Laser-induced Resonance Fluorescence Research Articles

New Instrument for Time-Resolved OH and HO2 Quantification in High-Pressure Laboratory Kinetics Studies.

We have constructed a new time-resolved high-pressure fluorescence assay by gas expansion (HP-FAGE) apparatus, optimized for the detection of OH and HO2 radicals in complex gas-phase reactions. The new instrument fills a gap in the existing experimental toolkit for chemical kinetics by enabling the quantification of two key reactive species with microsecond time resolution from high-pressure sources, which was previously not attainable. The HP-FAGE is interfaced with a flow reactor, designed for pressures up to 100 bar and temperatures up to 1000 K, in which reactions are initiated by laser photolysis of radical precursors at repetition rates of 1-10 Hz. The HP-FAGE samples gas out of the reactor into a miniature FAGE chamber, where OH is detected by resonant laser-induced fluorescence using a time-delayed probe laser pulse. HO2 is converted to OH via reaction with NO and then detected by OH fluorescence. The novel FAGE design places the probe region very close to the gas expansion, minimizing the transport time of sampled molecules and resulting in time resolution better than 20 μs for both OH and HO2. We calibrate the sensitivity of HP-FAGE, validate its performance with measurements of well-known reaction kinetics (OH + CH4, OH + OH, OH + HO2, and HO2 + HO2), and discuss prospects for its future use.

Development of a laser-photofragmentation laser-induced fluorescence instrument for the detection of nitrous acid and hydroxyl radicals in the atmosphere

Abstract. A new instrument for the measurement of atmospheric nitrous acid (HONO) and hydroxyl radicals (OH) has been developed using laser photofragmentation (LP) of HONO at 355 nm after expansion into a low-pressure cell, followed by resonant laser-induced fluorescence (LIF) of the resulting OH radical fragment at 308 nm similar to the fluorescence assay by gas expansion technique (FAGE). The LP/LIF instrument is calibrated by determining the photofragmentation efficiency of HONO and calibrating the instrument sensitivity for detection of the OH fragment. In this method, a known concentration of OH from the photo-dissociation of water vapor is titrated with nitric oxide to produce a known concentration of HONO. Measurement of the concentration of the OH radical fragment relative to the concentration of HONO provides a measurement of the photofragmentation efficiency. The LP/LIF instrument has demonstrated a 1σ detection limit for HONO of 9 ppt for a 10 min integration time. Ambient measurements of HONO and OH from a forested environment and an urban setting are presented along with indoor measurements to demonstrate the performance of the instrument.

Optical processes for middle atmospheric Doppler lidars: Cabannes scattering and laser-induced resonance fluorescence: erratum

Optical processes for middle atmospheric Doppler lidars: Cabannes scattering and laser-induced resonance fluorescence: erratum

Optical processes for middle atmospheric Doppler lidars: Cabannes scattering and laser-induced resonance fluorescence

The problem of light scattering from a monochromatic laser beam by an ensemble of atmospheric atoms or molecules in thermal equilibrium under ambient wind and temperature is revisited with two emphases. First, the essence of and difference between laser Cabannes scattering (CS) and laser-induced resonance fluorescence (LIF), which permit atmospheric temperature and wind measurements, are simply presented. A classical model is shown to yield the same angular distribution in scattered power for both, a factor of two larger Doppler shift for the former (CS), in the backward direction with the same line-of-sight (LOS) wind, and a dramatic 17 order larger backscattering cross section for the latter (LIF). Second, the LIF process is developed from first principles, and we present the relations between absorption cross section, differential scattering cross sections, and LIF emission spectrum for atoms with hyperfine structure. By doing this, we (1) equate the decoherence rate among the mixed excited substates to half of the Einstein coefficient to comply with energy conservation, and (2) present a clear physical insight into the origin of the Hanle effect, whose quantitative evaluation for these atoms requires quantum mechanical treatment. We then derive LIF emission rates for each of the eight and 10 pathways associated with the D1 and D2 transitions, and deduce the temperature and LOS wind dependence in differential backscattering cross sections of atmospheric sodium and potassium atoms. Using these cross sections allows retrieval of atmospheric parameters in the mesopause region (80–110 km in altitude) by a metal resonance lidar.

