Adaptive dual-comb spectroscopy in the green region.

Dual-comb spectroscopy is extended to the visible spectral range with two short-pulse frequency-doubled free-running ytterbium-doped fiber lasers. When the spectrum is shifted to other domains by nonlinear frequency conversion, tracking the relative fluctuations of the femtosecond oscillators at their fundamental wavelength automatically produces the correction signal needed for the recording of distortion-free spectra. The dense rovibronic spectrum of iodine around 19,240 cm(-1) is recorded within 12 ms at Doppler-limited resolution.

Raman-induced Kerr-effect dual-comb spectroscopy.

We report on the first (to our knowledge) demonstration of nonlinear dual-frequency-comb spectroscopy. In multi-heterodyne femtosecond Raman-induced Kerr-effect spectroscopy, the Raman gain resulting from the coherent excitation of molecular vibrations by a spectrally narrow pump is imprinted onto the femtosecond laser frequency comb probe spectrum. The birefringence signal induced by the nonlinear interaction of these beams and the sample is heterodyned against a frequency comb local oscillator with a repetition frequency slightly different from that of the comb probe. Such time-domain interference provides multiplex access to the phase and amplitude Raman spectra over a broad spectral bandwidth within a short measurement time.

Continuous-wave 1.55 microm 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.

Infrared frequency combs and supercontinua for multiplex high sensitivity spectroscopy.

An infrared high-brightness light source based on supercontinuum generation through a SF6 photonic crystal fiber seeded by a Cr(4+):YAG femtosecond oscillator is developed for high resolution multiplex spectroscopy in the 1.5 µm region. Moreover, a multiplex high resolution approach based on a Cr(4+):YAG frequency comb enables to probe large spectral domains, with simultaneous sensitive measurement of the absorption and the dispersion associated with all individual spectral features.

Sensitive multiplex spectroscopy in the molecular fingerprint 2.4 mum region with a Cr(2+):ZnSe femtosecond laser.

An ultrashort-pulse Cr(2+):ZnSe laser is a novel broadband source for sensitive high resolution molecular spectroscopy. A 130-fs pulse allows covering of up to 380 cm(-1) spectral domain around 2.4 mum which is analyzed simultaneously with a 0.12 cm(-1) (3.6 GHz) resolution by a Fouriertransform spectrometer. Recorded in 13 s, from 70-cm length absorption around 4150 cm(-1), acetylene and ammonia spectra exhibit a 3800 signal-tonoise ratio and a 2.4.10(-7) cm(-1).Hz(-1/2) noise equivalent absorption coefficient at one second averaging per spectral element, suggesting a 0.2 ppbv detection level for HF molecule. With the widely practiced classical tungsten lamp source instead of the laser, identical spectra would have taken more than one hour.

Emission spectra of HCN/HNC in the 2-5 microm range of astrophysical interest.

Emission from H12C14N observed with a Fourier transform spectrometer from a radio-frequency excited plasma is reported in the 2400-3400 cm(-1) spectral range of astrophysical interest. The molecular constants, for 21 vibration-rotation bands are given, as well as estimates of the first-order Herman-Wallis coefficients for 11 bands. These constants are derived from about 900 observed transitions in HCN, and are used to generate a sequential linelist of about 1400 calculated line positions, within a standard deviation equal to 3 x 10(-4) cm(-1). The relative intensities of the observed lines are also reported, as well as those for the nu1 band of H14N12C, at 3650 cm(-1), simultaneously observed from the same plasma.

Line Intensities of (12)C(16)O(2) in the 1.2-1.4 µm Spectral Region.

The 7000-8500 cm(-1) spectral region of (12)C(16)O(2) has been investigated using the high-resolution FT spectrometer of LPPM in Orsay. The two strongest bands in this region are the 10031 <-- 00001 and 10032 <-- 00001 bands centered at 8294 and 8192 cm(-1). Line intensities in these two bands and in the 40013 <-- 00001 and 40014 <-- 00001 bands have been measured. Using the method of effective operators, these line intensities have been included in a new fit of effective dipole-moment parameters to all available experimental data in the same spectral region of (12)C(16)O(2). The corresponding calculated line intensities of the 10031 <-- 00001 and 10032 <-- 00001 bands are compared with the experimental ones. Copyright 2000 Academic Press.

