Long-path Absorption Research Articles

Hot bands in the [formula omitted] spectrum of HCBr

Spectra of both the Ã(0, 0, 0)– X̃(0, 1, 0) and Ã(0, 0, 0)– X̃(0, 0, 1) bands in the Ö X̃ band system of HCBr have been recorded and rotationally assigned. The HCBr radical was formed by 193 nm excimer laser photolysis of bromoform. We found no signals from HCBr when the photolysis laser wavelength was changed to 248 nm, although strong CH product emission was observed by eye at both wavelengths. Most of the spectra were recorded in absorption at ambient temperature in a long-path absorption cell using a single frequency Ti:sapphire laser as the light source, but some congested sections were recorded under jet-cooled conditions. Analysis yielded accurate values for the vibrational fundamentals and rotational constants for the vibrationally excited ground state levels determined. A combination of experimental measurements and ab initio vibration–rotation constants was used to estimate equilibrium parameters and a ground state structure for HCBr.

Laser-Induced Fluorescence Instrument for the Detection of Tropospheric OH Radicals

Abstract An instrument for the measurement of troposphric OH radical concentrations using a laser-induced fluorescence technique has been developed. Ambient air is expanded through a pinhole into a low-pressure fluorescence cell and irradiated by the second harmonic of a dye laser at a high repetition rate of 1 kHz. The OH radicals are electronically excited using A2Σ+(ν′ = 0) ← X2Π (ν′′ = 0) transitions around 308 nm. The fluorescence from OH radicals is collected by a lens system and detected by a photomultiplier. The photoelectron pulses from the photomultiplier are processed by a photon-counting system. The dynode gate of the photomultiplier and counting gate systems are made to minimize the detection of the chamber and Rayleigh scattering signal. The sensitivity of the developed instrument is calibrated with two methods: one is a long-path absorption technique and the other is a titration technique with simultaneous photolysis of water vapor and oxygen. It is found that the background signal is mainly produced by after-pulse effects in the photomultiplier for the fluorescence detection with a dynode gate system. The minimum detection limit is determined to be 7.0×105 molecules cm-3 for OH radicals with a signal-to-noise ratio of 2 and a signal integration time of 60 s.

New water vapor line parameters in the 26000– [formula omitted] region

The radiative properties of water vapor play an important role in the physical and chemical processes occurring in the atmosphere. Accurate knowledge of the line parameters for this species is therefore needed. This work presents new measurements of water vapor line parameters in the 26 000– 13 000 cm −1 spectral region. The measurements were obtained by combining a high-resolution Fourier transform spectrometer with a long-path absorption cell, thus allowing the observation of very weak, previously unobserved, lines. A total of more than 9000 lines have been identified and their position, integrated cross section and self-broadening parameter have been determined. The dependence of the line parameters on nitrogen buffer gas pressure (0– 800 hPa ) has also been studied. The complete line list presented here is primarily compared to the HITRAN spectroscopic database, most frequently used in atmospheric calculations.

Optical Design of a Space Retroreflector Using a Hybrid Search

Retroreflectors on satellites are used as targets for laser ranging and laser long-path absorption measurements of atmospheric trace species. To compensate for the velocity aberration, slight tuning of the design is necessary. The curvatures of mirrors, the angles between mirrors, and the direction of the retroreflector on the satellite must be tuned to optimize the optical efficiency of the retroreflector. An enumerative scheme is not applicable when the number of parameters is large. We proposed earlier a technique using a genetic algorithm for the optical design of a space retroreflector.1) In this paper, we have improved the method introducing a hybrid technique that is a combination of a genetic algorithm and simulated annealing.

Optical monitoring of volcanic sulphur dioxide emissions—comparison between four different remote-sensing spectroscopic techniques

The emissions of sulphur dioxide from the Italian volcanoes Mt. Etna and Stromboli were studied in ship-borne underpasses of their plumes. Four different optical spectroscopy techniques were used and inter-compared. All techniques utilise the absorption signature of the gas in the wavelength region of around 300 nm. A differential absorption lidar was employed in active gas concentration assessment. In parallel, a differential optical absorption spectroscopy system (DOAS) provided spectrally resolved absorption spectra. In one configuration the DOAS used a vertically looking telescope and the absorption of the sky-light was studied, while a different DOAS implementation utilised the sun disc as the light source in slant-angle, long-path absorption measurements. Parallel measurements with the customary correlation spectroscopy method were also performed. Path length Monte Carlo simulations of the down-welling radiation through the volcanic plume at different sun altitude and azimuth angles have been performed taking into account also the effects of other geometric parameters as the plume height and extension. The results are discussed with special emphasis on systematic effects due to scattering.

