Diode Laser Measurements Research Articles

Tunable diode laser measurements on nitric oxide in a hypersonic wind tunnel

A tunable infrared diode laser was used in the F4 high-enthalpy hypersonic wind tunnel at ONERA to make line-of-sight measurements on nitric oxide (NO). The NO is generated in the arc chamber and remains frozen in the hypersonic flow. We report the results of a medium enthalpy run. The Ω (14.5), Ω = 3/2 doublet was recorded simultaneously in two views : one normal to the flow and the other biased at 63-deg to the flow axis. Spectra were recorded at 2 ms intervals during a tunnel run, and 30 sets of spectra were analyzed by a least-squares fitting procedure. The mean of the velocity measurements inferred from the Doppler shift of the doublet was 2740 m/s (±5%). The velocity decayed about 10% over 60 ms. The temperature and NO column density determined by the least-squares fits were subject to much larger uncertainties. The static temperature of the freestream is estimated to be 168 K (±35%). The median inferred NO column density of 4 X 10 17 molecule/cm 2 corresponds to an NO number density of order 5 x 10 15 molecules/cm 3 in the freestream.

Oxidation of fullerene C 60 behind shock waves: tunable diode laser measurements of CO and CO 2

Submicron fullerene C 60 particles dispersed in a carrier gas and shock heated to temperatures of about 1500 K evaporate in a few μs to form C 60 vapour. The high temperature oxidation of fullerene in O 2/Ar mixtures was studied at temperatures 1655 K ⩽ T ⩽ 2705 K and pressures between 1.5 and 2.6 bar. The progress in the oxidation was measured by tunable IR-diode laser absorption of CO and CO 2. A multipass cell incorporated into the shock tube was used to enhance the initial absorption path length. Additionally the spectral and time resolved emission during the fullerene oxidation was recorded by an intensified CCD camera. In a wavelength range 280 nm ⩽ λ ⩽ 530 nm typical spectra for C 60 and C 2 were detected. From the time dependent increase of CO concentration and the time dependent decrease of the spectral emission apparent rate coefficients for the formation of CO and the disappearance of C 60 were determined.

Tunable diode laser measurements of methane fluxes from an irrigated rice paddy field in the Philippines

This paper presents the first micrometeorological‐based measurements of methane (CH4) emissions from Asian rice paddies of which we are aware. The research features the tunable diode laser trace gas analyzer system (TGAS) recently developed at the University of Guelph. CH4 fluxes were measured between March 9 and 24, 1992, from an irrigated rice paddy field at the International Rice Research Institute (IRRI), the Philippines. The daytime CH4 flux averaged 6.0 μg m−2 s−1. The CH4 fluxes displayed a diurnal trend similar to daily soil temperature curves, with peak emissions of about 8 μg m−2 s−1 in the early afternoon. A tenfold increase in CH4 emissions (to about 70μg m−2 s−1) during a brief weeding experiment resulted from soil disturbance. Up to 25 μg m−2 s−1 of CH4 were released during a drying of the field, after which unsuitable soil redox potentials apparently suppressed methanogenesis. The CH4 flux was also arrested when the field was flooded with oxygen‐rich water during a heavy rainstorm.

The laser magnetic resonance spectrum of the ν1 band of the CCN radical in its state

Vibration–rotation transitions in the ν1, band of the CCN free radical in the [Formula: see text] state have been observed by CO laser magnetic resonance (LMR) spectroscopy. The observations have been combined with existing diode laser data to determine the parameters of an N2 effective Hamiltonian modified to include the Renner–Teller effect in the [Formula: see text] state explicitly. The LMR observations are well modelled by estimated Zeeman parameters that take into account non-adiabatic vibronic interactions. The analysis suggests a small systematic error in the diode laser measurements and leads to a revision of the band origin, ν1, to 1923.252 92 (42) cm−1.

Diode Laser Measurements of Self-Broadening Coefficients in the v5-Band of C2H2 at Low Temperature

Collisional self-broadening coefficients for 21 lines in the P and R branches of the v5 band of C2H2 at low temperature (173 K) have been measured using a tunable diode laser spectrometer and a low-temperature variable path length absorption cell. These lines, with J values ranging from 1 to 25, are located in the spectral range 670-790 cm−1. A semiclassical line-broadening calculation provides satisfactory results when the electrostatic interactions arising from the hexadecapole moment of C2H2 are taken into account. We have also examined the validity of a simple power law governing the temperature dependence of the broadening coefficients.

Diode-laser measurements of self-broadening coefficients and line strengths in the v3 Band of CH 335Cl

Self-broadened half-widths of 29 lines (with J values ranging from 3 to 50 and K from 2 to 9) have been measured in the P- and R-branches of the v 3 parallel-type band of CH 3 35Cl. These lines, located in the spectral range 684–767 cm -1, were recorded using a tunable diode-laser spectrometer. The collisional half-widths were estimated along with line strengths by fitting Voigt profiles to the measured shapes of the individual lines. The results obtained for line strengths are generally larger than those derived from previous measurements.

