M. Chenevier

Quantitative measurements of very weak H2O absorption lines by time resolved intracavity laser spectroscopy

Time resolved quasicontinuous intracavity dye laser absorption spectroscopy (ICLAS) experiments have been made without and with absorbing species inside the cavity. In the absence of an absorber, a near Gaussian shape with a half‐width decreasing as the square root of the time has been found for the spectrum. With atmospheric water vapor as the absorber, absorption lines in the 600 nm range appear on the spectrum, and the absorption intensity increases with time. This time evolution leads to a quantitative determination of the absorption coefficient of the lines. We confirm that ICLAS follows the Lambert–Beer law with an equivalent length of the cell given by Leq = ct (c is light velocity and t is the time between the laser turn on and the time of observation). This experiment result is in good agreement with time‐dependent theories.

Jet cooled NO2 intra cavity laser absorption spectroscopy (ICLAS) between 11200 and 16150 cm−1

We have combined the high sensitivity of the ICLAS technique with the rotational cooling effect of a slit jet expansion in order to observe and to understand the visible and near infrared NO2 spectrum. By this way, an equivalent absorption pathlength of several kilometers through rotationally cooled molecules has been achieved. Due to the vibronic interaction between the two lowest electronic states, X2A1 and A 2B2, this spectrum is vibronically dense and complex. Moreover, the dense room temperature rotational structure is perturbed by additional rovibronic interactions. In contrast, the rotational analysis of our jet cooled spectrum is straightforward. The NO2 absorption spectrum is vanishing to the IR but, owing to the high sensitivity of the ICLAS technique, we have been able to record the NO2 spectrum down to 11200 cm−1 with a new Ti:sapphire ICLAS spectrometer. As a result 249 2B2 vibronic bands have been observed (175 cold bands and 74 hot bands) in the 11200–16150 cm−1 energy range. Due to the cooling effect of the slit jet we have reduced the rotational temperature down to about 12 K and at this temperature the K = 0 subbands are dominant. Consequently, we have analysed only the K = 0 manifold for N ⩽ 7 of each vibronic band. The dynamical range of the band intensities is about one thousand. Due to the strong vibronic interaction between the X 2A1 and A 2B2 electronic states, we observed not only the a1 vibrational levels of the A 2B2 state but also the b2 vibrational levels of the X 2A1 state interacting with the previous ones. By comparison with the calculated density of states, we conclude that we have observed about 65% of the total number of 2B2 vibronic levels located in the studied range. However, there are more missing levels in the IR because of the weakness of the spectrum in this range. The correlation properties of this set of vibronic levels have been analysed calculating the power spectrum of the absorption stick spectrum which displays periodic motions: the dominant period, at 714 ± 20 cm−1, corresponds to the bending motion of the A 2B2 state. The other observed periods remain unassigned. In contrast the next neighbor spacing distribution (NNSD) shows a strong level repulsion, i.e. a manifestation of quantum chaos. These two observations, apparently contradictory, can be rationalized as follows: the short time dynamics, for t < 10−12 s, is “regular” while for longer times the dynamics becomes “chaotic”. We suggest that this behavior may be observed directly with a pump and probe fs laser experiment.

Gas temperature measurements by laser spectroscopic techniques and by optical emission spectroscopy

Abstract H-atom temperatures were measured using two-photon allowed transition laser induced fluorescence in a microwave plasmareactor used for diamond deposition. These temperatures are compared with the rotational temperatures of ground state molecular hydrogen, equal to the gas temperature, measured previously by coherent anti-stokes Raman spectroscopy. We observed an excellent agreement between the two temperatures. Rotational temperatures of the G 1 {g} + electronically excited state of molecular hydrogen were also obtained by measuring the relative line intensities in emission of the R -branch of the {itG 1 {g} + B 1 {u} + (0,0) band by optical emission spectroscopy. These measurements yielded lower temperatures than the gas temperature by 550 K, but followed its variation as a function of the power density. We defined then {itT R (G 1 {g} + ) as a spectroscopic parameter useable for optimizing and monitoring the plasma conditions for diamond deposition, within the range of conditions tested here.

Intracavity-laser-absorption spectroscopy of the visible overtone transition of methane in a supersonically cooled jet

Abstract 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.

