as 1 mT, the Fg = 2 to Fe = 3 transition for 87Rb can be used as the reference for laser locking. Level crossing is observed in 85Rb at fields less than 5 mT.

4W continuous-wave narrow-linewidth tunable solid-state laser source at 546nm by externally frequency doubling a ytterbium-doped single-mode fiber laser system

A high-power continuous-wave coherent light source at 545.5nm is described. We use 8.3W from a solid-state ytterbium-doped single-mode fiber oscillator/amplifier system as input into an external frequency doubling stage. This system produces up to 4.1W of stable green single-frequency laser radiation. We characterize the light source by performing absorption spectroscopy on iodine across the full tuning range of the fiber laser and saturation spectroscopy on one strong iodine line of the doppler-broadened spectrum.

Absence of positronium formation in clean buried nanocavities inp-type silicon

Buried nanocavities at about 350 nm depth in Si were produced by thermal treatment of He implanted p-type (100) Si. The internal surfaces of the nanocavities were found free of impurity decorations by examining the high-momentum part of the Doppler-broadened positron annihilation spectra. Positron lifetime measurements with a pulsed slow positron beam show neither a short lifetime (125-150 ps) ascribable to parapositronium nor a longer lifetime (2-4 ns) ascribable to pick-off annihilation of orthopositronium. The lifetime of positrons trapped into nanocavities was found to be about 500 ps. The absence of positronium formation could be explained by an insufficient electron density and a lack of electron states in the band gap at the nanocavities internal surfaces produced in the p-type silicon.

Ultracold neutral plasmas

Ultracold neutral plasmas are formed by photo-ionizing laser-cooled atoms near the ionization threshold. Through the application of atomic physics techniques and diagnostics, these experiments stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas ranges from 1 to 1000 K and the ion temperature is around 1 K. The density can approach 1011 cm−3. Fundamental interest stems from the possibility of creating strongly coupled plasmas, but recombination, collective modes, and thermalization in these systems have also been studied. Optical absorption images of a strontium plasma, using the Sr+ 2S1/2 → 2P1/2 transition at 422 nm, depict the density profile of the plasma, and probe kinetics on a 50 ns time-scale. The Doppler-broadened ion absorption spectrum measures the ion velocity distribution, which gives an accurate measure of the ion dynamics in the first microsecond after photo-ionization.

Absorption imaging and spectroscopy of ultracold neutral plasmas

Absorption imaging and spectroscopy can probe the dynamics of an ultracold neutral plasma during the first few microseconds after its creation. Quantitative analysis of the data, however, is complicated by the inhomogeneous density distribution, expansion of the plasma and possible lack of global thermal equilibrium for the ions. In this paper, we describe methods for addressing these issues. Using simple assumptions about the underlying temperature distribution and ion motion, the Doppler-broadened absorption spectrum obtained from plasma images can be related to the average temperature in the plasma.

Positron annihilation lifetime and coincidence-gated Doppler broadening studies of FeS2 nanostructures

Positron lifetimes were measured in different nanosystems of FeS2—granular samples, ribbons, oxidized ribbons, rods, and a mixture of wires and tubes. The initial characterization and size determination of the synthesized samples were carried out using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. To identify the positron trapping sites, the 511–511keV gamma rays coincidence-gated Doppler-broadened spectra were recorded and it appeared that the trapping of positrons took place mainly in the vacancies created by the absence of Fe2+ ions. The positron lifetimes in the nanogranular sample were conspicuously larger compared to those in the coarse-grained bulk due to trapping and annihilation at the grain surfaces. The difference in the measured lifetimes in the nanoribbons from those of the nanogranular samples is explained as due to the annihilation of positrons in the additional trapping sites available at the inter-ribbon-layered cavities. Positron annihilation in nanotubes differed distinctly from solid nanorods because of the trapping of positrons in the inner channel surfaces of the former and a significant fraction of the positrons formed orthopositronium inside these open holes.

Velocity-selective resonance dips in the probe absorption spectra of Rb D 2 transitions induced by a pump laser

We report experimental observation of velocity-selective resonances in the Doppler-broadened probe absorption spectra of 85Rb and 87Rb D 2 transitions in the presence of a strong co-propagating pump laser locked to a frequency within the Doppler profile of the transition. The set of three dips having the separation of allowed hyperfine transitions move along the Doppler profile while the pump laser frequency is tuned. This indicates a resonance between the pump laser frequency and the velocity shifted probe laser frequency.

