Time-resolved Laser-induced Fluorescence Measurements Research Articles

Experimental analysis of a non-isothermal confined impinging single plume using time-resolved particle image velocimetry and planar laser induced fluorescence measurements

In this study, the experimental results of the flow behavior of a non-isothermal confined impinging single plume were analyzed. Experimental measurements were performed in the upper plenum of a high-temperature gas-cooled reactor (HTGR) that was scaled, designed, and assembled at Texas A&M University. The goal was to investigate flow mixing, thermal stratification, and plume impingement in the upper plenum of the HTGR under a loss-of-forced-coolant accident and provide a high-fidelity experimental database for validating computational fluid dynamics (CFD) and system codes. Time-resolved particle image velocimetry (TR-PIV) and planar laser-induced fluorescence (P-LIF) measurements were performed in the central plane of the plume flow in the upper plenum. A total power of 0.4 kW of heat was applied to the system. Convergence was achieved for the statistical results from the TR-PIV and P-LIF snapshots. The TR-PIV velocity vectors presented statistical characteristics of the plume flow, such as the mean velocity, root-mean-square fluctuating velocity, and Reynolds stress. The results revealed that the flow reached the maximum velocity far from the jet inlet at y/Dj=2. Furthermore, two-point velocity cross-correlation was performed on the TR-PIV velocity vector fields. Finally, the statistical results were calculated from the P-LIF temperature snapshots and the mean temperature. Furthermore, the temperature profiles were plotted. It was observed that the flow reached the maximum temperature near the plume inlet and decreased downstream.

Time-resolved ion velocity measurements in a high-power Hall thruster using laser-induced fluorescence with transfer function averaging

We present time-resolved laser-induced fluorescence measurements of ion velocity distributions in a 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS) operating in both quasi-periodic and aperiodic oscillation regimes. Transfer function averaging in Fourier space is used to obtain useable signal-to-noise ratios and synchronize data traces taken at different laser wavelengths, measurement axes, and positions in the plasma, achieving a measurement bandwidth of ∼100 kHz. For breathing-mode like global oscillations, the results are shown to be robust to the choice of either discharge current Id(t) or cathode-to-ground voltage Vcg(t) as the reference waveform input to the transfer function. At discharge voltage Vd=600 V, a nearly periodic, impulsive oscillation in the acceleration zone position was accompanied by a ≳100 V peak-to-peak oscillation in the near-plume plasma potential. Smaller amplitude, aperiodic oscillations in the mean ion velocities were detected at Vd=300 V.

Optical properties and fluorescence of quantum dots CdSe/ZnS-PMMA composite films with interface modifications

Processing and characterization of polymer nanocomposites based on poly (methyl methacrylate) (PMMA) matrix with embedded core-shell CdSe/ZnS quantum dots (QD's) were presented in this work. The aim is to preserve the optical activity of QD's in nanocomposite processing with improved mechanical properties. The modification of QD's CdSe/ZnS-PMMA interface was achieved with 3-mercaptopropyltrimethoxysilane (MPTMS). Nanocomposites were processed by solution casting. The samples of pure PMMA, and composites with unmodified and silane modified QD's CdSe/ZnS were processed. FTIR analysis revealed modification of QD's with silane and characteristic bonding with PMMA matrix. Raman spectrum confirms a presence of alloyed layer at the core-shell interface. Time-resolved laser induced fluorescence measurements confirmed slight blue shift in QD's CdSe/ZnS-PMMA composite without interface modification, while there was a slight red shift with modified particles. Oxidation effects in quantum dots were removed with the use of PMMA as a host, and the core remained active, which was confirmed with FTIR analysis and time-resolved laser induced fluorescence measurements.

