Diode Laser Spectrometer Research Articles

Self-broadening coefficients in the ν2 and ν5 bands of 12CH 3F at 183 and 298 K

The self-broadening coefficients of 33 rovibrational lines in the ν 2 and ν 5 bands of 12CH 3F were measured at a sample temperature of 183 K using a diode-laser spectrometer. We have also realized the measurement of these coefficients at room temperature for 10 of these lines in order to determine their temperature dependence. These results were obtained by fitting to the experimental profile the Voigt lineshape and the Rautian and Galatry models taking into account the collisional narrowing. Calculations of the self-broadening coefficients were also performed for the same temperatures from a semiclassical model involving only electrostatic interactions in the intermolecular potential. The calculated values are significantly larger than the experimental data for both temperatures but the J-dependences of the self-broadenings are well reproduced. Moreover, the theoretical temperature dependence of these coefficients is in good agreement with that derived from the measurements.

Lightweight diode laser spectrometer CHILD (Compact High-altitude In-situ Laser Diode) for balloonborne measurements of water vapor and methane

A new lightweight near-infrared tunable diode laser spectrometer CHILD (Compact High-altitude In-situ Laser Diode spectrometer) was developed for flights to the stratosphere as an additional in situ sensor on existing balloonborne payloads. Free-air absorption measurements in the near infrared are made with an open-path Herriott cell with new design features. It offers two individual absorption path lengths optimized for CH4 with 74 m (136 pass) and H2O with 36 m (66 pass). New electronic features include a real-time gain control loop that provides an autocalibration function. In flight-ready configuration the instrument mass is approximately 20 kg, including batteries. It successfully measured stratospheric CH4 and H2O profiles on high-altitude balloons on four balloon campaigns (Environmental Satellite validation) between October 2001 and June 2003. On these first flights, in situ spectra were recorded from ground level to 32,000-m altitude with a sensitivity of 0.1 ppm [(parts per million), ground] to 0.4 ppm (32,000 m) for methane and 0.15-0.5 ppm for water.

Highly Sensitive Tunable Diode Laser Absorption Spectroscopy ofCO2 Around 1.57 μm

The absolute absorption spectrum intensities of carbon dioxide sampleshas been recorded with a tunable diode laser spectrometer in thespectral range 6350–6364 cm−1, which is suitable for the in-situsensing of carbon dioxide in the lower stratosphere studied using acommercial telecommunication-type diode laser. It was found that thetypical uncertainty of experimental line intensities is about 1%compared with the values listed in the HITRAN database, which arecalculated by direct numerical diagonalization. The intensity ofthe weakest line we detected in this experiment is 1.116×10−26 cm−1/(molecule.cm−2) at the pressure of0.5 Torr, and the corresponding absorption is 3.2×10−6 with SNRof 4.53.

Diode-laser measurements and calculations of N 2-broadening coefficients in the ν7 band of ethylene

N 2-broadening coefficients have been measured for 41 lines of C 2H 4 at 173.2 K in the P, Q, and R branches of the ν 7 fundamental band near 10 μm, using a tunable diode-laser spectrometer. These lines were individually fitted with a Voigt and a Rautian profile in order to determine their collisional widths. The resulting broadening coefficients, as well as those previously measured at room temperature for 35 transitions, are compared with values calculated on the basis of a semiclassical model of interacting linear molecules, using an atom–atom Lennard-Jones potential in addition to the electrostatic contributions. An overall satisfactory agreement is obtained for all the results at room and low temperatures. Finally the temperature dependence of the broadening coefficients has been determined through the temperature exponent n, experimentally for the 23 common transitions as well as theoretically for all the studied transitions.

