Nathalie Picqué

Femtosecond laser Fourier transform absorption spectroscopy

A femtosecond mode-locked laser is used for what is believed to be the first time as a broadband infrared source for high-resolution Fourier transform absorption spectroscopy. A demonstration is made with a Cr(4+):YAG laser. The entire nu(1)+nu(3) vibration-rotation band region of acetylene, observed after passing through a single-pass 80-cm-long cell, is simultaneously recorded between 1480 and 1600 nm, in 7.9 s with a signal-to-noise ratio equal to 1000. Two hot bands of the most abundant acetylene isotopologue and the nu(1)+nu(3) band of the (13)C(12)CH(2) are also present. Replacement of the usual conventional tungsten lamp by the bright laser source reduces by about a factor of 150 the recording time needed to get similar results. The noise equivalent absorption coefficient at 1 s averaging is equal to 7x10(-7) cm(-1)Hz(-1/2) per spectral element.

Time-resolved Fourier transform intracavity spectroscopy with a Cr 2+ :ZnSe laser

Intracavity laser absorption spectroscopy (ICLAS) with an evacuated Cr2+:ZnSe laser is performed with a high-resolution time-resolved Fourier transform interferometer with a minimum detectable absorption coefficient equal to 4 x 10(-9) cm(-1) Hz(-1/2) in the 2.5 microm region. This represents the extreme limit currently reached in the infrared by ICLAS with Doppler-limited resolution. The broad gain band of the crystal allows a spectral coverage at most equal to 125 nm, wide enough to see entire vibration bands. Weak CO2 bands observed up to now only in the Venusian atmosphere are recorded for the first time, to our knowledge, in a laboratory. An H2O detection limit down to 0.9 parts per billion by volume is also demonstrated.

Continuous-wave 1.55 μm diode-pumped surface emitting semiconductor laser for broadband multiplex spectroscopy

A room-temperature-operating vertical external cavity surface emitting laser is applied around 1550 nm to intracavity laser absorption spectroscopy analyzed by time-resolved Fourier-transform interferometry. At an equivalent path length of 15 km, the high-resolution spectrum of the semiconductor disk laser emission covers 17 nm simultaneously. A noise-equivalent absorption coefficient at 1 s averaging equal to 1.5 x 10(-10) cm(-1)Hz(-1/2) per spectral element is reported for 65 km, the longest path length employed.

Absolute line intensities, vibrational transition moment, and self-broadening coefficients for the 3-0 band of 12C16O☆

Abstract Absolute line intensities, vibrational transition moment, Herman–Wallis factor and self-broadening coefficients for the 3-0 vibration-rotation band of 12 C 16 O are determined from absorption Fourier transform spectra with about 6×10 −3 xa0cm −1 unapodized resolution. The range of the CO measurements extends from P(20) at 6254xa0cm −1 to R(20) at 6407xa0cm −1 . The fitted squared transition dipole moment | μ 0 3 | 2 and Herman–Wallis factor coefficients C and D are respectively equal to (1.6727±0.0014)×10 −7 xa0Debye 2 , and (1.204±0.005)×10 −2 and (1.08±0.05)×10 −4 when given with one standard deviation. With the absolute uncertainty, | μ 0 3 | 2 is equal to (1.67±0.1)×10 −7 xa0Debye 2 .

Experimental transition dipole moment for the four lowest Δv=1 bands of ArH+ in the 1Σ+ fundamental state

Vibration–rotation bands of the Δv=1 sequence of ArH+ are recorded from the emission of a discharge tube with a Fourier spectrometer, between 1800 and 2830 cm−1. The analysis of the relative line intensities of the four lowest transitions made possible the determination of their Herman–Wallis factors. The derived ratios of first- and second-order coefficients of the dipole moment function to the permanent dipole moment M0 are equal to M1/M0=1.89, and M2/M0=0.96. The absolute values of the vibrational transition moments are respectively found, with no need of the ion concentration, equal to 0.194, 0.565, 1.049, and 1.623 debye for the 1–0, 2–1, 3–2, 4–3 transitions, with the assumption of M0 equal to 1.42 debye as experimentally given by K. B. Laughlin et al., Phys. Rev. Lett. 58, 996, (1987).