Time-resolved laser-induced fluorescence diagnostics for electric propulsion and their application to breathing mode dynamics

Several techniques have been developed recently for performing time-resolved laser-induced fluorescence (LIF) measurements in oscillating plasmas. One of the primary applications is characterizing plasma fluctuations in devices like Hall thrusters used for space propulsion. Optical measurements such as LIF are nonintrusive and can resolve properties like ion velocity distribution functions with high resolution in velocity and physical space. The goals of this paper are twofold. First, the various methods proposed by the community for introducing time resolution into the standard LIF measurement of electric propulsion devices are reviewed and compared in detail. Second, one of the methods, the sample-hold technique, is enhanced by parallelizing the measurement hardware into several signal processing channels that vastly increases the data acquisition rate. The new system is applied to study the dynamics of ionization and ion acceleration in a commercial BHT-600 Hall thruster undergoing unforced breathing mode oscillations in the 44–49 kHz range. A very detailed experimental picture of the common breathing mode ionization instability emerges, in close agreement with established theory and numerical simulations.

Plasma-assisted CO2 conversion: optimizing performance via microwave power modulation

Significant improvement in the energy efficiency of plasma-assisted CO2 conversion is achieved with applied power modulation in a surfaguide microwave discharge. The obtained values of CO2 conversion and energy efficiency are, respectively, 0.23 and 0.33 for a 0.95 CO2 + 0.05 N2 gas mixture. Analysis of the energy relaxation mechanisms shows that power modulation can potentially affect the vibrational–translational energy exchange in plasma. In our case, however, this mechanism does not play a major role, likely due to the low degree of plasma non-equilibrium in the considered pressure range. Instead, the gas residence time in the discharge active zone together with plasma pulse duration are found to be the main factors affecting the CO2 conversion efficiency at low plasma pulse repetition rates. This effect is confirmed experimentally by the in situ time-resolved two-photon absorption laser-induced fluorescence measurements of CO molecular density produced in the discharge as a result of CO2 decomposition.

Spectroscopic observation of neutral carbon during photodissociation of explosive-related compounds in the vapor phase

We perform time-resolved laser-induced fluorescence measurements of mononitrotoluenes (MNTs) and dinitrotoluenes (DNTs) in nitrogen and air. We observe the multipeak emission spectrum of NO and find that the emission peak intensity in the 247-248nm range is stronger than expected compared to the other NO emission peak intensities. This increased emission intensity is believed to be due to neutral carbon [C(I)], which has a strong emission peak at 247.85nm. By comparing the ratios of integrated emission peak intensities with those expected from the Franck-Condon factors for NO, we are able to identify samples that exhibit C(I) emission. We show that the DNTs exhibit C(I) emission for gate delays of 1500ns and beyond, while the MNTs exhibit C(I) emission for gate delays of only up to about 500ns. Carbon deposits in the analysis chamber confirm the presence of C. Ambient NO in air enhances the observed NO+C(I) signal from MNTs and DNTs.

Photon counting technique applied to time-resolved laser-induced fluorescence measurements on a stabilized discharge

A novel approach to perform time-resolved laser-induced fluorescence (LIF) measurements in plasma discharges is presented. The LIF technique relies on a photon counting method associated with a sinusoidal potential modulation on a floating electrode located in the plasma to ensure time coherence. By tuning the modulation frequency, resonance can be reached with the discharge current in order to guarantee repeatable measurement conditions. Time-averaged characteristics of the discharge (such as Te, ne, Vp, and Vion) remain unaffected by the modulation. As an example, the association of the photon counting method with the modulation system is employed to determine the time evolution of several ion velocity groups inside an E × B discharge. Interesting features of the velocity oscillations are examined and pave the way for more focused studies.

Atomic transition probabilities of Nd I

Fourier transform spectra are used to determine emission branching fractions for 236 lines of the first spectrum of neodymium (Nd i). These branching fractions are converted to absolute atomic transition probabilities using radiative lifetimes from time-resolved laser-induced fluorescence measurements (Den Hartog et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 225001). The wavelength range of the data set is from 390 to 950 nm. These transition probabilities from emission and laser measurements are compared to relative absorption measurements in order to assess the importance of unobserved infrared branches from selected upper levels.

