Detection of N2O by photoacoustic spectroscopy with a compact, pulsed optical parametric oscillator

A pulsed optical parametric oscillator (OPO) operated in an optical cavity with a grazing-incidence grating configuration (GIOPO) was used for sensitive photoacoustic detection of trace quantities of dinitrogen oxide (N₂O). The (ν₁+ν₃) combination vibration band of N₂O was excited with the idler beam of the GIOPO at 2.86 μm using an optical cavity optimized for the idler beam. The linewidth of the GIOPO could be reduced to 0.4 cm^-1, allowing the rotational structure of the absorption spectrum to be resolved. A concentration sensitivity (signal-to-noise ratio=3) of 60 parts in 10⁹ by volume (60 ppb V) N₂O in synthetic air was obtained. This may be sufficient for continuous monitoring of N₂O in the atmosphere. Received: 29 April 2002 / Revised version: 4 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +49-6221/54-4255, E-mail: peter.hess@urz.uni-heidelberg.de     

Pulsed cavity ring-down spectroscopy of NO and NO2 in the exhaust of a diesel engine

The application of pulsed cavity ring-down spectroscopy has been demonstrated for the in situ quantitative determination of NO and NO₂ in the exhaust of a diesel engine. NO absorption has been monitored at the transition from the Χ²Π ground state to the A²Σ⁺ state at 226 nm. For NO₂, absorption bands in the spectral region from 438 nm to 450 nm were used. At the selected engine conditions, concentrations of 212±22 ppm and 29±4 ppm have been measured for NO and NO₂, respectively, in good agreement with separate chemical exhaust gas analysis. The method is sensitive enough to meet the European Euro V standard directive on NO_x emissions. This communication discusses the relatively simple setup needed for this type of measurement, the problems encountered, as well as the prospects for single-stroke, simultaneous measurements of both NO and NO₂ at the sub-ppm level. Received: 30 November 2001 / Revised version: 18 February 2002 / Published online: 14 March 2002     

Quantification of stable minor species in confined flames by cavity ring-down spectroscopy: application to NO

Cavity ring-down spectroscopy (CRDS) is used to measure the NO mole fraction formed in the burnt gases of low-pressure premixed flames. It is shown that the line-of-sight absorption is greatly increased by the contribution of the NO molecules surrounding the burner. This contribution has been quantified by developing a mathematical procedure taking into account the spatial and spectral features of the CRDS measurement. Calculations have been undertaken in the general case of a stable species not consumed in the flame. The most sensitive parameter is the temperature both in the flame and outside the flame. Simulations allow the selection of the best spectroscopic transitions for a given flame (i.e. a given temperature profile), ensuring the weakest influence of the inaccuracy affecting the temperature determination. High quantum states belonging to the A–X (0–1) band of NO have been found to be the most valuable and have led to a NO mole fraction determination with an accuracy of ±13%. NO absorption in the flame was completely masked using the A–X (0–0) band. Finally, the prompt-NO mole fraction formed in a methane/air flame stabilized at 33 Torr is obtained by combining CRDS and laser induced fluorescence techniques. Received: 12 October / Revised version: 1 February 2002 / Published online: 14 March 2002     

Optimization of the mode matching in pulsed cavity ringdown spectroscopy by monitoring non-degenerate transverse mode beating

A simple and reliable method is presented for optimizing the mode matching of a laser beam to the high-finesse cavity used in pulsed cavity ringdown spectroscopy (CRDS). The method is based on minimizing the excitation of higher-order transverse cavity modes through monitoring the non-degenerate transverse mode beating which becomes visible with induced cavity asymmetry caused by slight misalignment. No additional instrument is required other than a pinhole aperture, thus this method can be applied for CRDS experiments in the whole wavelength range. Measurements of the CRDS absorption spectrum of acetylene (C₂H₂) near 571 nm demonstrate that the mode-matching optimization improves the sensitivity of pulsed CRDS. Received: 22 October 2001 / Revised version: 16 January 2002 / Published online: 14 March 2002     

