Guy Guelachvili

The 1997 spectroscopic GEISA databank

The current version GEISA-97 of the computer-accessible database system GEISA (Gestion et Etude des Informations Spectroscopiques Atmospheriques: Management and Study of Atmospheric Spectroscopic Information) is described. This catalogue contains 1,346,266 entries. These are spectroscopic parameters required to describe adequately the individual spectral lines belonging to 42 molecules (96 isotopic species) and located between 0 and 22,656 cm-1. The featured molecules are of interest in studies of the terrestrial as well as the other planetary atmospheres, especially those of the Giant Planets. GEISA-97 contains also a catalog of absorption cross-sections of molecules such as chlorofluorocarbons which exhibit unresolvable spectra. The modifications and improvements made to the earlier edition (GEISA-92) and the data management software are described. GEISA-97 and the associated management software are accessible from the ARA/LMD (Laboratoire de Meteorologie Dynamique du CNRS, France) web site: http://ara01.polytechnique.fr/registration.

Coherent Raman spectro-imaging with laser frequency combs

Advances in optical spectroscopy and microscopy have had a profound impact throughout the physical, chemical and biological sciences. One example is coherent Raman spectroscopy, a versatile technique interrogating vibrational transitions in molecules. It offers high spatial resolution and three-dimensional sectioning capabilities that make it a label-free tool for the non-destructive and chemically selective probing of complex systems. Indeed, single-colour Raman bands have been imaged in biological tissue at video rates by using ultra-short-pulse lasers. However, identifying multiple, and possibly unknown, molecules requires broad spectral bandwidth and high resolution. Moderate spectral spans combined with high-speed acquisition are now within reach using multichannel detection or frequency-swept laser beams. Laser frequency combs are finding increasing use for broadband molecular linear absorption spectroscopy. Here we show, by exploring their potential for nonlinear spectroscopy, that they can be harnessed for coherent anti-Stokes Raman spectroscopy and spectro-imaging. The method uses two combs and can simultaneously measure, on the microsecond timescale, all spectral elements over a wide bandwidth and with high resolution on a single photodetector. Although the overall measurement time in our proof-of-principle experiments is limited by the waiting times between successive spectral acquisitions, this limitation can be overcome with further system development. We therefore expect that our approach of using laser frequency combs will not only enable new applications for nonlinear microscopy but also benefit other nonlinear spectroscopic techniques.

Adaptive real-time dual-comb spectroscopy

The spectrum of a laser frequency comb consists of several hundred thousand equally spaced lines over a broad spectral bandwidth. Such frequency combs have revolutionized optical frequency metrology and they now hold much promise for significant advances in a growing number of applications including molecular spectroscopy. Despite an intriguing potential for the measurement of molecular spectra spanning tens of nanometres within tens of microseconds at Doppler-limited resolution, the development of dual-comb spectroscopy is hindered by the demanding stability requirements of the laser combs. Here we overcome this difficulty and experimentally demonstrate a concept of real-time dual-comb spectroscopy, which compensates for laser instabilities by electronic signal processing. It only uses free-running mode-locked lasers without any phase-lock electronics. We record spectra spanning the full bandwidth of near-infrared fibre lasers with Doppler-limited line profiles highly suitable for measurements of concentrations or line intensities. Our new technique of adaptive dual-comb spectroscopy offers a powerful transdisciplinary instrument for analytical sciences.

The ground state of methane 12CH4 through the forbidden lines of the ν3 band

Abstract High resolution spectra of the ν3 band of methane, 12CH4, were recorded by using a “third generation vacuum Fourier interferometer”; a large pressure range (from 0.009 to 10 Torr) with a sample path fixed at eight meters was used, enabling observation of transitions with intensity ratios as low as 1 10 000 . More than 350 forbidden transitions of the ν3 band, including about 125 transitions of the Q+ branch, were unambiguously identified. Of the 277 transitions retained for computations, one-hundred have 11 ≤ J ≤ 16. From combination difference relations using pairs of transitions having the same upper state energy level (forbidden-allowed and forbidden-forbidden pairs were used), 276 independent differences between ground state energy levels could be determined with uncertainties of about 0.001 cm−1. These data yielded the following values for the ground state structure constants of 12CH4 along with their standard deviations (in cm−1): β o hc =5.2410356±0.0000096 , γ o hc =(−1±0.00074) 10 −4 , π o hc =(5.78±0.18) 10 −9 , ϵ o hc =(−1.4485±0.0023) 10 −6 , ϱ o hc =(1.768±0.126) 10 −10 , ξ o hc =(−1.602±0.067) 10 −11 , Thus, for the first time, the scalar constant π0 has been evaluated and ir values have been obtained for the two tetrahedral constants ϱ0 and ξ0; furthermore, these values are in very good agreement with the ones recently determined from radiofrequency data, i.e., in cm−1: ϵ o hc =(−1.45061±0.00014) 10 −6 , ϱ o hc =(1.7634±0.0068) 10 −10 , ξ o hc =(−1.5432±0.0040) 10 −11 From these values, the 276 differences can be reproduced with an overall rms deviation equal to 0.0009 cm−1. Finally, the ground state energies of 12CH4 have been calculated for J ≤ 16.

