Diode Laser Spectrometer Research Articles

Free-jet infrared diode laser spectroscopy of the ν2-band of the Ar–N2O van der Waals complex

The vibration–rotational spectrum of the Ar–N2O van der Waals complex has been recorded in the low-frequency stretching mode region of the N2O-monomer (ν1∼1285cm−1). Almost 400 absorption lines were detected using a free supersonic slit-jet expansion in combination with an infrared tunable diode laser spectrometer. The recorded spectrum was assigned to the ν2-band of Ar–N2O. Rotational constants as well as a full set of quartic centrifugal distortion constants are reported. The effective geometrical parameters of this T-shaped complex in the ν2-excited state are calculated and found to be R=3.46(2)Å and θ=82.9(7)°. The band-origin of the spectrum is observed at ν0=1285.11907(14)cm−1 and shows a blue-shift compared to the N2O-monomer spectrum of 0.2158(2)cm−1.

Line intensity and self-broadening investigations in the ν1 and ν3 bands of SO2

Line intensities and self-broadening coefficients were measured for 37 transitions in the ν1 band of SO2 between 1072 and 1149cm−1 in the rotational quantum number range between 3≤J″≤44 and 0≤Ka″≤23 and for 12 transitions in the ν3 band of SO2 near 1321cm−1 in the rotational quantum number range between 50≤J″≤61 and 2≤Ka″≤18. The experiments were carried out with a dual-channel lead–salt diode-laser spectrometer, which was operated in pulsed mode. The spectroscopic parameters for the individual lines were determined from the analysis of spectra measured in the pressure range between 1 and 12Torr. The measured data of a complete pressure series were analyzed at once. Therefore, the fit delivered directly the self-broadening coefficients and the line intensities. The determined self-broadening coefficients are within the uncertainties in good agreement with previously published data and confirm the typical decrease of the self-broadening coefficient γSO2–SO2 with increasing rotational quantum number Ka″. The broadening coefficients decrease from a γSO2–SO2 by about 0.4cm−1/atm for lines with small Ka″down to values of about 0.2cm−1/atm for lines with a Ka″>18. The line intensities determined here were slightly smaller compared to the data listed in the HITRAN 1996 database. The deviations do not show any remarkable quantum number dependence. The average values of the ratio SiExp/SiHITRAN 1996 were calculated to 0.83±0.02 (ν1 band) and 0.75±0.03 (ν3 band). The integrated band intensities were determined to (3.41±0.11)×10−18cm−1/moleculecm−2 for the ν1 band and to (2.40±0.15)×10−17cm−1/moleculecm−2 for the ν3 band, respectively.

Line-Mixing Effects in He- and N2-Broadened Q Branches of C2H2 at Low Temperatures

Two Q branches of C2H2 near 716.3 and 719.9 cm−1 of Σ←Π symmetry have been studied for He and N2 perturbers at pressures ranging from 0.05 to 0.8 atm and low temperatures (−75 and −90°C) using a tunable diode-laser spectrometer. To interpret the strong line-mixing effects observed in these spectra, we have applied a model based on the energy corrected sudden approximation (ECS) whose parameters are derived from those previously determined at room temperature through a fitting procedure of line-broadening coefficients. These coefficients have been measured for a few lines of C2H2 perturbed by He at −100°C. The ECS model provides satisfactory agreement with experimental bandshapes for both bands, temperatures and perturbers at pressures generally higher than 0.1 atm.

A Near-Infrared Diode Laser Spectrometer for the In Situ Measurement of Methane and Water Vapor from Stratospheric Balloons

The Spectrometre a Diodes Laser Accordables (SDLA), a balloonborne near-infrared diode laser spectrometer, was developed to provide simultaneous in situ measurements of methane and water vapor in the troposphere and the lower stratosphere. The instrument was flown several times from stratospheric balloons operated by the Centre National d'Etudes Spatiales within the framework of the Third European Stratospheric Experiment on Ozone in 1998–2000. The SDLA is based on a multipass optical cell open to the atmosphere. Two near-infrared telecommunication-type laser diodes are connected with optical fibers to the cell to take in situ absorption spectra of methane (in the 6047 cm−1 spectral region) and water vapor (in the 7181 cm−1 spectral region) at 1-s intervals. Mixing ratios are obtained, with a precision error of within 5%–10%, from a nonlinear fit to the full molecular line shape in conjunction with in situ pressure and temperature measurements. The SDLA is described, and achieved atmospheric methane and water vapor vertical concentration profiles are reported.