Laser diagnostics of NO reburning in fuel-rich propene flames

Absolute NO concentrations were measured by laser-induced fluorescence (LIF) in three different fuel-rich non-sooting propene flames (φ=1.5, 1.8 and 2.3). The experiments were performed in low-pressure premixed propene flames with 0.2%-1% NO added. Laser diagnostics was applied as a tool for investigating reburn chemistry. The Q₂(25.5) line in the A-X(0,0) band was excited because of the small temperature dependence of its ground state population. The NO fluorescence lifetimes were measured directly and compared to theoretical values. The initial NO levels are strongly reduced in all three flames. According to modeling results, the HCN mole fraction increases strongly with stoichiometry. As guidelines for laser diagnostics applications in such systems, the modeling results were analyzed with respect to the main reaction channels and reaction partners in fuel-rich flames. Received: 1 March 2000 / Revised version: 20 April 2000 / Published online: 20 September 2000     

Laser intracavity absorption spectroscopy

Emission spectra of multimode lasers are very sensitive to spectrally selective extinction in their cavity. This phenomenon allows the quantitative measurement of absorption. The sensitivity of measurements of intracavity absorption grows with the laser pulse duration. The ultimate sensitivity obtained with a cw laser is set by various perturbations of the light coherence, such as quantum noise, Rayleigh scattering, four-wave mixing by population pulsations, and stimulated Brillouin scattering. It depends on the particular laser type used, and on its operative parameters, for example pump power, cavity loss, cavity length, and length of the gain medium. Nonlinear mode-coupling dominates the dynamics of lasers that feature a thin gain medium, such as dye lasers, whereas Rayleigh scattering is more important in lasers with a long gain medium, such as doped fibre lasers, or the Ti:sapphire laser. The highest sensitivity so far has been obtained with a cw dye laser. It corresponds to 70000 km effective length of the absorption path. The ultimate spectral resolution is determined by the spectral width of mode emission, which is 0.7 Hz in this dye laser. High sensitivity and high temporal and spectral resolution allow various practical applications of laser intracavity spectroscopy, such as measurements and simulations of atmospheric absorption, molecular and atomic spectroscopy, process control, isotope separation, study of free radicals and chemical reactions, combustion diagnostics, spectroscopy of excited states and nonlinear processes, measurements of gain and of spectrally narrow light emission. Intracavity absorption in single-mode lasers shows enhanced sensitivity as well, although not as high as in multimode lasers. Received: 10 May 1999 / Published online: 29 July 1999     

High-resolution spectroscopy of velocity-selected atoms in a thin cell

We propose a novel technique of sub-Doppler spectroscopy using a thin vapor cell. Optical pumping in a thin cell transfers atoms with small velocity components to a specific quantum state. The resultant velocity distribution appears as a sub-Doppler structure in the absorption spectrum of a probe light. A single laser beam from a laser diode is split into two paths: one beam optically pumps Cs atoms on the D₂ line, and the other probes the absorption on the same line from a perpendicular direction. Observed hyperfine-resolved spectra and their parameter dependence are analyzed on the basis of rate equations. Received: 16 January 2002 / Revised version: 11 February 2002 / Published online: 24 April 2002     

A pulsed optical parametric oscillator, based on periodically poled lithium niobate (PPLN), for high-resolution spectroscopy

-1 ), approaching the limit imposed by the Fourier transform of the pulse duration. Received: 31 August 1998     

Thermal grating and broadband degenerate four-wave mixing spectroscopy of OH in high-pressure flames

2 ∑–X²Π(0,0) band of OH has been studied in premixed methane/air flames using a cw Ar⁺ laser probe. Measurements of flame temperature and pressure were derived from fits of theoretical simulations to the observed time variation of signals over a pressure range of 10 to 40 bar and for different stoichiometry that were in agreement with independent measurements using N₂ CARS and predictions of a one-dimensional flame calculation. Broadband DFWM spectra in the same band of OH were observed up to a pressure of 9 bar, above which signals were obtained only from scattering from thermal gratings. Received: 10 November 1997/Revised version: 28 May 1998     