Formation of Peroxy Radicals from OH−Toluene Adducts and O2

The gas-phase reaction of OH radicals with toluene and toluene-d8 and reactions of the resulting OH adducts with O2 were studied in N2/O2 mixtures at atmospheric pressure and room temperature. OH production was performed by pulsed 248 nm photolysis of H2O2. Cw UV-laser long-path absorption at 308 nm was used for time-resolved detection of OH and OH−toluene adducts. The reaction OH + toluene was studied in N2 and O2 and similar rate constants of (5.70 ± 0.19) × 10-12 cm3 s-1 (N2) and (5.60 ± 0.14) × 10-12 cm3 s-1 (O2) were obtained at 100 kPa. For toluene-d8 the corresponding rate constants are (5.34 ± 0.34) × 10-12 cm3 s-1 (N2) and (5.47 ± 0.14) × 10-12 cm3 s-1 (O2). Absorption cross-sections of (1.1 ± 0.2) × 10-17 cm2 were determined for both the OH−toluene and OH−toluene-d8 adduct at 308 nm. Rate constants of (4.7 ± 1.4) × 10-11 cm3 s-1 and (1.8 ± 0.5) × 10-10 cm3 s-1 were determined for the OH−toluene adduct self-reaction and the OH−toluene adduct + HO2 reaction, respectively. Upper limits ≤8 × 10-15 cm3 s-1 were estimated for any reactions of the OH−toluene adduct with H2O2 or toluene. Adduct kinetics in the presence of O2 is consistent with reversible formation of peroxy radicals with an estimated rate constant of (3 ± 2) × 10-15 cm3 s-1 and an equilibrium constant of (3.25 ± 0.33) × 10-19 cm3 (toluene-d8: (3.1 ± 1.2) × 10-19 cm3). The effective adduct loss from the equilibrium can be explained by (i) an additional, irreversible reaction of the adduct with O2 with a rate constant of (6.0 ± 0.5) × 10-16 cm3 s-1 (toluene-d8: (4.7 ± 1.2) × 10-16 cm3 s-1), or (ii) a unimolecular reaction of the peroxy radical, with a rate constant of (1.85 ± 0.15) × 103 s-1 (toluene-d8: (1.5 ± 0.4) × 103 s-1). Consequences for the atmospheric degradation of toluene will be discussed.

Optical design of a hollow cube-corner retroreflector for a geosynchronous satellite

The optical characteristics of a single-element hollow cube-corner retroreflector for a geosynchronous satellite were numerically evaluated for laser ranging and laser long-path absorption measurements of atmospheric species. An optical design with spherical surfaces and tuned dihedral angles was considered to compensate for velocity aberrations. The parameters for the retroreflector were optimized with genetic algorithms for different retroreflector sizes and wavelengths (500 nm and 1, 3, and 10 microm). We found that 20-cm retroreflectors are sufficient for realistic measurements when the laser wavelength is 500 nm or 1 microm. However, a larger retroreflector is necessary to overcome the detector noise level at 3 and 10 microm.

Diode laser spectroscopy of the weakly bound complex Ne–CH4

The infrared absorption spectrum of the weakly bound rare-gas–spherical-top complex Ne–CH4 was discovered and analyzed for the first time. Measurements were made with tunable diode laser spectrometers using a pulsed supersonic jet and a long-path low-temperature absorption cell. Close to the R(0) transition of the methane ν4 fundamental band at 1311.430 cm–1, the Ne–CH4 spectrum was recorded as a very compact absorption pattern. Within a total wave-number range of about 0.1 cm–1, P-, Q-, and R-branches are located. As the first step, the Ne–CH4 spectrum was recorded and analyzed in a supersonic jet at low rotational temperature of about 5 K. Three branches were identified, of which the P- and R-branches were partially resolved and the Q-branch remained unresolved. Compared with the previously measured spectra of Ar–CH4 and Kr–CH4 [Z. Naturforsch. A, 53, 725 (1998).], the absorption pattern in the spectrum of Ne–CH4 is much denser and considerably more compact. However, by analogy with the spectra of Ar–CH4 and Kr–CH4, assignment and analysis were carried out using a Hamiltonian model that incorporates a Coriolis interaction between the total angular momentum of the complex and the angular momentum of the methane monomer. This analysis then allowed us to assign the same spectrum as recorded in a long-path (160 m) cell at a higher temperature of 62 K. The observed rotational constant for Ne–CH4, B"= 0.129(9) cm–1, corresponds to an effective intermolecular separation of 3.8 Å. PACS Nos.: 33.20E, 34.25, 35.20P, 36.40

Remote Sensing of Atmospheric Environment by Means of Laser Spectroscopy.

