Search

The very weak absorption spectra of 18 O 3 and 16 O 3 have been recorded between 7700 and 7920 cm −1 by high sensitivity cavity ring down spectroscopy ( α min ∼10 −10 cm −1 ). The upper limit of the investigated region is only 6% below the dissociation energy. The spectrum consists in the superposition of very weak vibrational bands and unexpected broad absorption features which dominate in the case of the 18 O 3 isotopologue. From recordings at temperatures ranging from 294 to 354 K, the broad features were identified as due to the 3 A 2 (0 0 0)–X 1 A 1 (1 1 0) hot band. The rovibronic levels of the 3 A 2 (0 0 0) state are predissociated and the widths of the corresponding lines range from 0.02 to at least 0.5 cm −1 .

The high sensitivity absorption spectrum of the a 1 Δ g - X 3 ∑ g - band of 16O17O, 17O18O and 17O2 has been recorded by CW-Cavity Ring Down Spectroscopy near 1.27 μm. The spectra were obtained between 7640 and 7917 cm−1 with a 17O-enriched sample at room temperature and at 80 K. Due to the I = 5/2 nuclear spin of the 17O atom, the nuclear hyperfine structure of the transitions could be partly resolved at low temperature. The parameter coupling of the nuclear spin to the electronic angular momentum in the a1Δg state has been derived for the three studied isotopologues of oxygen.

More than 5050 rovibrational transitions were assigned in the very sensitive CW-CRDS 18 O 3 spectra in the 5900–7000 cm −1 range leading to the first determination of 14 new vibrational band centres. Three bands correspond to the highest ozone states observed so far under high resolution. The good agreement between predicted and observed centres ( rms deviation of 1.1 cm −1 ) confirms the accuracy of the potential function previously determined from 16 O 3 data. At this energy range situated ∼20% below the dissociation energy, the small changes of masses associated with the 16 O 3 → 18 O 3 homogeneous isotopic substitution result in irregular isotopic shifts.

The high resolution absorption spectrum of methane has been recorded at room temperature by high sensitivity CW-cavity ring down spectroscopy between 1.50 and 1.60 μm (6250–6680 cm −1 ). This spectral region has a low opacity and is then of particular interest for the spectral analysis of the outer planets and Titan. The recordings were performed with a sensitivity corresponding to a noise equivalent absorption of α min ∼ 5 × 10 −10 cm −1 and allowed for the detection of lines with an intensity smaller than 10 −28 cm/molecule far beyond the weakest lines provided by the HITRAN database. The spectrum exhibits a highly complex and congested structure with a kind of ‘fractal’ character and a density of transitions of a few tens per cm −1 . The impact of the weak newly observed transitions on the total absorption in the region is discussed.

A few absorption lines of the Q Q branch of the a 1 Δ g ( 0 ) – X 3 Σ g - ( 1 ) hot band of pure O 2 have been detected near 1.58 μm by CW-cavity ring down spectroscopy based on a fibred DFB laser. A sensitivity of the order of 2 × 10 −11 cm −1 has been achieved by averaging a large number of spectra, allowing for the measurement of line intensities as weak as 5 × 10 −31 cm/molecule. The integrated band intensity is estimated to be S 01 = 4.5(7) × 10 −29 cm/molecule which differs by a factor of about 6 from the value provided by the HITRAN database. Our intensity value is supported by a significantly better agreement with the value deduced from the intensity of the a 1 Δ g ( 0 ) – X 3 Σ g - ( 0 ) cold band and the respective Franck–Condon factors.

The acetylene absorption spectrum has been recorded between 1.10 and 1.05 μm by Fourier Transform Spectroscopy (FTS) and Intracavity Laser Absorption Spectroscopy (ICLAS) based on a Vertical external Cavity Surface Emitting Laser (VeCSEL). Eleven bands of 12 C 2 H 2 were detected, six of them being newly reported. The spectroscopic parameters retrieved from the rovibrational analyses improve significantly the accuracy of parameter values available in the literature. One additional band is assigned to the 13 C 12 CH 2 isotopologue, present in natural abundance in the sample. Finally, three bands newly detected near 12 300 cm −1 by ICLAS based on a Ti:Sapphire are also reported. The vibrational upper states are unambiguously assigned on the basis of the effective Hamiltonian model and the observed and predicted vibrational term values are found in good agreement.