Atomic transition probabilities of Er i

Atomic transition probabilities for 562 lines of the first spectrum of erbium (Er i) are reported. These data are from new branching fraction measurements on Fourier transform spectra normalized with previously reported radiative lifetimes from time-resolved laser-induced fluorescence measurements (Den Hartog et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 155004). The wavelength range of the data set is from 298 to 1981 nm. In this work we explore the utility of parametric fits based on the Cowan code in assessing branching fraction errors due to lines connecting to unobserved lower levels.

Time-resolved laser-induced saturated fluorescence measurements in a thallium see-through hollow cathode discharge: Evaluation of ground state number density and quantum efficiency

A thallium see-through hollow cathode lamp, or galvatron, was studied to evaluate its potential as a narrow band atomic line filter. Time-resolved laser-induced saturated fluorescence was used to evaluate the ground state number density of this glow discharge as a function of current. It was found to produce a sufficient number density at 16.0 mA to absorb 99.9% of incident light from a line source based on an absorption curve-of-growth calculation. A saturation curve was experimentally obtained and modeled with a time-dependent two-level model, as well as a time-dependent three-level model in the presence of a trap. The three-level model showed excellent agreement with the experimental data when a 10 ns pulse duration was used and when collisional rate constants were set to zero. The quantum efficiency of the system was found to be limited only by the spontaneous transition probabilities. Evaluation of these two parameters has shown that a thallium galvatron is an attractive atom reservoir for the applications as a narrow band atomic line filter.

Influence of gas temperature on the loss mechanisms of H-atoms in a pulsed microwave discharge identified by time-resolved LIF measurements

The present work deals with investigating the loss kinetics of atomic hydrogen in an H2/CH4 pulsed microwave discharge used for diamond chemical vapour deposition. The temporal evolution of the hydrogen atom density, particularly in the afterglow phase, was investigated in order to determine the fundamental loss processes occurring in such discharge. Time resolved two-photon laser induced fluorescence (LIF) measurements of the relative concentration of H-atoms in the ground state and observed by Hα transition have been established. The gas temperature was estimated from the Doppler broadening of the LIF Gaussian profile obtained by scanning the laser frequency. The wavelength calibration and the estimation of the effective bandwidth of the LIF profile were accomplished by measuring the spectral response of atomic hydrogen and deuterium. The decay of H-atom density in the afterglow is characterized by a fast decrease in the early afterglow up to 2 ms, followed by a slow evolution. The effect of the gas cooling on the diffusion coefficient and then on the atom density decay in the afterglow was studied. It is found that the fundamental loss processes of atomic hydrogen are governed by the diffusion phenomenon and surface recombinations, especially on the diamond substrate.

Two-photon laser-induced fluorescence of atomic hydrogen in a diamond-depositing dc arcjet

Atomic hydrogen in the plume of a dc-arcjet plasma is monitored by use of two-photon excited laser-induced fluorescence (LIF) during the deposition of diamond film. The effluent of a dc-arc discharge in hydrogen and argon forms a luminous plume as it flows through a converging-diverging nozzle into a reactor. When a trace of methane (< 2%) is added to the flow in the diverging part of the nozzle, diamond thin film grows on a water-cooled molybdenum substrate from the reactive mixture. LIF of atomic hydrogen in the arcjet plume is excited to the 3S and 3D levels with two photons near 205 nm, and the subsequent fluorescence is observed at Balmer-alpha near 656 nm. Spatially resolved LIF measurements of atomic hydrogen are made as a function of the ratio of hydrogen to argon feedstock gas, methane addition, and reactor pressure. At lower reactor pressures, time-resolved LIF measurements are used to verify our collisional quenching correction algorithm. The quenching rate coefficients for collisions with the major species in the arcjet (Ar, H, and H2) do not change with gas temperature variations in the plume (T < 2300 K). Corrections of the LIF intensity measurements for the spatial variation of collisional quenching are important to determine relative distributions of the atomic hydrogen concentration. The relative atomic hydrogen concentrations measured here are calibrated with an earlier calorimetric determination of the feedstock hydrogen dissociation to provide quantitative hydrogen-atom concentration distributions.