The MVACS tunable diode laser spectrometers

Two independent tunable diode laser spectrometers are resident aboard the Mars Polar Lander as part of the Mars Volatiles and Climate Surveyor payload. One spectrometer is located on the meteorological mast for measurements of H2O and CO2 in the free atmosphere, and the other serves as the H2O and CO2 analyzer for the Thermal and Evolved Gas Analyzer. Water vapor is measured using a tunable diode laser operating at 1.37 μm, while CO2 is measured using a second laser operating near 2.05 μm. The 2.05 μm laser also has isotopic analysis capability. In addition to the major CO2 isotopomer (12C16O16O), analyses of 13C16O16O and 12C18O16O in the atmosphere and in the Thermal and Evolved Gas Analyzer are possible under certain conditions. The spectrometers were designed and built at the Jet Propulsion Laboratory and have their heritage in a series of tunable diode laser spectrometers developed for Earth atmospheric studies using high‐altitude aircraft and balloon platforms. The 1.37 μm diode laser on the meteorological mast will provide the first in situ measurements of water vapor in the Martian boundary layer, with a detection sensitivity an order of magnitude greater than the water vapor abundances inferred from the remote‐sensing observations by the Viking Orbiters.

Balloon-borne near-infrared diode laser spectroscopy for in situ measurements of atmospheric CH 4 and H 2O

Absorption spectroscopy with near-infrared telecommunication laser diodes is a very convenient technique to measure in situ methane and water vapor in both the troposphere and the lower stratosphere (LS) and thereby to address many topics in the science of the atmosphere. This technique offers a high temporal resolution that ranges from 10 ms to 1 s, a precision error in the concentration retrieval of within a few percents and a dynamic range for the measurements of four orders of magnitude. A balloon-borne near-infrared diode laser spectrometer is described that provides simultaneous in situ methane (in the 1.65-μm region) and water vapor (in the 1.39-μm region) measurements at 1 s intervals. Tropospheric and stratospheric vertical concentration profiles of methane and water vapor are reported.

Diode-Laser Measurements of Absolute Line Intensities in the ν1 Band of Cyanogen Chloride

With a tunable diode-laser spectrometer, absolute line intensity measurements have been made in the spectrum of cyanogen chloride in the vicinity of 710 cm−1. The intensity of the fundamental ν1 band has been found to be 6.088 cm−2 atm−1 for the 35ClCN isotopomer and 7.121 cm−2 atm−1 for the 37ClCN form. The influence of the Fermi resonance on the band intensity has been analyzed.

Diode-Laser Spectroscopy: Line Profiles of H2O in the Region of 1.39 μm

Using a stabilized diode-laser spectrometer, we have studied the line profiles of water vapor near 7185.59 cm−1. With low pressures of pure gas (≤1,Torr) in a White-type cell, we have deduced the line intensities using the Voigt profile. This study shows clear differences between experimental and calculated lines described by a Voigt profile when the pressure increases. For water vapor in mixtures with N2, O2, Ar, or He, we have determined the collisional broadening coefficients, taking into account the collisional narrowing (Dicke effect). The narrowing parameters have been determined using soft (Galatry) or hard (Rautian and Sobel'man) collision models and compared to the dynamic friction coefficient.

N2-Broadening for Methyl Chloride at Low Temperature by Diode-Laser Spectroscopy

Using a tunable diode-laser spectrometer, we have measured N2-broadening coefficients of 87 individual rovibrational lines in the P and R branches of the ν3 band of methyl chloride (CH335Cl) at 203.2 K. These lines with J values ranging from 2 to 22 and K from 0 to 6 are located in the range 711–751 cm−1. Most of the collisional widths are obtained by modeling the spectral region from the superposition of overlapping Voigt or Rautian profiles. We have reviewed and completed previous measurements of N2-broadening coefficients at room temperature in the ν3 band, in order to compare them to the low-temperature data. A semiclassical calculation, including electrostatic, induction, and dispersion energy contributions, has provided results that are in reasonable agreement with the experimental data at room temperature, but generally larger than the data at low temperature for small or medium J values. The temperature dependence of the collisional broadenings derived from a simple power law has been determined both experimentally and theoretically.