The capabilities of intra-cavity laser absorption spectroscopy associated with a high-resolution Fourier-transform spectrometer (FT-ICLAS) are investigated with a Ti:Sapphire laser. Weak absorption lines of atmospheric water were used to test the accuracy of absolute intensity measurements by FT-ICLAS leading to an excellent agreement (a few %) with the HITRAN data. The performances in terms of spectral resolution (0.028 cm � 1 ) and sensitivity (2� 10 � 9 cm � 1 ) are illustrated by the spectroscopic study of the overtone spectrum of 12 C2H2 between 12 250 and 12 400 cm � 1 which allowed for a significant improvement of recent cavity ring-down measurements. Among the three P–R bands rotationally analyzed, one is newly observed. The absolute intensity values of the bands are given. 2003 Elsevier Science B.V. All rights reserved.

Collision induced absorption (CIA) bands in molecular oxygen, at 630 and 577 nm, were measured at low temperatures, down to 132 K. Besides providing confirmation of previous room temperature data, and new information about the temperature dependence of CIA bands, these measurements demonstrate the performance of cavity ring down spectroscopy in a rather unusual configuration. The cavity ring down spectroscopy (CRDS) setup includes a broadly tunable commercial pulsed optically pumped oscillator (OPO) laser coupled with a ring down cell, a section of which can be cooled down to liquid nitrogen temperature. In addition, a spectrograph is used as a wavelength-selective element at the cavity output, before measuring the ring down signal. This was needed given the broad OPO lineshape and the coexistence of other wavelengths generated by the OPO system.

The Intracavity Laser Absorption Spectrum (ICLAS) of nitrous oxide, 12 N 2 16 O , has been recorded between 9560 and 9980 cm −1 with a Vertical External Cavity Surface Emitting Laser (VECSEL). Thirteen bands were observed, 11 of which are new. The rovibrational analysis has revealed the occurrence of a number of local perturbations due to anharmonic or Coriolis couplings, which could be analysed on the basis of the polyad model of effective Hamiltonian.

The absorption spectrum of the germylene radical produced in a continuous flow discharge of germane diluted in argon has been recorded in the 17 090–17 135 cm −1 region. This range corresponds to the central part of the A 1 B 1 (0, 1, 0)− X 1 A 1 (0, 0, 0) rovibronic transition, which has been analysed for the first time. The spectrum was measured by Intracavity Laser Absorption Spectroscopy with an equivalent pathlength of 15 km. The experimental resolution and wavenumber accuracy are of the order of 0.06 and 0.015 cm −1 , respectively. Each rovibronic absorption is made up of five lines corresponding to the five germanium isotopomers appearing in a natural abundance sample. We have assigned around 25 transitions for each species, from which the rotational constants of the excited state and the band origin of the transition have been calculated. The equilibrium structure of the excited electronic state has been estimated with the help of ab initio data for some vib-rotational parameters. Good agreement is found with previous ab initio determinations of this structure.

A new compact and low-power-consuming cw intracavity laser absorption spectroscopy (ICLAS) spectrometer with a broad tunability near 1.06 μm has been developed. This ICLAS spectrometer uses a thin plate of Nd:glass as the amplification medium and allows pumping by a laser diode. The absorption spectrum of nitrous oxide 14 N 2 16 O has been recorded near 9400 cm−1 with this new ICLAS apparatus. It reveals an extremely weak absorption band centred at 9398.848 cm−1. The corresponding upper term value, vibrational assignment and Bv rotational constant, as well as the band intensity, are quantitatively discussed in the frame of the vibrational cluster model. The observed atmospheric spectrum in the region is assigned to H 2 16 O and discussed.