Pressure-broadening coefficients and line strengths of H 2O near 1.39 μm: application to the in situ sensing of the middle atmosphere with balloonborne diode lasers

Since 1998, we have developed two balloonborne diode laser spectrometers, SDLA and micro-SDLA, to yield in situ concentration data of H 2O in the middle atmosphere by absorption spectroscopy in the 1.39-μm spectral range. In this paper, we revisit the molecular parameters, line intensities and pressure-broadening coefficients, for several lines of the ν 1+ν 3 and 2ν 1 bands, which are useful for the monitoring of atmospheric H 2O. The new values are thoroughly compared to existing molecular database and previous determinations. Calculations are performed to yield the pressure-effect coefficients: the theoretical model is described and the ab initio predictions are compared to laboratory measurements achieved with a laser diode spectrometer. The improvement in the H 2O monitoring obtained with the new set of molecular data is evaluated directly from vertical H 2O concentration profiles.

Vibrational hot bands of linear C 4 and C 5 arising from a bending vibration with two quanta in the lowest bend: the ( ν3 + 2 ν5) − 2 ν5 band of C 4 and the ( ν3 + 2 ν7) − 2 ν7 band of C 5

We report the complete characterization of the hot band ( ν 3 + 2 ν 5) − 2 ν 5 for linear C 4 and the analogous band ( ν 3 + 2 ν 7) − 2 ν 7 for linear C 5 via gas phase, rotationally resolved spectra. C 4 and C 5 were generated using a flowing mixture of acetylene and helium subjected to a hollow cathode discharge. The spectra were recorded using a diode laser spectrometer. Σ–Σ and Δ–Δ transitions were observed for both molecular species. Analyses of these hot bands yielded the following lower state molecular parameters: ν 0 = 1547.3164(5) cm −1, B = 5024.92(18) MHz, D = 0.924(52) kHz, x L = 0.5802(92) cm −1 and q 5 = 10.8890(26) MHz for C 4 and ν 0 = 2166.2331(3) cm −1, B = 2576.524(87) MHz, D = 0.143(13) kHz, x L = 0.409(10) cm −1 and q 7 = 3.9845(15) MHz for C 5. The results were completely consistent with linear structures and support our previous conclusions with regard to semi-rigid linear nature of these two molecular species. This work represents the first conclusive study of a vibrational state with two quanta in the bend for any carbon cluster with four or more atoms.

Line-shape study of water vapour by tunable diode laser spectrometer in the 822?832�nm wavelength region

A near-infrared tunable diode laser absorption spectrometer is set up to measure the air-induced broadening coefficients and the line-strength parameters of water-vapour overtone transitions within the (2,1,1)←(0,0,0) band in the 822–832 nm wavelength region. A Hitachi HL8311 E double hetero-junction structure diode laser is used as a probe. The diode laser controller is home-built and stable within ±10 μA and ±10 mK, respectively. The laser-head mount has a simple design and provides easy access whenever changing of the laser head is required. The diode laser emission wavelength is thermally tuned between 50 °C and 12 °C. Thermal tuning of the diode laser emission wavelength is used to reveal the mode structure of the diode laser and to probe the overtone-band transitions of water vapour within its operating wavelength range. Current tuning of the diode laser is used at a fixed laser temperature to study the transitions one at a time. A balanced detector is used to improve the S/N ratio of the spectrum. A phase sensitive detection technique is followed to obtain the first-derivative spectra of the overtone transitions. The first-derivative spectra have been recorded at different air pressures inside the sample cell while the water-vapour pressure is kept fixed. The first-derivative spectrum is numerically integrated to obtain the original line shape. The original line shape is fitted with a Voigt profile by using a nonlinear least-squares fit program to extract the air-broadening coefficient and the line-strength parameter. The data obtained in our work is compared with the results of the HITRAN database.

Infrared spectra of carbon monoxide–hydrogen sulfide van der Waals complexes in the C–O stretching region

The CO-stretching vibration–rotation spectra of CO–H 2S, CO–D 2S, and CO–HDS complexes have been studied in the 2150 cm −1 region using a supersonic slit-jet expansion and a tunable diode laser spectrometer. The spectra were analyzed with the help of very recent microwave pure rotational studies of the same complexes. Two bands were assigned for each of the symmetric hydrogen sulfide isotopes, corresponding to the two nuclear spin modifications, para and ortho. The band origins were blue shifted, relative to the free CO molecule, by about 3.8 cm −1 for CO–H 2S and 4.3 cm −1 for CO–D 2S. These are considerably smaller shifts than exhibited by the related CO–water complexes, indicating that the intermolecular forces in CO–H 2S are weaker and more isotropic.