Laser spectroscopy with a pulsed, narrowband infrared optical parametric oscillator system: a practical, modular approach

-1 in a single scan. The potential of the OPO system for linear and nonlinear-optical spectroscopy is demonstrated by recording high-resolution photoacoustic absorption and coherent anti-Stokes Raman spectra of methane, as a gas and in a pulsed supersonic free jet. This narrowband tunable infrared source is shown to scan reliably with an optical bandwidth as small as 0.007 cm^-1 (210 MHz) full width half maximum (fwhm), close to the fourier-transform limit. Received: 5 February 1998/Revised version: 6 March 1998     

Cavity ring-down spectroscopy for atmospheric trace gas detection: application to the nitrate radical (NO3)

Cavity ring-down spectroscopy is a relatively new and quite sensitive technique for the measurement of gas-phase optical extinction. It holds the potential for simple, direct and sensitive measurement of the concentrations of a variety of trace gases in the atmosphere. For example, detection of the nitrate radical, NO₃, and its companion, dinitrogen pentoxide, N₂O₅, has been demonstrated with a sensitivity of 0.25 pptv (1σ). This paper considers several of the requirements for the application of cavity ring-down spectroscopy to concentration measurements of trace gases in ambient air. These include detection sensitivity, measurement of an accurate zero in the presence of competing absorbers, cavity stability and mirror cleanliness, laser line-width effects, saturation effects, Rayleigh scattering, the influence of atmospheric aerosols and sampling issues for reactive species. Examples drawn from our work on NO₃ and N₂O₅ detection in the field illustrate these considerations. Received: 1 April 2002 / Revised version: 5 June 2002 / Published online: 12 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-5822, E-mail: sbrown@al.noaa.gov     

Cavity ring-down spectroscopy of photochemically produced NaH for the determination of relative dipole transition moments

-30 Cm of isolated lines in the A¹Σ⁺←X¹Σ⁺ NaH transition band were measured. Received: 18 November 1996/Revised version: 10 May 1997     

High-stability reference setup for photoacoustic spectroscopy

2 laser at 9.676 μm. Received: 2 December 1996/Revised version: 2 April 1997     

Bichromatic pulsed source in the visible and infrared

A source of bichromatic coherent radiation is described with independent tunability in wavelength, energy and pulse duration. The system is able to deliver pulses in the visible to infrared region with possible extension into the ultraviolet. Pulse durations can be tailored to be from 15 ps to 310 fs with collimated beam fluences of several mJ/cm². Received: 28 January 2002 / Revised version: 24 April 2002 / Published online: 12 July 2002     

Artifacts in femtosecond transient absorption spectroscopy

The paper discusses three different artifacts related to two-photon absorption (TPA), stimulated Raman amplification (SRA) and cross-phase modulation (XPM), all intrinsic to transient absorption measurements with femtosecond time resolution. Certain properties of these signals are analysed and shown to superimpose onto measured transient absorption spectra. Ways of reducing the influence of the artifacts discussed are suggested. A simple correcting procedure based on the linear intensity dependence of the artifacts discussed is proposed. Received: 29 May 2001 / Final version: 15 October 2001 / Published online: 29 November 2001     

Detection of NO in a spark-ignition research engine using degenerate four-wave mixing

We report the first application of degenerate four-wave mixing (DFWM) to combustion diagnostics in a methane-fuelled internal combustion research engine. Combustion-generated NO in the spark-ignited engine was detected using scanning narrowband DFWM in a modified forward folded BOXCARS geometry. The resulting spectra of the X²Π-A²Σ⁺(0,0) band at 226 nm display an excellent signal-to-noise ratio. Extension of the technique to different engine operating conditions and to time-resolved multiplex DFWM is discussed. Received: 3 May 2001 / Revised version: 1 October 2001 / Published online: 29 November 2001     

Resonant photoacoustic simultaneous detection of methane and ethylene by means. of a 1.63-μm diode laser