Diode Laser Measurements of H 2O Line Intensities and Self-Broadening Coefficients in the 1.4-μM Region

Spectrally resolved measurements of pure water vapor absorption spectra have been performed with a tunable diode laser. The laser, a distributed feedback InGaAsP diode, emits in the 1.4-μm region. A total of 12 lines were studied corresponding to rovibrational transitions within the ν 1 + ν 3 and 2ν 1 vibrational bands. A Voigt profile analysis of lineshape is used to infer both intensities and self-collision-broadening coefficients of the water vapor. Good agreement is found between the observed line intensities and those recently measured by Toth. These results are apparently the first published measurements of the broadening coefficient within this spectral range.

Tunable diode-laser measurement of carbon monoxide concentration and temperature in a laminar methane–air diffusion flame

The application of tunable diode lasers for in situ diagnostics in laminar hydrocarbon diffusion flames is demonstrated. By the use of both direct-absorption and wavelength-modulation (second-derivative) techniques, carbon monoxide concentrations and the local flame temperature are determined for a laminar methane-air diffusion flame supported on a Wolfhard-Parker slot burner. In both cases the results are found to be in excellent agreement with prior measurements of these quantities using bothrobe and optical techniques.

Diode Laser Measurements of CO 2 Hot Band Line Intensities at High Temperature Near 4.3 μm

Diode laser measurements of CO 2 absorption spectra have been made in the 2270-2295 cm −1 spectral region at low pressure (<25 mbar) and at temperatures up to 800 K. Line intensities of several parallel hot bands (20013 ← 20003, 12212 ← 12202, 20012 ← 20002, 12211 ← 12201, 20011 ← 20001,and 01121 ← 01111 bands of 12C 16O 2, and 01111 ← 01101 band of 13C 16O 2) have been obtained. The data are analyzed to derive squared dipole-moment matrix elements and band intensities. Our results for the 01111 ← 01101 band of 13C 16O 2 are consistent with previous measurements performed at room temperature. The values obtained for the 12C 16O 2 bands investigated in this work, which to our knowledge have not been analyzed previously, are in good agreement with available theoretical predictions.

Diode-Laser Measurements of N 2-Broadening Coefficients in the ν 2 Band of HCN at Low Temperature

N 2-broadening coefficients have been measured for 28 lines of HCN at 203 K in the P, Q, and R branches of the ν 2 band, using a tunable diode-laser spectrometer. Semiclassical calculations of these collisional broadenings have been performed using two intermolecular anisotropic potentials as well as two isotropic potentials to describe the trajectory model: one is the Lennard-Jones potential, and the other is a spherical average of the atom-atom model. The parameters of the atom-atom model and those of the Lennard-Jones potentials have been calculated for HCN by fitting experimental values of second viral coefficients. The calculated broadening coefficients are significantly larger, especially for the lines with J > 10, than the experimental data. By comparing the results obtained at room and low temperatures. the temperature dependence of line widths has been determined both theoretically and experimentally.

Absolute absorption of ozone in the midinfrared

The absolute absorption of ozone in the midinfrared has been measured relative to 253.7‐nm ultraviolet absorption to an accuracy of 1 % using a Bruker Fourier transform spectrometer. These new results agree with previous measurements by diode laser measurements after the latter are corrected for more recent infrared calculations and ultraviolet cross sections. They also agree with a determination based on refractive index measurements in the infrared.

Tunable diode laser measurements of trace gases during the 1988 Polarstern cruise and intercomparisons with other methods

Measurements of NO2, HCHO, and H2O2 were made by the highly specific method of mid infra-red absorption spectroscopy using tunable diode lasers (TDLAS) during the 1988 Polarstern expedition. The TDLAS data are compared to those obtained during the cruise using less direct methods. Southern Hemisphere NO2 levels suggest nett photochemical destruction of O3 in the boundary layer. Northern Hemisphere HCHO averaged 0.47±0.2 ppbv; the HCHO measurements are used in a simple calculation to estimate OH noontime maxima of 3–6×106 cm-3.

Tunable diode laser measurements of carbon monoxide energy distributions from acrolein photodissociation at 193 nm

Rovibrational energy distributions and average translational energies for CO photofragments produced by the 193-nm photolysis of gas-phase acrolein have been measured. A tunable infrared diode laser system was used to probe the CO fragment. Energy release to the CO translational [P 1,0 (16)], rotational (=0), and vibrational degrees of freedom can be described by the effective temperatures 1380, 2750, and 2230 K, respectively. A simple phase space model, consistent with a two-step mechanism where radical pair formation occurs prior to the formation of free CO, accurately predicts rotational and vibrational energy distributions

Tunable diode laser measurements on the 951.7393 cm -1 line of 12C 2H 4 at planetary atmospheric temperatures

The absolute intensity and collision-broadened half-widths of the 5 0.5→5 1,5 transition at 951.7393 cm -1 in the v 7-fundamental band of 12C 2H 4 have been measured at 152, 202 252, and 295 K with the Doppler-limited spectral resolution (∼10 -4cm -1) of a tunable diode laser spectrometer. The temperature dependence of the half-width has been determined for line broadening by the planetary atmospheric gases He, H 2, and N 2.