Measurements of air and noble-gas broadening and shift coefficients of the methane R3 triplet of the 2ν3 band

By using a tunable diode laser spectrometer with one absorption White cell for low pressure and one photoacoustic cell for high pressure, line shape parameters of the R3 triplet of the 2ν3 band of methane at 1.65 μm were measured. The absorption line was recorded by using the wavelength modulation spectroscopy technique with first harmonic detection. The broadening and shift coefficients were obtained by fitting the first harmonic absorption signal while varying the pressure of different perturbing gases: air and noble gases (helium, neon, argon, krypton and xenon). We present here the results for the R3 triplet. The observed shift and broadening coefficients behaviors are discussed.

Rovibrational Study of the CH2 Wagging Fundamental of Monofluoroacetonitrile

The vapor-phase infrared spectrum of monofluoroacetonitrile (CH2FCN) has been recorded at high resolution in the ν4 band region (1363–1398 cm−1) using a tunable diode laser spectrometer. A detailed assignment of the rotational structure of the expected a-/b-hybrid band has been made for a-type transitions with Ka≤19, J≤61. The b-type transitions were too weak to be observed. Upper state spectroscopic constants including the sextic terms have been obtained from a set of unperturbed data employing the Watson A-reduced Hamiltonian in the Ir representation. A number of perturbed transitions have been accounted for, including b- and c-type Coriolis resonances and anharmonic resonances between ν4 and the nearby levels ν5+ν12 and ν11+ν12. The vibrational energies of these perturbing levels have been obtained from the reanalysis of the low-resolution infrared spectrum. Rotational constants and the band origin of the perturbers along with the interaction parameters with the v4=1 state have also been determined.

Eddy flux measurements of methane over the fen “Murnauer Moos”, 11°11′E, 47°39′N, using a fast tunable diode laser spectrometer

Eddy covariance measurements of methane were carried out over the fen “Murnauer Moos” in the south of Germany in order to evaluate the performance of a newly developed eddy covariance measurement system, based on a frequency-modulated tunable diode laser spectrometer as a fast chemical sensor. During a six-day period, an average daytime methane emission of (5.4±1.8) mg CH 4 m −2 h −1 was measured. We find this value moderate, considering the favorable meteorological and soil conditions for methane emission. Diurnal cycles of the fluxes of methane and carbon dioxide as well as of sensible and latent heat are presented. Results are discussed in terms of relevant micrometeorological quantities, and quality control procedures based on Allan variance and spectral analysis are discussed.

Absolute Line Intensities and Self-Broadening Coefficients in the ν1 Band of 35Cl12C14N

The self-broadening coefficients and the intensities of 29 lines in the ν1 band of cyanogen chloride (35Cl12C14N) have been measured at high resolution in the range 699–736 cm−1, using a tunable diode-laser spectrometer. The collisional widths and most of the intensities are obtained by fitting Voigt and Rautian profiles to the measured shapes of the lines. From the analysis of the line intensities we determine the absolute strength as well as the Herman–Wallis factors for the ν1 band. A semiclassical calculation of the self-broadening coefficients, performed by considering the main electrostatic interactions only, has provided larger results than the experimental data.

Line broadening and line shift of H 2S absorption lines in the ν2 band — collisions with H 2O and Ar in a three component mixture

Lorentzian half-widths (HWHM) and shifts of the H 2S absorption line at 1294.5083 cm −1 (ν 2 band) induced by the pressure of H 2O and Ar in binary (H 2S– H 2 O, H 2 S –Ar) and three-component mixtures (H 2S–Ar–H 2O) were measured using a high-resolution diode-laser spectrometer with a spectral resolution better than 5×10 −4 cm −1 and two multipass Herriott absorption cells. The measurements were carried out to check experimentally the additivity of the binary collisions contributions to the broadening and shift of the same absorption line in the mixture of H 2S–Ar–H 2O. Due to the atmospheric relevance as one collisional partner water vapor was selected. The mixtures of H 2S–H 2O–Ar were prepared with a fixed partial pressure of H 2S and Ar. The H 2S pressure was selected to about 2 Torr, the Argon pressure was varied between 2.3 and 6.3 Torr, whereas the H 2O pressure varied from 0 till 12 Torr. The averaged H 2S– H 2 O, H 2 S –Ar broadening coefficient γ and shift coefficient δ were measured to be: γ H 2 S– H 2 O =(0.265±0.001) cm −1/ atm, δ H 2 S– H 2 O =+(0.016±0.001) cm −1/ atm, δ H 2 S– Ar =(0.0381±0.0002) cm −1/ atm , and δ H 2 S– Ar =−(0.0016±0.0001) cm −1/ atm . Additivity concerning broadening was found to be proved with a deviation less than 1.3%. Concerning the line shift it is shown that for mixtures H 2S–Ar–H 2O with pressures larger than 10 Torr the additivity of the H 2S–H 2O and H 2S–Ar shift coefficients could be proved with an averaged deviation less than 13% which is comparable to the uncertainty of the shift measurements.