Kinetics of the NCN + NO Reaction over a Broad Temperature and Pressure Range

Rate coefficients for the reaction (3)NCN + NO → products (R3) were measured in the temperature range 251-487 K at pressures from 10 mbar up to 50 bar with helium as the bath gas. The experiments were carried out in slow-flow reactors by using pulsed excimer laser photolysis of NCN(3) at 193 or 248 nm for the production of NCN. Pseudo-first-order conditions ([NCN](0) ≪ NO) were applied, and NCN was detected time-resolved by resonant laser-induced fluorescence excited near 329 nm. The measurements at the highest pressures yielded values of k(3) ∼ 8 × 10(-12) cm(3) s(-1) virtually independent of temperature and pressure, which indicates a substantially smaller high-pressure limiting value of k(3) than predicted in earlier works. Our experiments at pressures below 1 bar confirm the negative temperature and positive pressure dependence of the rate coefficient k(3) found in previous investigations. The falloff behavior of k(3) was rationalized by a master equation analysis based on a barrierless association step (3)NCN + NO ↔ NCNNO((2)A″) followed by a fast internal conversion NCNNO((2)A″) ↔ NCNNO((2)A'). From 251-487 K and above 30 mbar, the rate coefficient k(3) is well represented by a Troe parametrization for a recombination/dissociation reaction, k(3)(T,P) = k(4)(∞)k(4)(0)[M]F(k(4)(0)[M] + k(4)(∞))(-1), where k(4) represents the rate coefficient for the recombination reaction (3)NCN + NO. The following parameters were determined (30% estimated error of the absolute value of k(3)): k(4)(0)[M=He] = 1.91 × 10(-30)(T/300 K)(-3.3) cm(6) s(-1)[He], k(4)(∞) = 1.12 × 10(-11) exp(-23 K/T) cm(3) s(-1), and F(C) = 0.28 exp(173 K/T).

Lifetime measurement of metastable fluorine atoms using electron cyclotron resonance plasma source

The authors have measured the lifetime of metastable state (3sP5∕24) of F atoms by resonant laser-induced fluorescence method. For this experiment, a special 2.45GHz electron cyclotron resonance (ECR) plasma source, which is highly efficient in F radical generation and free from magnetic field leakage in front of the beam-emitting orifice, has been developed. Using the ECR plasma gun, the authors observed a precision fluorescence spectrum related to 3pD7∕204(F=4)→3sP5∕24(F=3) transition of F radicals, which made it possible to experimentally determine the longitudinal velocity distribution and the angular spread of the F radical beam. Based on these measured beam characteristics, the authors extracted a true decay curve of fluorescence intensity as a function of distance from the source and determined the lifetime of F metastable state (3sP5∕24) as 7.3±0.5μs.

Optical–optical double resonance LIF measurement of N 2(C 3Π u, v = 2) deactivation rate constant by N 2 collisions

This Letter reports on the first direct measurement of N 2 (C 3Π u, v = 2) electronic quenching rate constant by optical–optical double resonance laser-induced fluorescence. The value found, (4.19 ± 0.13)×10 −11 cm −3 s −1, solves the discrepancies between the literature data. The technique opens the way to a complete and definitive investigation of electronic quenching and V–V relaxation in the whole C 3Π u state vibrational manifold by means of a minimal laser configuration.

Isotope shift in chromium

Thirty-three spectral lines of chromium atom in the blue-violet region (425–465 nm) have been investigated with the method of laser-induced resonance fluorescence on an atomic beam. For all the lines, the isotope shifts for every pair of chromium isotopes have been determined. The lines can be divided into six groups, according to the configuration of the upper and lower levels. Electronic factors of the field shift and the specific mass shift ( F ik and M ik SMS, respectively) have been evaluated and the values for each pure configuration involved have been determined. Comparison of the values F ik and M ik SMS to the ab initio calculations results has been performed. The presence of crossed second order (CSO) effects has been observed.

Hyperfine magnetic anomaly in the atomic spectra of rare-earth elements

Hyperfine-splitting constants in the optical spectra of rare-earth elements, Nd, Eu, Gd, and Lu, were measured using the laser-induced resonance fluorescence in a low-divergence atomic beam. Values of the hyperfine magnetic anomaly for different atomic levels were determined by comparing the ratios of the magnetic dipole constants of neighboring isotopes with odd numbers of protons or neutrons. The relation of these values to the special features of the atomic and nuclear structure of the investigated elements is discussed.