Multispectrum processing approach of weak H2O profiles recorded with absorption paths ranging from 20 to 120 km

Abstract A new powerful approach to intracavity laser absorption spectroscopy is explored and evaluated. The laser emission is recorded with a high-resolution time-resolved step-scan Fourier transform interferometer. Time-resolved spectra are obtained from an intracavity laser set-up based on a vertical-cavity surface-emitting semiconductor laser located in the open air of the laboratory. A restricted set of H 2 16 O lines is used for the evaluation of the method. The lines are measured around 9625 cm −1 in more than one hundred time-component spectra simultaneously recorded with absorption path lengths varying in arithmetic progression from 20 up to about 120 km . Data processing is performed with a multispectrum fitting program. The procedure is shown to be efficient for the quantitative determination of molecular parameters of ultra weak transitions.

Quantitative wideband spectroscopy with kilometric absorption paths

Combining Intra Cavity Laser Absorption and Time Resolved Fourier transform spectro-scopies (ICLAS-TRFTS) brings original advantages that are derived from the ability to obtain consistent series of Fourier spectra recorded according to a well-defined time sequence on the same accurate wavenumber scale. These advantages, mainly centred on the possibility to perform quantitative measurements are discussed and evaluated. Accurate determination of line positions and intensities of ultra weak transitions previously hardly measurable becomes possible. Ultimate limits are estimated for the present experiment in the neqr infrared range for H2O.

Sensitive instrumental developments in high-resolution laser and Fourier transform spectroscopies

Abstract Two recent instrumental developments involving laser and Fourier transform spectroscopies, and their applications are discussed. The first one is of metrological relevance, and provides a stable frequency reference in the near-infrared region thanks to diode laser instrumentation. The second one combines intracavity laser absorption spectroscopy (ICLAS) and time-resolved Fourier transform spectroscopy (TRFTS). Spectra around 1xa0μm, exhibiting kilometric absorption path length, are obtained from the high-resolution time-resolved step-scan interferometer of the Laboratoire de Photophysique Moleculaire (LPPM), with an intracavity laser absorption set-up based on a vertical-external-cavity surface-emitting semiconductor laser (VECSEL).

Time-resolved Fourier transform spectroscopy applied to collisional relaxation study of the B 3Πgv = 0 level of N2 in a pulsed electrical discharge

Time-resolved Fourier transform spectroscopy, with temporal and spectral resolutions respectively at best 1.6 μs and 0.014 cm−1 (Doppler-limited), is applied as a new diagnostic tool to the characterization of nitrogen plasma. A spectral range extending from 0.93 to 1.85 μm is recorded at once. The rotational lines of the B 3Πg→A 3Σu+, Δv = 0,−1,−2,−3 vibronic sequences are fully resolved. Thermodynamical plasma parameters are obtained from line intensity and width analysis. The relaxation of the B 3Πg state of N2 in a pulsed electrical discharge is observed under various pressures and pulse repetition rates. Evolutions of the low-lying vibrational levels B 3Πg,v = 0,1,2 during the early afterglow (post-discharge time period ≤150 μs) are analysed. A simplified kinetic model based on the strong couplings between B 3Πg and A 3Σu+ states and between B 3Πg and W 3Δu states is adopted. It is found appropriate for the interpretation of the spectra within a restricted range (pressure 16–53 Pa (0.12–0.40 Torr), repetition rate 50–200 Hz) of the experimental conditions. Collisional rate constants coupling the B v = 0 level to the levels A v = 7 and W v = 0 are estimated to be kBW = 9×10−12 cm3 s−1 and kBA = 3×10−12 cm3 s−1, in accordance with existing results.

Wide-band spectroscopic investigation of the state-to-state dependence of the ArH+ ion average mobility in a Ar/He plasma

Abstract A wide-band experimental study on the quantum dependence of an ion average mobility in the positive column of a glow discharge is reported: no vibrational nor rotational energy dependence of the ArH + ion average mobility in a mixture Ar/He is observed, within an experimental uncertainty of 5%. This conclusion is obtained from electric field-induced Doppler-shift measurements in the Δ v =1 sequence vibration–rotation bands, around 5 μm. Two approaches, involving high-resolution Fourier transform spectroscopy, are employed. The two results agree well.