High-information time-resolved Fourier transform spectroscopy at work.

The first instrumental setup, to our knowledge, that is capable of recording in a few hours the time-resolved Fourier transform (TRFT) interferograms of gas-phase spectra that cover several thousands of inverse centimeters with spectral- and time-resolution limits that are equal, at best, to 2.5 x 10(-3) cm(-1) and 2 ns, respectively, is reported. It was developed on the stepping-mode Connes-type interferometer of the Laboratoire de Photophysique Moléculaire Université de Paris Sud. Also, for the first time, to our knowledge, these high-resolution TRFT spectra, illustrated with the Doppler-limited emission spectra of the N(2) transitions (B-A) and (B'-B) between 5500 and 11 000 cm(-1) and of the atomic Ar lines between 1800 and 4000 cm(-1), are recorded in the infrared spectral range. To obtain identical results that have the same signal-to-noise ratio, we should have increased the recording time of our unique previous high-information TRFT spectra by approximately 50,000. In other words, one hour is now long enough to obtain what would previously have required six years to record.

High-Resolution Fourier Transform Spectroscopy of (14)N(2): Analysis of the (1-0), (2-1) Bands of the B(3)Pi(g)-W(3)Delta(u) System.

The extension of high-resolution observation of the electronic emission spectrum of (14)N(2) toward the infrared domain is presented. To date, rotational analysis of the widely investigated spectrum of the N(2) molecule was done in a spectral domain ranging from 2500 cm(-1) to the UV. We have recorded for the first time the infrared part of the (14)N(2) spectrum from 1250 to 2250 cm(-1), using the Fourier transform spectrometer of Laboratoire de Photophysique Moléculaire (LPPM) at an unapodized resolution of 0.0043 cm(-1). A complete rotational analysis is performed for the (1 --> 0), (2 --> 1), (0 <-- 1), (1 <-- 2) bands of the B(3)Pi(g)-W(3)Delta(u) system, not included in any previous analysis. Spectroscopic parameters for the v = 0, v = 1, v = 2 levels of the B(3)Pi(g) and the W(3)Delta(u) states, consistent with those previously reported but with improved accuracy, are obtained from the experimental wavenumbers by a nonlinear least-squares procedure. Copyright 1999 Academic Press.

Positions and Intensities in the 2nu4/nu1/nu3 Vibrational System of 14NH3 Near 3 µm.

Line positions and line intensities of the nu1, nu3, and 2nu4 bands of 14NH3 were analyzed using line positions from 0.0054 cm-1 apodized resolution FT spectra recorded at Orsay and using line intensities from 0.011 cm-1 unapodized resolution FT spectra recorded at Kitt Peak National Observatory. About 2110 lines with J'

High-resolution multimodulation Fourier-transform spectroscopy.

High-resolution double-modulation Fourier-transform (FT) spectroscopy (FTS) is demonstrated for what is, to our knowledge, the first time. Two high-resolution FT interferograms are simultaneously recorded. The first one is nonselective and contains all the spectral information from the observed source, and the other one is made of the samples that are sensitive to only a specific source modulation. General formulations and practical recording procedures are given for phase- and intensity-modulated spectra. The advantage of selectivity is illustrated by velocity-modulated emission spectra of the Dv = 1 sequence of the Doppler-shifted ArH(+) ion. It is also shown that for a source perturbation of small amplitude, only the product of intensity x shift can be retrieved from the selective line shapes obtained in a phase-modulated laser or FT spectra. Thanks to the multimodulation FTS approach, the intensity and the shift of the transitions are measured in a single experiment.