Detection of NH2 Radical in Ammonia Radio-Frequency Plasmas byLaser-Induced Resonance Fluorescence

By means of one photon absorption laser-induced resonance fluorescence at599.5 nm the relative concentrations of the amidogen (NH2) radical inthe ammonia (NH3) radio-frequency (rf) plasma source were measured underdifferent discharge pressures and rf powers. The time dependence of thefluorescence which comes from the radiation 101-211 of theP-branches of the Σ vibronic sub-bands can be described by asingle-exponential decay. The decay time of NH2(A2A1)Σ (0, 9, 0) rovibronic state was determined. The spatialdependence of the NH2 density in the discharge tube was measured.

An OH Monitoring Method Using Infrared-Ultraviolet Double Resonance Laser-Induced Fluorescence

Abstract We have applied an infrared-ultraviolet double resonance laser-induced fluorescence (IR-UV LIF) technique to the measurement of OH concentrations in the troposphere. This OH exists in extremely low concentrations, ca. 106 molecules cm-3, and plays a central role in the atmospheric chemistry. The detection limit of this new technique using our laser and detection system was experimentally determined to be 1.3×109 molecules cm-3 at 9.2 mTorr in Ar buffer gas. We have found that the method is free from chemical interferences but less sensitive than the single photon LIF method. A simple model is formulated and differential equations are numerically solved after the selection and determination of necessary physical constants. The method leads to the simulated (single photon LIF)/(IR-UV LIF) sensitivity ratio of 1.4×103, comparable with the experimental value of 2.2×103. On the basis of the proposed kinetic model, we discuss the optimum experimental parameters (sample pressure, IR and UV laser powers, and delay time) in this IR-UV LIF method by taking account of rotational energy transfer, fluorescence quenching, and saturation of absorption.

Hyperfine splitting and isotope shift in the optical transition of Eu isotopes and electromagnetic moments of Eu

The laser-induced resonance fluorescence in an atomic beam was used in order to measure the hyperfine splitting of the 4f7 6s2\(\) and 4f7 6s6p\(\) levels in 151,153,155Eu isotopes. The hfs constants A and B of the unstable 155Eu were determined for the first time: \(\) MHz, \(\) MHz and \(\) MHz. With these data and after corrections for second-order hyperfine structure perturbations the nuclear moments of 155Eu were deduced: \(\) n.m. and \(\) b. In addition new and more precise values of the hfs constants of the excited state for the stable 151,153Eu were obtained. They are as follows: \(\) MHz, \(\) MHz and \(\) MHz, \(\) MHz. The hyperfine anomalies \(\)% and \(\)% were extracted from the corrected hfs constants.

Probability distribution of laser-induced breakdown and ignition of ammonia

Laser-induced breakdown and ignition of atmospheric pressure ammonia and ammonia/oxygen mixtures were studied using a nominal 7-ns pulsed Nd : YAG laser source. The probability distribution functions (pdf) for the breakdown and ignition events were determined. For the pdf measurements, the NH-radical ground electronic state was observed using resonant laser-induced fluorescence (LIF), and comparison of the LIF and spontaneous emission of the NH A-state emission is shown. The NH X-state concentration is shown to give a reliable indicator of ignition at 5 μ s following breakdown.

Double resonance laser-induced fluorescence of Cadmium and Zinc in the inductively coupled plasma and electrothermal atomizer

Double resonance laser-induced fluorescence (LIF) spectrometry studies of Zn and Cd have been performed in the inductively coupled plasma (ICP) and electrothermal atomizer (ETA). Stepwise excitation of Zn is accomplished at 213.856 nm and 636.235 nm with fluorescence emissions observed near 334.5 nm. Excitation of Cd is accomplished at 228.802 nm and 643.847 nm with fluorescence emissions observed near 361.1 nm and 347.6 nm. Calibration studies demonstrate that the double resonance LIF approaches provide high sensitivity and linearity over several orders of magnitude in both atomizers. Limits of detection for Zn and Cd in the ICP are 1.7 ng/ml and 5 ng/ml, respectively. Excellent sensitivity is observed in the ETA resulting in limits of detection of 70 pg/ml (700 fg absolute mass) and 4 pg/ml (40 fg absolute mass) for Zn and Cd, respectively. The Zn content of a bovine serum standard reference material (NIST SRM #1598) has been determined by ETA-LIF and found to be 940±60 ng/g, which is in very good agreement with the certified value of 890±60 ng/g. Low-level ETA-LIF measurements of Zn in these studies are strongly limited by the high background level observed for this element.