High resolution Fourier transform spectrum of water between 2930 and 4255 cm-1

The water vapour spectrum recorded with a resolution of 5 × 10-3 cm-1 between 2930 cm-1 and 4255 cm-1 on a Fourier transform spectrometer is presented. Wavenumbers, attributions and equivalent widths of about 1500 lines are given. Lines of the 2v 2, v 1 and v 3 bands of H2 16O, lines of the hot band v 2 + v 3 - v 2 of H2 16O and lines of the v 1 and v 3 bands of H2 18O and H2 17O are observed.

Absolute wavenumbers and molecular constants of the fundamental bands of 12C16O, 12C17O, 12C18O, 13C16O, 13C17O, 13C18O and of the 2-1 bands of 12C16O and 13C16O, around 5μm, by Fourier spectroscopy under vacuum

Abstract This paper is part of a more general work undertaken on the determination of Dunham coefficients for all the isotopic species of CO. The absolute wavenumbers of 466 CO lines obtained from absorption spectra of six different isotopic forms in the 5 μm region are reported with an accuracy of the order of 3 MHz or less for about 300 lines. Absolutely measured within 9 MHz, 43 lines of 13 C 17 O (a molecule whose spectrum has not been observed in the infrared before) are given together with 59 wavenumbers measured within 3.2 MHz of 12 C 17 O, also reported for the first time. The wavenumbers extend from 1939 to 2271 cm −1 . More than 50 accurate molecular constants are available allowing precise determination of pure rotational transitions. These metrologic data should be helpful both for microwave or infrared identification in astronomy, for instance, and for more specific molecular purposes. The high self-consistency of these Fourier results measured versus the Krypton line, and their consistency with all the other accurate measurements suggest that the reported values be adopted as secondary standards.

High resolution wavenumber standards for the infrared (Technical Report)

The calibration of high resolution infrared spectra is generally more precise than accurate. This is the case even when they are recorded with Fourier transform interferometers. The present document aims at improving the accuracy of wavenumber measurements in the infrared by recommending a selection of spectral lines as wavenumber standards for absolute calibration in the range from about 4 to about 7000 cm(-1). The uncertainties of these wavenumber standards range from 4+/-1x10(-3) to +/-1x10(-6) cm(-1). Sources of frequency standards, on which the wavenumber determinations are based, are also given.

Determination of A0 for CH335Cl and CH337Cl from the ν4 infrared and Raman bands

Abstract The ν4 infrared and Raman bands of CH3Cl were analyzed simultaneously. A direct fit yielded a complete set of constants for CH335Cl, including A0 = 5.20530 ± 0.00010 cm−1 and DK = (8.85 ± 0.13) × 10−5cm−1. For CH337Cl an incomplete set of constants was obtained from the infrared band, and A0 = 5.2182 ± 0.0010 cm−1 was estimated by curve fitting of the Raman spectrum. The resulting equilibrium structure is r( CH) = 1.0854 ± 0.0005 A , r( CCl) = 1.7760 ± 0.0003 A , and

Improved Dunham coefficients for CO from infrared solar lines of high rotational excitation

Abstract About 4500 unblended CO lines have been selected and their wavenumbers accurately measured on high resolution solar spectra obtained from space with the ATMOS Fourier transform spectrometer. Half of these lines are of high rotational excitation energy and have never been observed before in the laboratory. Line positions of the fundamental bands of 12 C 16 O have been measured up to J = 133, those of 13 C 16 O and of 12 C 18 O up to J = 103 and 91, respectively. The first overtone bands of 12 C 16 O have been measured up to J = 110. These new solar CO wavenumbers, with an additional selected set of about 14 000 accurate laboratory measurements, have been simultaneously fitted to the Dunham expression utilizing 10 recently published relations between isotopically invariant parameters U ij . The present set of coefficients reproduces all accurate laboratory positions and our solar measurements of high rotational excitation with a standard deviation of about 10 −5 cm −1 (300kHz). This new set is particularly recommended for all high resolution studies of infrared laboratory and stellar spectra showing CO lines of high J -values.

Infrared bands of HCl and DCl by Fourier transform spectroscopy: Dunham coefficients for HCl, DCl, and TCl

Abstract Fourier absorption spectra of HCl and DCl are recorded simultaneously in the spectral range [2840 cm−1–8450 cm−1]. A Uij reduced Dunham coefficient set is deduced by fitting all the available data and then used together with four mass-scaling parameters to predict the TCl Yij Dunham coefficients.