Pressure broadening and shift coefficients of H2O due to perturbation by N2, O2, H2 and He in the 1.39 μm region: experiment and calculations

Near-infrared diode laser spectrometry was used to determine the pressure broadening and shift effect on H2O due to N2, O2, H2 and He in the 1.39 µm region. Halfwidths and shifts of water vapour were measured for six transitions. These lines are from the ν 1+ν 3 and 2ν 1 bands. A complete set of H2O transitions with various J values was investigated, including the lines selected to monitor in situ stratospheric H2O with the SDLA, a balloon-borne diode laser spectrometer. Experimental results are compared with theoretical calculations based on a semi-empiric technique that incorporates various corrections to the Anderson theory. This approach is performed in the framework of the impact approximation, which makes interpretation of the collision process simpler and allows reliable results to be obtained.

Line profile study down to 80 K of R(7) in the 13CO (1–0) band perturbed by Ar and 13CO collisional cooling with He at 6.9 K

We present a line profile study at 150, 110 and 80 K for the R(7) line in the fundamental band of 13CO perturbed by Ar in a new stabilized low-temperature cell cooled by liquid nitrogen. The broadening, shifting and narrowing parameters are determined taking into account the absorber speed dependence by simultaneous least-squares fitting of spectra over a 4–300 Torr pressure range recorded using a frequency-stabilized diode laser spectrometer. When the cell is cooled by liquid helium the (1–0) R(2) line of 13CO in collision with helium is observed at a temperature of 6.9 K.

Modeling, measuring, and minimizing the instrument response function of a tunable diode laser spectrometer

The instrument response function (IRF) of a spectrometer limits the accuracy of measured spectroscopic parameters by broadening recorded spectral lines/features. We describe methods to model the effects of the IRF on spectral data, to minimize the IRF widths, and to measure the resulting width of the spectrometer IRF. We have modeled the IRF of our Tunable Diode Laser Spectrometer as a Voigt function. A real-time method of eliminating the effects of low-frequency spectrometer drift has been implemented and has resulted in a substantial reduction in the width of the IRF, its residual Gaussian component reduced from about 0.00045 cm - 1 to about 0.0002 cm - 1 . An accurate measurement of the IRF Gaussian width utilizes a computationally simple method making use of the spectral dependence of the RMS noise of each signal-averaged data point. Various noise sources affecting the spectrometer (preamp/detector noise, laser AM noise, and laser FM noise) are identified and separately quantified by use of the same method. The IRF Gaussian-width measurement can be automatically applied to each measured spectrum of an experimental data set. A related method is discussed which allows accurate determination of the spectral dependence of statistical noise appropriate for use in quantitative Chi-square fitting of absorption spectra. We explore simple, efficient numerical processes which can dramatically enhance the quality and usefulness of acquired spectral data, improving the ability to apply TDL spectroscopy to high-precision, quantitative measurements and the determination of detailed spectroscopic lineshape parameters. This paper provides a guide for interested readers to implement these developments in their own spectrometers.

Mars laser hygrometer.

We have designed and built a miniature near-IR tunable diode laser (TDL) spectrometer for measuring in situ the water vapor mixing ratio either in the Martian atmosphere or thermally evolved from Martian soil or ice samples. The laser hygrometer uses a thermoelectrically cooled single-mode distributed-feedback TDL at 1.87 microm to scan over a selected vibration-rotation line of both H2O and CO2 near 5327.3 cm(-1). A working prototype that weighs only 230 g has been built and used to generate spectra whose analysis demonstrates precision sensitivities as fine as 1 part in 10(6) by volume in 1 s or 0.1 part in 10(6) in 10 s at Martian pressures and temperatures. Absolute uncertainties of approximately 5% are calculated.

Infrared spectra of seeded hydrogen clusters: (paraH2)N - OCS, (orthoH2)N - OCS, and (HD)N - OCS, N = 2 - 7.