Diode-laser frequency stabilization by two-frequency Doppler-broadened absorption spectroscopy

We demonstrate a robust method for frequency stabilization of a diode laser by two Doppler-broadened absorption spectra of the cesium D2 line. This technique employs an acousto-optical modulator to generate another frequency component from a diode laser to perform the spectroscopy. The 852-nm diode laser with frequency stabilization at the zero crossing of the error signal showed a peak-to-peak fluctuation of 800 kHz compared with a frequency-stabilized femtosecond laser over a 2-h period. This frequency-locking method is free of modulation and can be applied to frequency discriminators as well as to atomic resonances.

Electromagnetically induced absorption due to transfer of population in degenerate two-level systems

We predict the occurrence of electromagnetically induced absorption (EIA) in cycling degenerate two-level transitions where ${F}_{e}={F}_{g}+1$ and ${F}_{g}>0$, interacting with pump and probe lasers with the same polarization. The EIA is due to transfer of population (TOP) between the Zeeman levels of the ground hyperfine state, rather than transfer of coherence (TOC) which occurs for perpendicularly polarized lasers. We model EIA-TOP using a double two-level system (TLS) which we compare with the four-level $N$ system, which models EIA-TOC. When the pump intensity is low, both models give an EIA peak at line center. The effect of introducing phase-changing collisions is studied, in the presence and absence of Doppler broadening, for both the double TLS and $N$ systems. In the presence of phase-changing collisions, the central EIA peaks are narrowed in both models and persist to higher pump Rabi frequencies than in the absence of collisions. In the double TLS, in the presence of Doppler broadening, the central EIA-TOP peak becomes narrower and does not develop a dip in its center, in contrast to the $N$ system. The central dip that appears in the Doppler-broadened EIA-TOC spectrum can be wiped out by adding phase-changing collisions. We demonstrate that EIA-TOP can be obtained for realistic atomic transitions interacting with lasers that have the same polarization.

Optic nerve head circulation in nonarteritic anterior ischemic optic neuropathy and optic neuritis

Purpose To quantify optic nerve head circulatory abnormalities in patients with unilateral nonarteritic anterior ischemic optic neuropathy (NAION) or optic neuritis (ON), and to assess the potential of such measurements to differentiate NAION from ON. Design Prospective, cross-sectional, observational study. Participants Thirty consecutive patients with unilateral NAION, 22 consecutive patients with unilateral ON, and 50 healthy control subjects. Methods All subjects underwent a complete neuro-ophthalmologic evaluation. The widths of Doppler-broadened frequency spectra, which are directly proportional to the speed of blood cells flowing through the capillaries of the optic nerve head, were measured at multiple sites in both eyes of each subject. The variation of Doppler broadening (DB) with age was determined in the control subjects. Doppler broadening values in the patients were compared between similar sites in affected and contralateral eyes, and between both affected and contralateral eyes and the age-adjusted values determined in the control subjects. Main outcome measures The differences in DB between (1) the affected and contralateral eyes of the patients, (2) the patients and the control subjects, and (3) the patients with NAION and those with ON. Results In NAION, DB was decreased at both temporal (−20.2% and −18.5%) and nasal (−12.8% and −12.4%) sites of the nerve head in affected eyes compared with contralateral eyes or eyes of control subjects. In ON, DB was also decreased at temporal sites (−11.3% and −9.2%) in affected eyes compared with contralateral or control eyes. At nasal sites, there were no significant differences in DB in affected eyes of ON patients compared with contralateral or control eyes. The DB decreases were significantly greater in NAION patients than in ON patients. Conclusions Optic nerve head circulatory abnormalities are present in patients with NAION or ON. This is the first demonstration of such abnormalities in ON, a finding consistent with the recent attention given to the phenomenon of axonal loss in this disease. Although there are differences in the circulatory abnormalities between the 2 diseases that provide insights into the pathophysiological mechanisms at play, they are not large enough to enable the clinician to distinguish between ON and NAION in an individual patient.