Pegylation of 1,4,8,11-tetraazacyclotetradecane (cyclam) and its Cu(II) complexation

Two novel star-like cyclam derivatives appended with four amino groups and four PEG-arms have been synthesised. The complex formation of cyclam and the two cyclam derivatives with Cu(II) has been studied by UV–vis and time-resolved laser-induced fluorescence (TRLFS) measurements outlining 1:1 complexes between cyclam ligands and Cu(II). By TRLFS measurements a quench effect of the Cu(II) on the fluorescence of the ligand was also found to determine the complex formation between Cu(II) and the ligands.

The Measurement of Radiative Lifetimes Using Laser-Induced Fluorescence: Experimental Review and Astrophysical Application

One of the standard methods for determining atomic transition probabilities is to combine branching fractions measured using Fourier-transform spectrometry with radiative lifetimes measurements using laser-induced fluorescence (LIF). This combination of techniques provides an efficient method for measuring large sets of accurate, absolute transition probabilities. The radiative lifetimes, which provide the overall scaling for the transition probabilities, can be measured routinely to ± 5% accuracy using time-resolved LIF. Although the time-resolved LIF technique we use does not achieve the accuracy of fast-beam LIF, the time-resolved technique does enable us to make measurements at a far greater rate (hundreds of level lifetimes per year). Care must be taken, however, to understand and control the systematic effects in time-resolved LIF measurements to maintain ± 5% accuracy over a wide dynamic range and hundreds of lifetime measurements.Over the last 25 years, we have measured lifetimes for 47 spectra using time-resolved LIF. Our atomic beam source can produce a slow beam of neutral and singly ionized atoms of nearly any element. Lifetimes from 2 ns to ~2 µs can be measured for energy levels ranging from 15,000 to ~60,000 cm-1. In this review we will describe our method of measuring radiative lifetimes with an emphasis on possible errors and techniques used for controlling them. The electronic bandwidth, linearity, and overall fidelity of the fast photomultiplier, cable connections, and transient waveform digitizer are concerns. Possible errors from atomic collisions, radiation trapping, Zeeman quantum beats, hyperfine quantum beats, atoms/ions escaping from the observation region before radiating, and from radiative cascade through lower levels must be understood and controlled.We will then present a recent example of the application of our transition probability data to abundance determinations in the sun and in metal-poor halo stars (Den Hartog E A et al 2003 Astrophys. J. Suppl. 148 543). Our results have significantly reduced the uncertainty of abundance determinations for many elements including the rare-earths: La, Nd, Eu, Tb, Dy, Ho, Tm, and Lu. Better laboratory data have made it possible to cleanly separate the enhanced abundance of r(apid)-process neutron capture elements in metal poor Galactic halo stars. The long-term scientific payoff from this attention to detail in laboratory experiments is a better understanding of the r-process, of the Galactic chemical evolution, of the age of the oldest stars in the Galaxy, and indeed of the origins of the non-primordial chemical elements.

Hyperfine structure and homogeneous broadening inPr3+:KY(WO4)2

As part of a search for suitable materials for coherent quantum operations, relaxation times and hyperfine structure of the D-1(2)(1)-H-3(4)(1) transition in Pr3+:KY(WO4)(2) (0.29 at. %) at 4 K have been obtained using photon-echo and spectral hole burning techniques. The homogeneous linewidth and the effect of excitation-induced dephasing were measured using two-pulse photon-echo techniques. Linewidths of 23.4+/-1.0 and 17.6+/-0.9 kHz were obtained in the absence and presence of an external magnetic field of about 9 mT. The radiative lifetime (T-1) of the D-1(2) state was measured to be 43+/-2 mus using time-resolved laser-induced fluorescence and three-pulse photon-echo measurements. The transmission hole spectra were measured and directly yielded the quadrupole level splitting in the D-1(2) (3.77+/-0.03 and 4.58+/-0.04 MHz) and H-3(4) (17.1+/-0.1 and 33.2+/-0.3 MHz) states. The spectral hole lifetime due to population redistribution between the ground-state nuclear levels was also determined to be 70+/-10 ms. A strong dipole-dipole interaction observed in this crystal opens for potential applications in quantum computing schemes for performing quantum logic operations, but the short dephasing time makes it less useful in data storage applications. (Less)