Infrared spectra of hydrogen-carbonyl sulfide clusters containing paraH2, orthoH2, or HD have been studied in the 2060 cm(-1) region of the C-O stretching vibration. The clusters were formed in pulsed supersonic jet expansions and probed using a tunable infrared diode laser spectrometer. Simple symmetric rotor type spectra were observed and assigned for clusters containing up to N = 7 hydrogen molecules. There was no resolved K structure, and Q-branch features were present for orthoH2 and HD but absent for paraH2. These characteristics can be rationalized in terms of near symmetric rotor structures, very low effective rotational temperatures (0.15 to 0.6 K), and nuclear spin statistics. The observed vibrational shifts were compared with those from recent observations on the same clusters embedded in helium nanodroplets. The observed rotational constants for the paraH2 clusters are in good agreement with a recent quantum Monte Carlo simulation. Some mixed clusters were also observed, such as HD-HD-He-OCS and paraH2 - orthoH2 - OCS.

Photo-acoustic measurements of gas and aerosol absorption with diode lasers

The results of designing multipurpose high-sensitive photo-acoustic (PA) detectors and their application to high-resolution diode laser spectroscopy of molecular gases, gas analysis, and aerosol absorption measurements are summarized in this paper. The hardware and software of the diode laser spectrometer with a Helmholtz resonant PA detector providing an absorption sensitivity limit of better than 10 −7 W m −1 Hz −1/2 are described. A procedure is proposed for an experiment involving the measurements of the rotational structure of hot vibrational bands of molecules. The results of the application of the nonresonant PA cell with temporal resolution of signals to measurements of weak nonresonant absorption of gases and soot aerosols are presented, and the possibility of creating a broad-band PA laser diode aerosol-meter is discussed.

A compact NIR fiber-optic diode laser spectrometer for CO and CO 2: : analysis of observed 2f wavelength modulation spectroscopy line shapes

A compact fiber-optic diode laser spectrometer for the measurement of CO and CO 2 gas concentrations in the near infrared around 1580 nm is described. By use of a balanced receiver to suppress diode laser intensity noise a sensitivity of 6.4×10 −7 at 1 Hz system bandwidth was achieved. At a reduced pressure of 80 hPa this equals to a detection limit of 5.1 ppm CO and 9.1 ppm CO 2 with 1 m absorption path length. The observed line shapes of the 2f wavelength modulation spectroscopy (WMS) scheme are analyzed theoretically and experimentally. Accurate measurements of magnitude and phase of the diode laser frequency and intensity modulation responses were found critically for modeling the observed line shapes. In situ measurements of gas dissociation processes inside of a medium-power carbon dioxide laser are presented as an application example.

Spatial and temporal variability of high resolution reduced nitrogen concentrations in the Fraser Valley

We present high resolution measurements of ammonia (NH3) and particle-ammonium (NH4+) collected during the Pacific 2001 experiment at an urban, sub-urban and high elevation site. These data are used to conduct a detailed instrument inter-comparison and to characterize the sites. The data are interpreted with respect to; the degree to which the sites are representative, the degree of sensitivity to inorganic particle burdens to NH3 availability and controls on reduced nitrogen (NHx) variability and partitioning. Ammonia measurements from wet effluent diffusion denuders (WEDD), annular diffusion denuders and a tunable diode laser spectrometer generally lie within a factor of 1.5 across a range of observed concentrations of 0–0.25 μmol m−3, although individual observations differed by more than a factor of two. The high resolution (2 min average) data from the WEDD systems are used to calculate a coefficient of variability which is used to assess the degree to which the measurement sites are spatially representative. The results indicate the sub-urban site is the most spatially representative. Ammonia concentrations at this site exhibit variability on time scales of less than 4 h that is comparable to that of ozone. Data collected in the urban setting indicate persistent measurable NH3 concentrations and that NH4+ dominates NHx. Conversely, the high elevation site shows a much greater degree of variability due in part to the influence of meteorological coupling and decoupling from the underlying valley, domination of NHx by NH3 and a stronger coupling of NH3 and NH4+.