Line Intensities for the 8-µm Bands of SO2

Using both high-resolution (R = 0.003 cm-1) and medium-resolution (R = 0.12 cm-1) Fourier transform spectroscopy, it has been possible to measure accurately a large set of individual line intensities for the nu1 and nu3 bands of SO2 in the 950-1400 cm-1 spectral region. These intensities were introduced into a least-squares fit calculation allowing one to obtain the expansion of the transition moment operator of the nu1 and nu3 bands. For these intensity calculations, the theoretical model takes into account the vibration-rotation interactions linking the upper levels involved in the nu1, 2nu2, and nu3 interacting SO2 bands. Finally, a synthetic spectrum of the 8-µm SO2 bands has been generated using the dipole moment expansion determined in this work and the molecular parameters and the Hamiltonian matrix given in a previous analysis. Copyright 1998 Academic Press.

Absolute Wavenumbers and Self-Induced Pressure Lineshift Coefficients for the 3-0 Vibration-Rotation Band of 12C16O

The absolute wavenumbers for 36 lines of the 3-0 band of 12C16O are measured from P(17) at 6271 cm-1 to R(19) at 6405 cm-1, with an uncertainty of about +/-3 x 10(-5) cm-1 (about +/-1 MHz). The experimental positions are compared with the best predicted positions from recent Dunham coefficients. Self-induced pressure lineshifts are determined and reach, at most, about -8 x 10(-3) cm-1 atm-1. Copyright 1997 Academic Press. Copyright 1997Academic Press

The High-Resolution Fourier Transform Spectrum of Dideuterated Methane CH2D2 in the Region between 1900 and 2400 cm-1

The infrared spectrum of doubly deuterated methane CH2D2 has been recorded in the region from 1900 to 2400 cm-1 at almost Doppler-limited resolution by using two high-resolution Fourier transform spectrometers. The vibrational bands observed include 2nu4, nu4 + nu7, 2nu7, nu2, nu8, nu4 + nu9, and nu7 + nu9, which were analyzed by taking into account Coriolis and Fermi interactions among them and also those with nu4 + nu5, nu3 + nu7, and nu5 + nu7. Most of the centrifugal distortion constants were constrained to appropriate values, while the vibrational term value and three rotational constants in each of the seven excited states were adjusted along with Coriolis and Fermi interaction parameters by the least-squares analysis of the observed spectrum. The vibration-rotation interaction constants alphas thus determined for the nu2 and nu8 states were combined with those of other fundamental states already published to calculate the equilibrium C-H distance.

Selective detection of molecular ions in dc discharges by dual-beam Fourier transform spectroscopy.

A new method to detect molecular ions selectively in gas discharges by Fourier transform emission spectroscopy is proposed. This method is by dual-beam optical subtraction. The difference spectrum between blue and red Doppler-shifted components is obtained directly thus eliminating the large background that is due to neutral precursors.

High-information time-resolved step-scan Fourier interferometer.

The adaptation of a high-resolution stepping-mode Connes-type interferometer to the study of timevarying phenomena is described. Solutions that can be used to solve the problem of the limited dynamic range of the measurements involved in time-resolved experiments at high spectral resolution are proposed. To handle the millions of temporal and spectral samples, computer programs based on a matrix formalism and graphic techniques have been developed. A computer simulation of a timeresolved Fourier transform spectroscopy (TRFTS) experiment of a systematic study of the problem posed by the limited dynamic range of the measurements is presented. Advantages and inherent difficulties of the step-by-step TRFTS are reviewed.

Polarization modulation high resolution Fourier transform spectroscopy.

A new polarization modulation Zeeman method combined with high resolution Fourier transform spectroscopy is described. It selectively detects the paramagnetic species produced in a plasma under a constant magnetic field. The polarizing optics is convenient and easily covers wide spectral ranges. NO absorption and N(2)O, NO emission spectra are reported. With a magnetic field of the order of 560 G and a 40-cm long discharge tube, the P and R lines of the 1-0 band of NO around 5 microm are observed up to J = 28.5 whereas the much more intense N(2)O lines are not detected. Unexplained line shapes of the first Q(2) lines of NO are reported. A comparison with a recent frequency modulation Zeeman method coupled with high resolution Fourier transform spectroscopy is given.