Detection of atoms and molecules using laser spectroscopy is substantially an active sensing method, and can be applied for the detection of minor contents of the atmosphere. This report reviews remote sensing of atmospheric gas constituents using various kinds of laser spectroscopic techniques, such as absorption, fluorescence and Raman spectroscopy. The spatial distribution of a specific constituent can be determined using the laser radar (lidar) technique. The long-path absorption method or differential absorption lidar (DIAL) is the most useful technique to measure various kinds of molecules related to the global environmental problems, such as ozone, H2O, SO2, NO2, and many other molecules with their absorption bands in the infrared region. Application of nonlinear Raman spectroscopy to the remote sensing of atmospheric constituents is briefly discussed.

A Portable Remote Gas-Leak Detector Using Laser Spectroscopy.

A portable remote gas-leak detector using a 1.65μm InGaAsP DFB laser is developed. It is designed as a manportable long-path absorption methane lidar using a topographical target with a distance of several meters. An operator can search for gas leaks by hand scanning of the laser light. High sensitivity is accomplished by means of second-harmonic detection using wavelength-modulation spectroscopy. The experimental methane detection limit with a diffusive target of magnesium oxide (distance: 8 m, incidence: 60 deg) is 2.2 ppm·m with a time constant of 0.1 s. Measurement with a realistic target shows that the detector can findsmall gas leaks of 10 ml/min within the distance of several meters.

Temperature and Pressure Dependence of Line Widths and Integrated Absorption Intensities for the O2 aΔg − X3Σg- (0,0) Transition

The electric-dipole forbidden a1Δg − X3 (0,0) band of gas-phase O2 has been studied in absorption at wavelengths around 1.27 μm using Fourier transform spectroscopy and a long-path absorption cell. Experiments were conducted at temperatures of 294, 243, and 200 K and at pressures in the range 140−750 Torr. Both pure O2 and a mixture of 21% O2/79% N2 were studied, and line widths, integrated line intensities, and integrated absorption intensities (AIs) for the (0,0) vibrational band were measured. Integrated AIs were found to be independent of temperature, pressure, and gas composition, and the recommended value for the vibrational band from the current study is Sint = 3.210(15) × 10-24 cm molecule-1 (1σ error) for pure 16O2, corresponding to an Einstein A-coefficient of A = 2.256(10) × 10-4 s-1. The effect of including other oxygen isotopomers is to increase the integrated AI value for this origin band to Sint = 3.226(15) × 10-24 cm molecule-1. Widths of individual spectroscopic lines decrease with increasing molecular rotational quantum number. The temperature dependence of Lorentzian line width components, γT (fwhm), is well represented by the expression γT = γ294K(T/294 K)-0.85. The effects of pressure broadening of lines by N2 are indistinguishable from those for pure O2 at the same temperature and pressure.

Flux measurements of NH 3, N 2O and CO 2 using dual beam FTIR spectroscopy and the flux–gradient technique

We describe the application of a dual beam Fourier transform infrared (FTIR) spectrometer and sampling system for simultaneous measurements of fluxes of several trace gases between the earth's surface and the atmosphere. The spectrometer is based on a commercial dual-output FTIR spectrometer with two long-path absorption cells, fully automated gas handling, data acquisition and quantitative spectrum analysis. The spectrometer may be operated in single or dual beam (optical subtraction) modes; the advantages and disadvantages of the two modes are tested and discussed. Measurements of fluxes of N 2O, CO 2 and NH 3 from agricultural landscapes were made by the flux–gradient technique in two field trials in Sweden and Denmark in 1993. Fluxes of NH 3 were determined following liquid manuring of a young wheat crop in early summer, and N 2O and CO 2 fluxes were measured from a recently harvested wheat stubble on an unfertilised organic soil in late summer. NH 3 fluxes of more than 5 μgN m −2 s −1 (4 kg ha −1 d −1) were measured a day after fertilisation, decreasing to <0.5 μgN m −2 s −1 two days later. N 2O fluxes averaged 42 ngN m −2 s −1 (36 g ha −1 d −1) over the six days of measurement and showed no significant diurnal or longer term variability. Minimum fluxes of 500 and 20 ngN m −2 s −1 for NH 3 and N 2O, respectively, were detectable with 20 min time resolution. The system is readily extendable to CH 4 measurement. Improvement for the future should improve the minimum detectable fluxes.