Optical isotope shifts and changes in nuclear charge radii of stable Ti isotopes

Isotope shifts and hyperfine structures in three optical transitions of TiI have been investigated by using laser induced resonance fluorescence in a collimated atomic beam. From the isotope shifts data, changes of the mean square charge radii of the stable titanium isotopes have been determined for the first time. Using a combined analysis with muonic atom data on root mean square nuclear charge radii, improved model independent 〈r2〉1/2 values for the odd-even Ti isotopes have been obtained.

Upper limits of higher nuclear moments of47Ti and49Ti

Within the framework of systematic hyperfine structure (hfs) investigation of titanium atom the hfs-splittings of 3d3(2G)4s b1G4 levels for two stable isotopes47Ti and49Ti have been studied. 5 hfs intervals for each isotope have been measured using ABMR-LIRF method (atomic beam magnetic resonance detected by laser induced resonance fluorescence) with an accuracy of about 300 Hz. TheJ-off-diagonal second order perturbation effects have been taken into account in the interpretation. Upper limit of the contributions from octupole and hexadecapole hyperfine structure interaction to hfs intervals of this level have been estimated to be about 400 Hz. However, for most of hfs intervals these contributions are almost within experimental error and in order to study such effects a more precise experimental method should be used.

High precision laser-RF spectroscopic measurements of the hyperfine structure of vanadium-51

The hyperfine structure (HFS) of the four high lying, metastable atomic states 3d 4 4s 2H 9 2 11 2 and 3d 3 4s 2 2D 3 2 5 2 of V-51 have been measured. The measurements are carried out using the ABMR-LIRF method (atomic beam magnetic resonance, detected by laser induced resonance fluorescence). The magnetic dipole and the electric quadrupole interaction constants A, B have been obtained from these measurements.

Vacuum ion source diagnostics at the Tokyo Institute of Technology

After a brief review of various diagnostics for vacuum ion sources measured data are presented for the particle distributions within the anode/cathode gap regions using passive spectroscopy, active laser interferometry, and resonant laser-induced fluorescence, while the extracted ion beams were observed by particle track detectors of both time-integrated and time-resolved types. Also measured were the prompt gamma-rays and the D-D neutrons when the beams hit the targets. The experimental arrangements of these diagnostics, together with some examples of the typical results obtained, are shown. It is discussed how various kinds of particles were distributed within the ion diodes, and how they were extracted from the source regions. >

Experimental evaluation of the autoionization cross section of the magnesium transition at 300.9 nm by laser induced fluorescence

The decrease in the laser induced resonance fluorescence of magnesium atoms at 285.213 nm, caused by photoionization of the 3 p −1 P 1 0 state with a second laser tuned at an autoionizing resonance at 300.9 nm, has been used to evaluate the autoionization cross section. This autoionizing resonance was found to have a cross section of 3 (±0.5) × 10 −16 cm 2 and a halfwidth of about 2.5 nm. The agreement with previous literature data, obtained by measuring the total number of charges produced, testifies to the reliability of our fluorescence method. The precision of the technique is also discussed and it is shown that our method works best for low-to-moderate values of the product of the autoionization cross section and the laser fluence.

On the J-dependence of the isotope shift in the ground term of Gd I

High-resolution laser-induced resonance fluorescence spectroscopy has been applied to investigate the isotope shift considering 19 lines in the isotope sequence 154Gd-158Gd, 160Gd. From this experimental data the level isotope shifts of the ground term a 9D of 4f75d6s2 were extracted and show a strong J-dependence. With a parametric description, including also higher-order isotope shift effects, this J-dependence was analysed and the following crossed-second-order isotope shift parameters (in MHz) were determined: z5d(160-158)=19.2 (1.0), z5d(160-157)=32.2 (1.0), z5d(160-156)=36.6 (1.5), z5d(160-155)=47.1 (1.5), z5d(160-154)=62.3 (2.0). The normalisation of these parameters with the corresponding mean square nuclear charge radii delta (r2) and the spin-orbit Slater integral zeta 5d was constant for Gd I:z5d,norm=z5d /( delta (r2). zeta 5d)=6.0(0.6)*10-6 fm-2. This indicates a crossed-second-order effect of the spin-orbit interaction and the field shift operator. The magnetic and electric hyperfine structure splitting constants for the levels 9F2-4 and 9D2-6 of 4f 75d6s6p for 155Gd are also presented.