On the accurate determination of pressure- induced line shifts in the 2ν<Subscript>3</Subscript> band of H<Subscript>2</Subscript>O at 1320 nm

A simple technique is demonstrated for the accurate determination of pressure-induced line shifts of water in air. High- and low-pressure water samples are simultaneously probed on selected overtone transitions at 1.32 μm using a current-modulated distributed-feedback diode laser and harmonic detection. The resultant profiles yield an average line shift of -293±30 MHz/atm for the 3,3,0 (002)2,2,1 (000)transition at 227251 GHz and -134±7 MHz/atm for the 3,2,1 (002)2,1,2 (000) transition at 227027 GHz. Comparisons are made between first- and second-harmonic detection, and wavelength- and frequency-modulation regimes. The effect of modulation broadening on the returned line shifts is quantified. Received: 12 August 2002 / Revised version: 21 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +44-01865/275410, E-mail: gus.hancock@chemistry.ox.ac.uk     

Collisional line shifting and broadening in the fundamental P-branch of CO in Ar between 214 and 324 K

The collisional shifts and widths of several P-branch spectral lines in the fundamental band of CO-Ar have been measured at temperatures between 214 and 324 K and pressures between 0.025 and 1 atm. The widths have been determined using a line shape model based on the solution of the transport/relaxation equation for the appropriate off-diagonal element of the density matrix. The model uses a realistic molecular potential energy surface to calculate the speed dependence of the collisional broadening, and a rigid sphere potential to calculate the translational motion. It is found that both the shifting and broadening coefficients follow a power law dependence on the temperature. Additionally, it is demonstrated that studies have tended to overestimate the accuracy of collisional widths when the line shape model used to obtain the widths involves multiple fitted line shape parameters or fails to fit the measured spectra within the experimental noise.     

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. Received: 17 November 2000 / Revised version: 19 February 2001 / Published online: 27 April 2001     

Differential Helmholtz resonant photoacoustic cell for spectroscopy and gas analysis with room-temperature diode lasers

The results of theoretical and experimental studies and the design of a multi-purpose differential Helmholtz resonant photoacoustic detector (DHRD) and its applications to high-resolution spectroscopy of molecular gases and gas analysis with a room-temperature diode laser in the near-IR region are summarized. The series of experiments and numerical analysis of the DHRD sensitivity were performed for both types (single-pass and multi-pass) of DHRDs within a wide pressure range 0.1–101 kPa, including the regime of a gas flowing through a DHRD cell. The hardware and electronic arrangement of DHRDs for diode laser spectrometers and gas analyzers providing a limiting absorption sensitivity better than 10^-7 Wm^-1 are described. The results of measurements of spectral line parameters of H₂O near 800 and 1390 nm and CH₄ near 1650 nm (intensities, line broadening and shifting by atomic and molecular gases) are presented and discussed. The problems and the ways of perfection of the methodology and accuracy of DHRD techniques with tunable diode lasers of near-IR and visible spectral ranges are discussed. Received: 1 April 2002 / Revised version: 20 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +7/382-225-8026, E-mail: kvan@asd.iao.ru     

Shape and width of IR absorption lines of ammonia expanded in a supersonic jet

Line-width and-shape of absorption lines of the ₂ vibration of ammonia were measured for a series of transitions starting from vibrational groundstate levels with a different rotational energy. An increase in linewidth was found for larger rotational energies of the initial states. This effect has been attributed to the spatial rotational non-equilibrium in the supersonic jet expansion.For expansion of NH₃ seeded in He and Ar a very pronounced dip in the absorption profile has been observed, due to the formation of clusters.     

Dicke-narrowed spectral line shapes of CO in Ar: Experimental results and a revised interpretation

The shapes of Dicke-narrowed spectral lines in the fundamental P-branch of CO in Ar are studied by comparing high-resolution measurements and theoretical calculations. The measured spectra were recorded at temperatures between 214 and 324 K, and at pressures between 0.025 and 1 atm. The calculations are based on solving a transport/relaxation equation for the appropriate off-diagonal element of the density matrix; they use a realistic intermolecular potential to determine the speed-dependent collisional broadening, and a rigid sphere potential to determine the Dicke narrowing. It is found that the calculations can reproduce the measured spectra within the experimental noise under all conditions, but that the magnitude of the Dicke narrowing in the measured spectra is 70-90% less than predicted from the mass diffusion constant. A revised view of the collision operator resolves the discrepancy in principle, and leads to a better understanding of the line shape problem in general.     