Development of a Lidar System for Measuring Methane Using a Gas Correlation Method

We developed a laser long-path absorption lidar system using a gas correlation method for measuring atmospheric methane. This technique uses a broad-band laser and a gas correlation cell. We developed a potassium titanyl arsenate (KTA) optical parametric oscillator at 3.416 µm for the laser source. The optical round-trip path length is 20 m. The experiment was carried out using this system. It was estimated that the error in the density of methane is 4.4 ppm for measurement within 1 s.

The Carbonate Radical (HCO3·/CO3-·) as a Reactive Intermediate in Water Chemistry: Kinetics and Modelling

Results from kinetic laboratory studies of reactions of the carbonate radical anion (CO - 3 .) with aromatic compounds in aqueous solution at T = 298 K are presented. Data were obtained in using a laser photolysis laser long-path absorption (LP-LPLA) apparatus which was designed for direct time-resolved studies of radical reactions. For the reactions of CO - 3 . with hydroquinone dimethyl ether (2), methyl anisole (3), benzene (4), p-xylene (5), toluene (6), chlorobenzene (7), nitrobenzene (8), and benzonitrile (9), rate coefficients of k 2 = (3.0 ± 0.6). 10 7 M-' s -1 , k 3 = (9.7 ± 1.7). 10 5 M -1 s -1 , k 4 = (3.2 ± 0.7). 10 5 M -1 s -1 , k 5 = (3.8 ± 0.9). 10 4 M -1 s -1 , k 6 = (6.8 ± 2.3). 10 4 M -1 s -1 , k 7 = (2.7 ± 0.6). 10 5 M -1 s -1 , k 8 = (1.4 ± 0.5). 10 4 M -1 s -1 , and k 9 < 1.3. 10 2 M -1 s -1 were obtained. In further studies the effect of temperature on the reactions (2), (4), and (5) has been studied. The kinetic data obtained for the reaction of the carbonate radical anion with aromatic compounds were compared to the corresponding reactions of the hydroxyl radical. Finally, these kinetic data were used within a simple model system to investigate the implications of carbonate radical anion kinetics within water treatment processes. It is shown that the degradation of organic pollutants in. OH-radical based water treatment may proceed via the CO - 3 ./HCO 3 . radical under certain conditions.

Integrated absorption intensity and Einstein coefficients for the O2 a 1Δg–X 3Σg− (0,0) transition: A comparison of cavity ringdown and high resolution Fourier transform spectroscopy with a long-path absorption cell

The two experimental techniques of cavity ringdown spectroscopy and high-resolution, long-path Fourier transform spectroscopy have been used to measure quantitative absorption spectra and determine the integrated absorption intensity (Sint,B) for the O2 a 1Δg–X 3Σg− (0,0) band. Einstein A-factors and radiative lifetimes for the O2 a 1Δg v=0 state have been derived from the Sint,B values. The two methods give values for the integrated absorption intensity that agree to within 2%. The value recommended from the results of this study is Sint,B=3.10±0.10×10−24 cm molecule−1, corresponding to an Einstein-A coefficient of A=2.19±0.07×10−4 s−1 and a radiative lifetime of τrad=76 min. The measurements are in excellent agreement with the recent absorption study of Lafferty et al. [Appl. Opt. 37, 2264 (1998)] and greatly reduce the uncertainty in these parameters, for which accurate values are required for determination of upper stratospheric and mesospheric ozone concentrations.

Earth-satellite-Earth laser long-path absorption experiment using the Retroreflector in Space (RIS) on the Advanced Earth Observing Satellite (ADEOS)

This paper reports the results of the laser long-path absorption experiments carried out with the Retroreflector in Space (RIS) on the Advanced Earth Observing Satellite (ADEOS). The RIS is a 0.5 m diameter single-element hollow retroreflector with a unique optical design which uses a curved mirror surface to correct velocity aberrations caused by the satellite movement. In the RIS experiments a laser beam was transmitted from a ground station, reflected by the RIS, and received back at the ground station. The absorption of the intervening atmosphere was measured in the round-trip optical path. After the launch of the ADEOS in August 1996, the optical characteristics of the RIS were tested, and it was confirmed that the RIS worked well in orbit. The spectroscopic measurement was carried out with the single-longitudinal-mode TEA lasers by means of the method utilizing the Doppler shift of the reflected beam caused by the movement of the satellite. The spectrum of ozone was successfully measured in the region, and the measurement of the column contents of ozone was validated with the simultaneous heterodyne spectrometer measurement. In June 1997, however, the experiment with the RIS was discontinued due to the malfunction of the ADEOS solar paddle.