Rotationally resolved overtone transitions of CHD3 in the visible range

Absorption spectra in the 14 900–18 700 cm−1 range have been recorded for gaseous CHD3 at high resolution using the intracavity laser absorption spectroscopy (ICLAS) technique. The observed transitions correspond to the N=6 and 7 overtones of the C–H chromophore. Five bands were identified and rotational constants determined for four of them. The comparison of our experimental data with the calculated band origins and rotational constants obtained by Lewerenz and Quack is discussed. Absolute band intensities are given for seven bands corresponding to the N=5, 6, and 7 polyads.

Rotational structure of overtone transitions of CHD3 near 13 500 cm−1

The absorption spectrum of CHD3 has been recorded for the N=5 polyad near 13 500 cm−1 by the intracavity laser absorption spectroscopy (ICLAS) technique with a spectral resolution of 0.02 cm−1. The rotational structure of the four observed bands has been resolved and the rotational constants given. The behavior of these constants versus the quantum number of the C–H stretching and bending modes are also discussed.

Measurement of trace gases by diode laser cavity ringdown spectroscopy

Optical spectroscopic methods based on direct absorption offer a quantitative measurement of the absorbance, which is the product of the concentration, the molar absorption coefficient of the transition being observed and the length of the absorption path. An absorption sensitivity adequate for trace detection may be achieved by increasing the path length. One solution is offered by cavity ringdown spectroscopy (CRDS), attractive for its simplicity. We recently demonstrated that an external cavity diode laser (ECDL) can be conveniently employed for CRDS instead of a pulsed laser, contrary to previous applications. Here we extend this result to distributed feed-back (DFB) diode lasers. Paying special attention to the coupling of the laser source to the cavity, we developed an extremely simplified CRDS scheme with a sensitivity of about 10-8/cm/(root)Hz. We then built detectors for methane and HF, working close to the optical wavelengths 1.65 and 1.31 micrometer, respectively With an optical assembly of about 50 cm length and a response time of about 1 s, these devices accurately measure atmospheric methane concentrations in the range 0.5 to 200 ppmv, and HF concentrations from 0.1 to 50 ppmv.

Rotationally resolved overtone transitions of 70GeH4 in the visible and near-infrared

Abstract The Δν GeH =6, 7 and 8 overtone transitions of natural germane and of mono isotopic germane 70 GeH 4 have been measured with rotational resolution by intracavity laser absorption spectroscopy. The Δν GeH =6 and 7 rotational structure is found to be that of a pseudo-symmetric top as expected for this molecule close to the local mode limit. On the other hand the Δν GeH =8 shows a more complicated structure which has not yet been analyzed.

Trace gas detection with DFB lasers and cavity ring-down spectroscopy

We recently demonstrated trace detection using Cavity Ring Down Spectroscopy (CRDS) coupled with telecom DFB diode lasers. Our scheme exploits optical feedback from a V-shaped cavity back to the laser. We built trace-gas detectors for CH4 and HF, characterized by a low cost, simplicity, compactness and sensitivity. Operating wavelength are 1.312 micrometers for HF and 1.65 micrometers for methane. The optical setup includes a distributed feed-back (DFB) diode laser, temperature stabilized by a Peltier, a collimating lens, 2 steering mirrors, a V-shaped optical resonator and a photodiode. The V-cavity is made of three low-cost super mirrors R 99.995%) and contains the air sample to be analyzed (20cm3). In standard atmospheric conditions the detection limits for 1 second integration time are of 50 ppbv for HF and 200 ppbv for methane. We present an analysis of the mechanisms of cavity injection and laser feedback, allowing to estimate the influence of various parameters on the performances of this type of apparatus. Calculations and results are given, with particular emphasis on the detection limit and the dynamic range.

Emission spectroscopy diagnostics of a d.c. plasma jet diamond reactor

Abstract We performed emission spectroscopy on a d.c. plasma jet (Ar, H2, CH4) chemical vapor deposition reactor. We used a spectrometer with a large spectral range (150–800 nm), a spectral resolution of 0.07 nm and a spatial resolution better than 1 mm. We were able to derive information concerning the gaseous species existing in the plasma (CH, C2, H, Ar, C) as well as their evolution with torch parameters. Studies of the CH (B2∑ → X2Π) rotational band and the C2 (A3Πg → X3 Πu) rovibrational bands allowed temperature measurements using both spectra simulation and the Boltzmann method. We thus obtained rotational temperature maps of the jet and information on the vibrational temperature. These data will be useful for modelling the plasma jet. Emission spectroscopy also provides information concerning the electron densities in the plasma. Indeed, significantly Stark broadened H Balmer lines were observed.