Nuclear dynamics in X-ray Raman scattering

Accepted: 6 March 1997     

N2- and O2-broadening coefficients in the ν2 and ν5 bands of CH3F at 183 and 298 K

The N₂- and O₂-broadening coefficients of 33 rovibrational lines in the ν₂ and ν₅ bands of ¹²CH₃F were measured at 183 K using a diode-laser spectrometer. The measurement of these coefficients was also realized at room temperature for 10 of these lines to determine their temperature dependence. The line parameters 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 pressure-broadening coefficients were also performed for the same temperatures from a semiclassical model involving electrostatic, induction and dispersion interactions in the intermolecular potential. The calculated values reproduce well the experimental data for both temperatures and both perturbers and the theoretical temperature dependence of the broadening coefficients is in satisfactory agreement with that derived from the measurements.     

Second-harmonic and sum-frequency generation in AgGaSe2 with tunable waveguide CO2 lasers

A tunable source is described, giving extensive coverage of a spectral region around 2000 cm^–1. A frequency self-calibration routine allows for the recording of Doppler-limited absorption profiles with an absolute accuracy of better than 10 MHz. The applicability of the source is demonstrated through measurements on the¹²C¹⁶O,¹²C¹⁷O, and¹²C¹⁸O isotopomers. Collision-broadening parameters and line strengths are determined, some of the data showing evidence of motional narrowing. Sensitivity limits are determined for linear absorption measurements as well as for photoacoustic absorption, and the feasibility of monitoring all three isotopomers with ppm sensitivity is demonstrated.Work supported by the Danish Science Research Council under grants no. 11-6866, 11-0116, and 11-0920     

CO2-broadening coefficients in the ν4 + ν5 band of acetylene

The CO₂-broadening coefficients of 24 P- and R-branch transitions in the ν₄ + ν₅ band of acetylene were measured at room temperature using a diode-laser spectrometer. These lines with J values up to 26, were located in the spectral range 1270 to 1400 cm⁻¹. The collisional broadening coefficients were retrieved by fitting the experimental profiles to the Voigt, Rautian, and Galatry lineshape models. Two experimental values for the narrowing coefficient were determined from the spectra and compared with the theoretical narrowing coefficient. The calculations of these broadenings were also performed in the frame of a semiclassical formalism involving a simple intermolecular potential with an adjustable parameter. The theoretical results are in good agreement with the experimental results and reproduce well the J dependence of the broadening coefficients.     

Systematic analysis of the Fourier transform spectrum of CH3OH between 400 and 950 cm

We have investigated a high-resolution Fourier transform (FT) absorption spectrum of the ¹³CH₃OH isotopomer of methanol from 400 to 950 cm⁻¹ with the “Ritz” program. We present the assignments of 7160 transitions, 3021 of which belong to A-symmetry, and 4139 to E-symmetry. These transitions occur between states labeled by K quantum numbers up to 14, and by torsional quantum numbers n up to 4. The “Ritz” program evaluated the energies of the 4684 involved levels with an accuracy of the order of 10⁻⁴ cm⁻¹. All of the assigned lines correspond to transitions involving torsionally excited levels within the ground small-amplitude vibrational state.     

The 3ν3 overtone bands of UF6 and UF6

The absorption specta of CO laser radiation by 3ν₃ overtone bands of ²³⁸UF₆ and ²³⁵UF₆ has been measured using photoacoustic spectroscopic techniques. For the two temperatures 250 K and 290 K, measured absorption coefficients and cross-sections of the 3ν₃ band of ²³⁵UF₆ are reported for the first time.     