The rotationally resolved absorption spectrum of two bands of the oxygen dimer near 630 and 578 nm have been recorded by intracavity laser absorption spectroscopy both in a supersonic slit expansion of pure O 2 and in a cell cooled at 77 K. These bands correspond to the transitions [ O 2 ( 1 Δ g ) v=0 ] 2 ←[ O 2 ( 3 Σ g − ) v=0 ] 2 and [ O 2 ( 1 Δ g ) v=0 – O 2 ( 1 Δ g ) v=1 ]←[ O 2 ( 3 Σ g − ) v=0 ] 2 . From the extension of the highly congested rotational structure, the dissociation energy of the ground and excited states are estimated to be 80 and 40 cm −1 , respectively. These values agree reasonably well with the results of ab initio calculations of the potential energy surface.

The near-infrared absorption spectrum of N2O has been recorded by intracavity laser absorption spectroscopy in the range 11200–12300 cm−1. Six bands have been observed and rotationally analysed. The corresponding upper-state vibrational levels and rotational constants are satisfactorily accounted for in the frame of the cluster model proposed in the literature, and supported by the effective Hamiltonian developed by Teffo, Perevalov and Lyulin.

The spectra of the 41 CH stretch-bend overtone band of 12CHF3 and 13CHF3 have been obtained at room temperature by intracavity laser absorption spectroscopy of a CHF3 sample with natural isotopic abundances. The two bands display a resolved and mostly unperturbed J rotational structure. At the present resolution of 0.02 cm−1 (half-width), and P and R branch J clumps have well resolved Lorentzian profiles with full-widths fluctuating between 0.06 and 0.15 cm−1, and an average value that agrees well with previous estimates. As pressure broadening was negligible, these are homogeneous linewidths given by intramolecular energy redistribution. The linewidth fluctuations are consistent with a two-step purely anharmonic coupling of the transition upper state to a dense rovibrational bath.

Three new weak bands of 12 C 2 H 2 at 13473.69, 13493.87 and 13787.97 cm −1 have been recorded by intracavity laser absorption spectroscopy and rotationally analyzed. Their presence was anticipated, on the basis of the cluster model recently presented by Abbouti Temsamani and Herman. The band energies and upper rotational parameters are well reproduced. The comparison between predicted and observed relative band intensities, including those previously reported in the literature in the range investigated, is briefly discussed.

The near-infrared and visible absorption spectrum of nitrous oxide 14Nz160 has been recorded by intracavity laser absorption spectroscopy from 11700 to 15000 cm-1. The absolute intensities of seven rotationally resolved overtone and combination bands are reported and discussed in relation to their vibrational assignment and compared with previous photoacoustic measurements.

The absorption spectra of three near-infrared bands of 12C 16O2 assigned to 00051, 10051 and 10052 have been obtained by intracavity laser absorption spectroscopy with a Ti:sapphire laser. Different procedures have been used to obtain absolute measurement of the intensity of the rotational lines and then to deduce the band intensity. Comparison with previous conflicting photoacoustic measurements is discussed. The values obtained could serve as secondary reference standards for other intensity measurements of weak absorptions in the near-infrared range.

By using a Ti: sapphire crystal as an amplifying medium the intracavity laser absorption spectroscopy technique (ICLAS) has been extended to the near-infrared range (10000–12000 cm −1 ). This range of energy is particularly interesting for the overtone spectroscopy of XH stretching vibrations (XC, N, O, Si, Ge). As an illustration of the possible spectroscopic applications of this spectrometer we present here the absorption spectrum of CHD 3 in the range of the Δν CH = 4 overtone transitions. Due to the Fermi interaction between the ν 1 stretching mode and the ν 5 bending mode, two bands have been observed at 11266.95 and 11062.6 cm −1 . Their rotational analysis is reported and discussed.

We report the CH-stretching overtone spectrum of jet-cooled methane in the visible range. Due to the very weak intensity of the transition, the intracavity-laser-absorption-spectroscopy technique (ICLAS) was used with a free jet of methane from a slit orifice placed inside a dye-laser cavity. In this way, an equivalent pathlength through the jet of 800 m was achieved. The rotational cooling drastically reduces the congestion of the spectrum: whereas the room-temperature spectrum is extremely congested and complicated, the jet spectrum is sparse and reduced to well-isolated lines. These results show that rotational congestion is mainly responsible for the overtone band shape of methane and that, in consequence, the contribution of the background vibrational states is not predominant. The experimental method developed in this work can be extended to study the overtone transitions of a wide class of jet-cooled compounds.