Hydrogen line broadening in the ν2 and ν4 bands of phosphine at low temperature

Using a tunable diode-laser spectrometer, we have measured H 2-broadening coefficients of PH 3 at low temperature (173.2 K) for 27 lines in the Q R branch of the ν 2 band and in the P P and R P branches of the ν 4 band. The recorded lines with J values ranging from 2 to 11 and K from 0 to 9 are located between 1016 and 1093 cm −1. The collisional widths are determined by fitting each spectral line with a Voigt profile and a speed-dependent Rautian profile which provides slightly larger broadening coefficients than the Voigt model. These coefficients have also been calculated on the basis of a semiclassical model of interacting linear molecules by considering an atom–atom Lennard–Jones potential in addition to the weak electrostatic contributions. Except for three Q R( J, K) lines, where K = J, the calculated broadening coefficients are in good agreement with the experimental data. By comparing the results obtained at room and low temperatures, the temperature dependence of linewidths has been determined both theoretically and experimentally.

Diode laser spectroscopy of ammonia and ethylene overtones

Some overtone absorption lines of ammonia and ethylene have been examined by using a tunable diode laser (TDL) spectrometer in the region around 12,650 and 11,800 cm −1, respectively. The spectrometer sources are commercially available double heterostructure InGaAlAs and AlGaAs TDLs operating in the “free-running” mode. The high resolving power (∼10 7) of the spectrometer permitted the detection and the study of the line positions of such molecules with a precision better than 0.01 cm −1. In order to maximize the signal to noise ratio and to extract the necessary informations either on the line width and on the line position for the detected molecular resonances, the wavelength modulation spectroscopy (WMS) along with the second harmonic detection techniques have been applied. For this purpose, the fitting procedure took into account the instrumental effects and the amplitude modulation (AM) always associated with the frequency modulation (FM) of these type of sources. This technique permitted also the measurement of the collisional-broadening and -shifting coefficients by different buffer gases at room temperature.

Development of a stabilized low temperature infrared absorption cell for use in low temperature and collisional cooling experiments

We have constructed a stabilized low temperature infrared absorption cell cooled by an open cycle refrigerator, which can run with liquid nitrogen from 250 to 80K or with liquid helium from 80K to a few kelvin. Several CO infrared spectra were recorded at low temperature using a tunable diode laser spectrometer. These spectra were analyzed taking into account the detailed effects of collisions on the line profile when the pressure increases. We also recorded spectra at very low pressure to accurately model the diode laser emission. Spectra of the R(2) line in the fundamental band of 13CO cooled by collisions with helium buffer gas at 10.5K and at pressures near 1Torr have been recorded. The He-pressure broadening parameter (γ0=0.3cm−1atm−1) has been derived from the simultaneous analysis of four spectra at different pressures.

Concentration measurements of ozone in the 1200–300 ppbv range: an intercomparison between the BNM ultraviolet standard and infrared methods

Simultaneous ultraviolet (UV) and infrared (IR) measurements of ozone concentration in air in the 1200–300 ppbv range have been performed using the ultraviolet absorption in the Hartley band at 0.2537 μm and the infrared absorption of a doublet at 9.507 μm in the ν 3 vibration–rotation band. Infrared concentration measurements were achieved using the tunable diode laser spectrometer of LPMA in Paris with interferometric control of the emitted wavelength while the UV concentration measurements were performed with the 49PS Megatec ozone generator of the Bureau National de Métrologie (BNM). The simultaneous recording of spectra of a reference cell filled with pure distilled ozone and of a low concentration mixture inside a long absorbing path Herriott cell allows to carry out infrared concentration measurements with an accuracy of the same order as the ultraviolet ones and provides the instrumental parameters of the spectrometer corresponding to each concentration measurement, which reduces systematic errors. Within the respective absolute uncertainties proper to the two techniques, no systematic discrepancy was evidenced between the IR and the UV measurements. The ozone ultraviolet absorption coefficient value determined by Hearn (308.3±4 cm −1 atm −1) and used by the BNM and the National Institute of Standards and Technology (NIST) is confirmed by the present work.