Simultaneous detection of CO and CO2 using a semiconductor DFB diode laser at 1.578 μm

2 and CO at a wavelength of λ=1578 nm. Sensitivity measurements under different conditions have been performed and the detection limit of the apparatus was measured to be less than 10 mTorr over a 1-m path length. In addition, we measured for the first time environmentally and spectroscopically relevant self-broadening and nitrogen-broadening coefficients for CO₂ and CO in this spectral region and we discuss different possibilities for increasing the sensitivity of the apparatus. Received: 8 November 1996/Revised version: 11 December 1996     

Pressure broadening measurement of submillimeter-wave lines of O3

The pressure broadening coefficients and their temperature dependences for two submillimeter-wave transitions of ozone, one being monitored with Odin and the other to be monitored with JEM/SMILES and EOS-MLS, have been determined by using a BWO based submillimeter-wave spectrometer. The measurements have also been extended to one of the symmetric isotopic species, ¹⁶O¹⁸O¹⁶O. The isotopic species is observed in natural abundance and as a consequence the temperature dependence is not determined due to weak signal intensity. The pressure broadening parameters are determined with better than 1% accuracy, while the temperature dependence exponents are obtained within 1.5-3% accuracy for the normal species transitions.     

High-resolution investigation of the weak ν1+3ν21-ν21+ν3 band of CO2 around 2 μm

Fundamental spectroscopical parameters of the weak ν₁+3ν₂ ¹-ν₂ ¹+ν₃ band of CO₂ are reported using a high-resolution, direct-absorption spectrometer, based on a distributed feed-back diode laser emitting at 2 μm. Line intensities and self-broadening coefficients have been measured for the first time with high accuracy, for nine lines of the R branch, from R(44) up to R(59). Comparison with available data has been made, and a generally good agreement has been found. Received: 30 August 1999 / Published online: 24 March 2000     

High-resolution measurements of line intensity, broadening and shift of CO around m

Fundamental spectroscopical parameters of CO₂ are reported using a high resolution, direct absorption spectrometer, based on a distributed feed-back diode laser emitting at 2 μm. Line intensity, self- and foreign broadening and shift, with N₂ and O₂ as foreign gases, have been measured with high accuracy in the combination band of CO₂, for 13 lines of the R branch, from R(22) up to R(46). Comparison with available data is made, when possible, and a generally good agreement has been found. Received: 24 April 1998 / Received in final form: 28 October 1998     

Hydrogen CARS thermometry in a high-pressure H2–air flame. Test of H2 temperature accuracy and influence of line width by comparison with N2 CARS as reference

This work describes a further step towards the determination of the temperature accuracy of H₂ Q-branch CARS (Coherent Anti-Stokes Raman Scattering) at high pressure with regard to the influence of the H₂ line widths. In laminar steady H₂/air flames in the pressure range 1–15 bar and at fuel-rich conditions with stoichiometries between two and four, quasi-simultaneous temperature measurements were performed with H₂ and N₂ CARS. The temperature values deduced from H₂ CARS are in good agreement with the reference temperature from N₂ CARS. The influence of different line-width contributions on the accuracy of H₂ Q-branch thermometry was investigated in detail. Received: 10 April 2001 / Revised version: 22 May 2001 / Published online: 18 July 2001     

Diode laser spectroscopy of overtone bands of acetylene

Overtone absorption lines of acetylene in the regions around 12700 and 11800 cm^–1 have been examined by the use of tunable diode lasers in free-running mode. The diode laser emission wavelength was scanned around the gas resonances by simply sweeping its injection current, permitting a direct observation of the absorption line-shapes. Weak overtone absorption lines have been detected by using the wavelength modulation (WM) spectroscopy with 2nd harmonic detection technique and the collisional broadening and shift coefficients have been obtained. The high resolving power and accuracy of the spectrometer permitted a wavenumber error of less than 0.01 cm^–1. The correct interpretation of the absorption signals when detecting the second harmonic in the presence of a sloping background is discussed.     

Isotopically invariant analysis of vibration-rotation transitions of HBr and its isotopologues

The vibration-rotation transition frequencies of HBr and its isotopologues reported in the literature were simultaneously analyzed in a least-squares fit to determine isotopically invariant molecular parameters U_kl and Born-Oppenheimer breakdown parameters Δ_kl. A total of 692 lines was fitted to Watson’s expression involving 25 adjustable parameters to a weighted unitless standard deviation of σ ≈ 0.91. From new data in the far-infrared region and those of improved precision in the mid-infrared and the near-infrared region, the breakdown parameters for the bromine atom were determined; the parameter for the harmonic frequency improved the fit and for the rotational constant enabled us to obtain the information on the adiabatic correction to the Born-Oppenheimer approximation.