Infrared spectroscopy of van der Waals modes in the intermolecular potential of Ar-CO: The K a, = 0 combination of stretch and bending

Using our diode laser spectrometer a new subband of Ar−CO in the 2167 cm−1 region has been detected. We have assigned 18 rotational transitions to a thus far unobserved K a = 0 van der Waals mode at 23.927cm−1 above the ground state. An exact analysis yields a Coriolis coupling of this new observed state to a previously detected van der Waals state at 26.187cm−1 [10]. The measurements and a complete analysis are presented.

Observation of a high lying van der Waals mode in the intermolecular potential of Ar-CO

A continuous two-dimensional jet expansion in combination with an infrared diode laser spectrometer has been used to record a new subband of the weakly bound complex Ar-CO in the 2168cm−1 region. Twenty-two transitions were assigned to a thus far unobserved K a = 0 van der Waals state at 36.765 cm−l above the ground state. This is the highest van der Waals mode of Ar-CO reported so far. These high lying energy levels will provide a very sensitive test for future ab initio and Semiempirical potential energy surfaces.

Semi-classical analysis of helium broadened acetylene (ν1+3ν3) band transitions measured by a NIR diode laser spectrometer

Helium-broadened line shapes of the rovibrational transitions of the ( ν 1 + 3ν 3 ) combination-overtone band of C2H2 have been studied by using a near IR diode laser spectrometer. The 2 f frequency detection technique has been employed for signal detection of these weak band transitions. The linestrength parameters and broadening coefficients (HWHM) have been extracted by fitting the observed line shapes with Voigt profiles. Helium, lightest noble gas perturber, exhibits the effect of close collisional dephasing on pressure broadened line profiles. Due to overlapping of atomic orbitals the repulsive interaction dominates over attractive interaction for collision diameter in the 3 to 4 Arange. In order to interpret this type of linear polyatomic-monatomic collision phenomena the Tipping-Herman potential has been chosen. Emphasis has been given to the analysis of observed data to obtain information about interaction potential, phase-shift and molecular constants. The results obtained from detailed semi-classical model calculations are found to be in good agreement with the observed data.

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

Detection of Hydrocarbons in Water by MIR Evanescent-Wave Spectroscopy with Flattened Silver Halide Fibers

IR transmitting AgCl xBr1- x, fibers were used as internal reflection elements for the spectroscopic detection of hydrocarbons that are dissolved in water in an attenuated total reflection (ATR) type of setup. The fibers were partly flattened by pressing and then coated with an analyte-enriching organic polymer in order to increase the sensor sensitivity. A portable tunable diode laser spectrometer served as a spectrometric unit. The dependence of the sensitivity on coupling conditions was investigated and lead to an optimized coupling. It is shown that the sensitivity of the flattened waveguide with a thickness of 170 μm can be enhanced by a factor of 5 compared to a fiber with 900 μm diameter. Aqueous toluene solutions with concentrations in the parts-per-million range were used to characterize the sensor system that showed a linear response with regard to analyte concentration. The noise equivalent concentration (NEC) was on the order of 1 ppm.

High quality infrared (8 μm) diode laser source design for high resolution spectroscopy with precise temperature and current control

The performance of a tunable diode laser (DL) spectrometer is directly determined by the DL qualities and its tunability control. Emission characteristics are very sensitive to current and temperature drifts. In order to obtain a high quality source for spectroscopy, we designed our own liquid nitrogen cryostat containing the DL. Moreover we compared current and temperature control and we found that temperature tuning can be more efficient. We check and demonstrate it by recording SO 2 spectra in the 1168–1169 cm –1 spectral region.