A compact multi-component trace-gas detector based on the resonant photoacoustic technique and a NIR external cavity diode laser has been developed. It has been characterized using a mixture of ethylene and methane diluted in ambient air. A spectroscopic investigation of combination bands and overtones between 5900 and 6250 cm^-1, obtained with an IR pulsed laser photoacoustic spectrometer, allowed us to find a wavelength region where the 2ν₃ overtone of CH₄ and the ν₅+ν₉ combination band of C₂H₄ show uncongested rotational lines. Using a single-mode scan of the diode laser in this region, around 6150 cm^-1, the sensitivity for the simultaneous detection of ethylene and methane is 8 ppm/mW and 40 ppm/mW respectively. Factors affecting the sensitivity and selectivity of the detection system and possible improvements suitable to reach the sub-ppm detection limit are discussed. Received: 1 August 2001 / Revised version: 28 November 2001 / Published online: 7 February 2002 An erratum to this article is available at .     

On-line monitoring of biogenic isoprene emissions using photoacoustic spectroscopy

Isoprene (C₅H₈) is one of the most important biogenic volatile organic compounds (VOCs) in the atmosphere. To calculate the impact of isoprene on atmospheric processes models have been developed that describe the isoprene release from plants. Measurements of this release require techniques for a fast, sensitive, on-line isoprene detection. Photoacoustic (PA) spectroscopy is applied here for the first time to monitor biogenic isoprene emissions. A CO overtone PA spectrometer is used for the detection, probing the C-H stretching vibrations in the 3 to 4 μm range. This allows us to detect isoprene down to a few ppb with a time resolution of one minute in a continuous gas flow. The number of laser lines can be adjusted to meet the requirements of the respective experiment in terms of time resolution and selectivity against other possibly interfering VOCs. This results in a highly versatile instrument for the isoprene detection in biological experiments. Furthermore, the infrared fingerprint offers the potential to detect different isoprene isotopomers simultaneously, thus allowing us to carry out on-line labelling experiments. The new apparatus was used to study the light dependence of isoprene emission from Eucalyptus globulus. The results demonstrate, that the detector system is a promising tool for the study of plant gas emissions. It allows the validation of existing emission models which are important for atmospheric processes. Received: 18 March 1999 / Revised version: 23 August 1999 / Published online: 30 November 1999     

Cavity ring-down spectroscopy of molecularly thin iodine layers

Cavity ring-down spectroscopy (CRDS) has so far mostly been used for measurements in the gas phase. Only in 1999 was a first spectrum of condensed phase published. This spectrum was measured by using a coated plate between the cavity mirrors. Rather than using this method, our measurements were made using the cavity mirrors as a substrate. This way, the scattering losses could be reduced by approximately a factor of 100. In our measurements we investigated molecularly thin layers of iodine. The iodine spectra were taken in the frequency range from 16200 to 17200 cm^-1 using pulsed CRDS. Received: 14 April 2000 / Revised version: 26 July 2000 / Published online: 22 November 2000     

Simultaneous online detection of isoprene and isoprene-d2 using infrared photoacoustic spectroscopy

A photoacoustic spectrometer for the simultaneous detection of isoprene and the deuterated species [4,4-²H]-2-methyl-1,3-butadiene (isoprene-d₂) is presented. Using a sealed-off ¹³CO₂ laser a single-component detection limit of 400 ppt (isoprene) and 600 ppt (isoprene-d₂) was achieved. Simultaneous monitoring of both compounds allowed the detection of labelling levels down to 6% (isoprene-d₂ in total isoprene) with a time resolution of 3 min. In emission studies with Eucalyptus globulus, the deuterated precursor [4,4-²H]-1-deoxy-D-xylulose was fed to a leaf through the transpiration stream. Emission of isoprene-d₂ started as early as 10 min after application of the precursor. Received: 3 May 2002 / Revised version: 31 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +49-228/733474, E-mail: frank.kuehnemann@iap.uni-bonn.de