Direct investigations of reactions of 1-butoxy and 1-pentoxy radicals using laser pulse initiated oxidation: reaction with O2 and isomerisation at 293 K and 50 mbar

The reactions of 1-butoxy and 1-pentoxy radicals were studied using time-resolved and simultaneous measurement of NO2 and OH concentrations in laser pulse initiated oxidation studies followed by numerical simulations of the concentration profiles. The alkoxy radicals were produced selectively by the excimer-laser photolysis of 1-butyl bromide and 1-pentyl bromide at 248 nm and subsequent reaction of the 1-alkyl radicals with O2 and NO. Whereas NO2 was detected by cw-LIF, OH was monitored by laser long-path absorption at 308 nm. All experiments were performed at 293±3 K and a total pressure of 50 mbar. The reactions with O2 and the isomerisations via a 1,5-H-shift, viz.,

Gas-phase reaction of the OH–benzene adduct with O2: reversibility and secondary formation of HO2

The reaction of OH radicals with benzene and consecutive reactions of benzene–OH adducts with O2 were studied in the gas phase in N2–O2 mixtures at atmospheric pressure and room temperature. OH was produced by pulsed 248 nm photolysis of H2O2. Time-resolved detection of both OH and benzene–OH adducts was performed by continuous-wave (cw) UV-laser long-path absorption at around 308 nm. The reaction: OH+benzene→products [reaction (1)] was not affected by the presence of O2. Rate constants k1=(1.10±0.07)×10-12 cm3 s-1 and (1.06±0.07)×10-12 cm3 s-1 were obtained in N2 and O2, respectively. In N2 addition of NO2 did not change k1, from which an upper limit of 5% is derived for formation of H atoms in reaction (1). An absorption cross-section of σ(308 nm)=(5.8±1.5)×10-18 cm2 and a self-reaction rate constant of (3.4±1.7)×10-11 cm3 s-1 were determined for the benzene–OH adduct. Upper limits of 5×10-15 cm3 s-1, 1×10-14 cm3 s-1 and 5×10-14 cm3 s-1 were obtained for reactions of the adduct with benzene, H2O2 and NO, respectively. The adduct kinetics in the presence of O2 is consistent with the reversible formation of a peroxy radical: adduct+O2↔adduct–O2 [reaction (2a/-2a)]. An equilibrium constant of K2a=(2.7±0.4)×10-19 cm3 was determined and a rate constant of k2a=(2±1)×10-15 cm3 s-1 was roughly estimated. The effective adduct loss from the equilibrium can be explained by (i) an additional irreversible reaction of the adduct with O2 with a rate constant of (2.1±0.2)×10-16 cm3 s-1, or (ii) a unimolecular reaction of the peroxy radical, with a rate constant of (7.6±0.8)×102 s-1. For a reaction of the peroxy radical with O2 an upper limit of 1×10-17 cm3 s-1 is estimated. Addition of NO reveals formation of HO2 in the presence of O2 by recovering OH via HO2+NO. Applying numerical methods, reaction models were tested to describe the observed complex kinetics of OH. The data are consistent with rapid HO2 formation following a peroxy radical+NO reaction with a rate constant of (1.1±0.4)×10-11 cm3 s-1. Extrapolation of HO2 formation rates to [NO]=0 points at a second source of HO2 not preceded by any RO2+NO reaction.

Optical design of space retroreflector using genetic algorithm

Hollow retroreflectors installed on satellites are used as targets for satellite laser ranging and earth-satellite laser long-path absorption measurements. The optical characteristics of a space retroreflector are determined by several parameters, including mirror curvatures, dihedral angles, and the direction of the retroreflector on the satellite. In this paper, we propose using a genetic algorithm to design space retroreflectors. We applied this method for design of a retroreflector 50 cm in diameter. We found that the optimum design of space retroreflectors having high optical efficiency is obtained in a short time by using the genetic algorithm. © 1999 Scripta Technica, Electron Comm Jpn Pt 2, 81(12): 10–16, 1998

The intermolecular vibrations of the NO dimer

The far infrared spectrum of the NO dimer in the gas phase has been observed for the first time, using a low temperature (100 K) long-path (180 m) absorption cell. The spectroscopic results are used, together with mid-infrared observations and high level CCSD(T) ab initio calculations, to locate the four low frequency intermolecular vibrational modes of (NO)2: ν2 (symmetric bend)=239.361 cm−1, ν3 (intermolecular stretch)=134.503 cm−1, ν4 (out-of-plane torsion)≈117 cm−1, and ν6 (antisymmetric bend)=429.140 cm−1. These values agree poorly with previous determinations based on condensed-phase spectra, indicating that there are large shifts in going from the isolated gas-phase dimer to the liquid, solid, or matrix-isolated environments.