Improved Atomic Data for Hoiiand New Holmium Abundances for the Sun and Three Metal‐poor Stars

Improved energy levels and hyperfine structure constants for a selected set of Ho ii levels were measured from spectra recorded Q1 using the 1 m Fourier transform spectrometer (FTS) at the US National Solar Observatory. Branching fractions for the strong blue-UV lines from these levels were also measured from the FTS data and combined with earlier radiative lifetimes from time-resolved laser-induced fluorescence measurements to determine accurate absolute transition probabilities for 22 lines of Ho ii. These new laboratory measurements, along with a recently reported partition function for ionized Ho, were used to improve Ho abundance determinations in the Sun and three metal-poor Galactic halo giant stars. The derived solar photospheric holmium abundance, log 10 � ðHo Þ¼þ 0:51 � 0:10, is consistent with its meteoritic value, log 10 � ðHo Þ¼þ 0:49 � 0: 02. In each of the metal-poor, neutron-capture‐rich program stars, the holmium abundance relative to those of other rare earth elements agrees well with solar system rapid‐neutron-capture abundance values.

Radiative lifetime, oscillator strength and Landé factor calculations in doubly ionized europium (Eu iii)

A set of transition probabilities has been calculated for Eu iii transitions of astrophysical interest particularly for the study of chemically peculiar stars. They were obtained taking configuration interaction and core-polarization effects into account. The accuracy of the new scale of oscillator strengths, which differs substantially from previous results, has been assessed through comparisons with recent time-resolved laser-induced fluorescence measurements of radiative lifetimes.

Time-resolved laser-induced fluorescence measurements of Rydberg states in Lu Iand comparison with theory

Time-resolved laser-induced fluorescence measurements have beenperformed for ten odd Rydberg states of neutral lutetium, belongingto the 6s2 (1S)np (n = 8– 9)and 6s2 (1S)nf(n = 5–8) series. For 6s2 (1S)8p and6s2 (1S)7f,the experimental lifetimes corresponding to the twoJvalues within the doublet differ substantially. Comparison with theoretical values,calculated with extensive configuration interaction and core-polarization effectsincluded, shows that the experimental trends are adequately reproduced both fornp (n = 8– 9) andnf (n = 5– 8)states.

Core-polarization effects in doubly ionized cerium (Ce III) for transitions of astrophysical interest

The importance of core-polarization effects on oscillator strength determination in Ce iii is emphasized. The inclusion of these effects in relativistic Hartree–Fock (HFR) calculations leads to theoretical lifetime values for some 4f6p levels of this ion in close agreement with recent time-resolved laser-induced fluorescence measurements. These new atomic data, the most accurate to date, have allowed the refinement of the stellar abundance determination of cerium in the chemically peculiar star HD 192913, but have not altered in a drastic way the conclusions derived in recent analyses regarding the overabundance of this element.

Experimental Radiative Lifetimes, Branching Fractions, and Oscillator Strengths for Laiiand a New Determination of the Solar Lanthanum Abundance

Radiative lifetimes, accurate in most cases to ±5%, from time-resolved laser-induced fluorescence measurements on a slow beam of lanthanum ions are reported for 31 odd-parity levels of La II. Experimental branching fractions for La II from emission spectra covering the near-ultraviolet to the near-infrared are also reported. The spectra were recorded using the US National Solar Observatory 1.0 m Fourier transform spectrometer. The branching fractions are combined with the radiative lifetimes to produce 84 experimentally determined transition probabilities or oscillator strengths, generally accurate to ±10%, for La II. These new experimental results are compared to older experimental and theoretical results. These data are applied to determine a new value for the solar photospheric lanthanum abundance, (La) = 1.13, with estimated internal errors of ±0.03 and external errors of ±0.03.