Measurement and analysis of the Fermi resonance between ν5 and 2 ν9 of nitric acid

New tunable diode laser measurements have been made on the ν 5 and 2 ν 9 infrared absorption bands of HNO 3 between 853 and 919 cm −1. These two bands are observed to be about 17 cm −1 apart and are coupled by Fermi resonance which causes a considerable displacement of all the energy levels and transition frequencies. An effective resonance interaction of the type 〈 ν 5, J, k + 2|2 ν 9, J, k〉 is particularly important for understanding the appearance of the spectrum. The Fermi resonance has been taken into account in a global fit of both bands that has allowed us to assign all of the strongest transitions. The Fermi interaction constant found is W f = 8.13 ± 0.14 cm −1 and the unperturbed band separation is 6.0 ± 0.4 cm −1. Other higher-order interactions were also considered in the analysis, including a Δ K = ±2 Coriolis interaction. Over 1400 diode laser transitions were fitted with a rms deviation of 0.00099 cm −1. Transition wavenumbers, assignments, and lower state energies are made available for the strong transitions of HNO 3 between 853 and 920 cm −1.

Diode-laser measurements of N 2-broadening coefficients in the ν2 band of HCN

N 2-broadening coefficients have been measured for 41 lines (with J values ranging from 2 to 33) of HCN in the P, Q, and R branches of the ν 2 band. These lines, located in the spectral range 610–810 cm −1, were recorded with a high-resolution tunable diode-laser spectrometer. A semiclassical impact calculation of these collisional broadenings provides results in the P and R branches as well as in the Q branch that for J > 10 are generally larger than the experimental data.

Diode laser measurements of CF 3Cl near 1258 cm −1: Rovibrational analysis of ν2 + ν3 combination and its hot bands

The gas-phase infrared spectrum of natural CF 3Cl has been investigated in the range 1244–1275 cm −1 with a resolution of about 0.002 cm −1 using a tunable diode laser spectrometer. The K structure of many P( J) and R( J) clusters of the ν 2 + ν 3 band has been resolved and analyzed: the maximum J values reached for individual transitions were 66 and 62 for CF 3 35Cl and CF 3 37Cl, respectively. Small irregularities found in the K = 15, 18 lines of most manifolds of the main species have been attributed to perturbations with the ν 5 + 2 ν 6 vibration, and different interaction mechanisms have been considered. From a least-squares fit of the assigned transitions, accurate parameters up to the quartic terms have been determined for the ν 2 + ν 3 combination of both the isotopomers. Residual features due to the hot bands ν 2 + 2 ν 3 − ν 3 and ν 2 + ν 3 + ν 6 − ν 6 of CF 3 35Cl have also been identified and spectroscopic constants have been derived by polynomial and manifold contour simulation methods.

Diode-laser measurements of H 2-broadening coefficients in the ν5 band of C 2H 2

H 2-broadening coefficients have been measured for 29 lines of acetylene in the P and R branches of the ν 5 band. These lines, located in the spectral range 640–810 cm −1, were recorded using a tunable diode-laser spectrometer. We obtain broadening coefficients that are significantly larger than previous experimental data. A comparison is also made with results calculated from a semiclassical theoretical model. The validity of this model for a light perturber such as H 2 is questioned.

Diode laser measurements of CO 2 line intensities at high temperature in the 4.3 μm region

Diode laser measurements of low pressure CO 2 transmission spectra have been made up to 800 K in the 4.3 μm region. Line intensities of several parallel hot bands with Δ v 3 = 1 of 12C 16O 2 and of the ν 3 band of 13C 16O 2 have been obtained. The data are analyzed to derive vibrational band intensities and dipole-moment matrix elements. Our results are consistent with previous measurements performed at room temperature by Fourier transform spectroscopy. The results obtained for three bands which, to our knowledge, have not been analyzed previously are in good agreement with available theoretical predictions.

Versatile integrated tunable diode laser system for high precision: application for ambient measurements of OCS

A versatile and integrated tunable diode laser system for high precision measurements of the important sulfur gas carbonyl sulfide is described. We explicitly address some of the-major factors affecting tunable diode laser measurement precision as well as accuracy and have implemented a number of new features for increased system control and versatility. The system described herein provides the capability for measuring ambient concentrations of this gas with a precision in the range from +/-0.3 to +/-1% over time periods of many hours. Such a precision provides us with an important new capability for measuring true spatial and temporal variations of carbonyl sulfide in the atmosphere.