Sophie Dixneuf

Long optical cavities for open-path monitoring of atmospheric trace gases and aerosol extinction

An incoherent broadband cavity-enhanced absorption spectroscopy setup employing a 20 m long optical cavity is described for sensitive in situ measurements of light extinction between 630 and 690 nm. The setup was installed at the SAPHIR atmospheric simulation chamber during an intercomparison of instruments for nitrate (NO(3)) radical detection. The long cavity was stable for the entire duration of the two week campaign. A detection limit of approximately 2 pptv for NO(3) in an acquisition time of 5 s was established during that time. In addition to monitoring NO(3), nitrogen dioxide (NO(2)) concentrations were simultaneously retrieved and compared against concurrent measurements by a chemiluminescence detector. Some results from the campaign are presented to demonstrate the performance of the instrument in an atmosphere containing water vapor and inorganic aerosol. The spectral analysis of NO(3) and NO(2), the concentration dependence of the water absorption cross sections, and the retrieval of aerosol extinction are discussed. The first deployment of the setup in the field is also briefly described.

Molecular iodine (I2) emission from two Laminaria species (Phaeophyceae) and impact of irradiance and temperature on I2 emission into air and iodide release into seawater from Laminaria digitata.

Kelps of the genus Laminaria accumulate iodine at high concentrations, but the iodine retaining capacity can be affected by emersion and physiological stress. In this study, I2 emission into the atmosphere from Laminaria digitata and Laminaria hyperborea was compared under controlled low irradiances and temperatures. The two species exhibited different I2 emission rates as blades of L. digitata emitted I2 at rates five times higher than those from newly-grown blades (current growth season) of L. hyperborea. I2 emission was not detectable from old blades (previous growth season) of L. hyperborea. Additionally, effects of irradiance and temperature on both I2 emission into air and net I(-) release into seawater where assessed for L. digitata while monitoring photo-physiological parameters as stress indicators. Irradiances between 30 and 120 μmol photons m(-2) s(-1) had only marginal effects on both I2 emission and I(-) release rates, but physiological stress, indicated by photoinhibition, was observed. The results suggest that the irradiances applied here were not stressful enough to impact on the iodine release. By contrast, at elevated temperatures (20 °C), photoinhibition was accompanied by an increase in I2 emission rates, but net I(-) release rates remained similar at 10-20 °C. High I2 emission rates into air and I(-) release into seawater observed from L. digitata underpin the fundamental function of this kelp as mediator of coastal iodine fluxes.

Anisotropic collision-induced Raman scattering by the Kr:Xe gas mixture

We report anisotropic collision-induced Raman scattering intensities by the Kr-Xe atomic pair recorded in a gas mixture of Kr and Xe at room temperature. We compare them to quantum-mechanical calculations on the basis of modern incremental polarizability models of either ab initio post-Hartree-Fock or density functional theory methods.

Induced anisotropy of Ar2: a state-of-the-art semiempirical model.

We report a semiempirical induced anisotropy model for Ar2 that optimally reproduces highly accurate experimental induced scattering band shapes over a large domain of frequency detunings. We believe that our model, which was derived (after an extensive compilation of the data) by using a systematic methodology and a novel reliable multiparameter equation solver (presented here), is the best induced anisotropy ever proposed. The applicability of our methodology to any induced band shape and its economical implementation make it a promising device for accessing various incremental properties for weakly interacting systems of an arbitrary complexity.

Collision-induced Raman scattering and the peculiar case of neon: Anisotropic spectrum, anisotropy, and the inverse scattering problem

Owing in part to the p orbitals of its filled L shell, neon has repeatedly come on stage for its peculiar properties. In the context of collision-induced Raman spectroscopy, in particular, we have shown, in a brief report published a few years ago [M. Chrysos et al., Phys. Rev. A 80, 054701 (2009)], that the room-temperature anisotropic Raman lineshape of Ne-Ne exhibits, in the far wing of the spectrum, a peculiar structure with an aspect other than a smooth wing (on a logarithmic plot) which contrasts with any of the existing studies, and whose explanation lies in the distinct way in which overlap and exchange interactions interfere with the classical electrostatic ones in making the polarizability anisotropy, α∥ - α⊥. Here, we delve deeper into that study by reporting data for that spectrum up to 450 cm(-1) and for even- and odd-order spectral moments up to M6, as well as quantum lineshapes, generated from SCF, CCSD, and CCSD(T) models for α∥ - α⊥, which are critically compared with the experiment. On account of the knowledge of the spectrum over the augmented frequency domain, we show how the inverse scattering problem can be tackled both effectively and economically, and we report an analytic function for the anisotropy whose quantum lineshape faithfully reproduces our observations.

Collision-induced Raman scattering by rare-gas atoms: The isotropic spectrum of Ne–Ne and its mean polarizability

We report the room-temperature isotropic collision-induced light scattering spectrum of Ne-Ne over a wide interval of Raman shifts, and we compare it with the only available experimental spectrum for that system as well as with spectra calculated quantum-mechanically with the employ of advanced ab initio-computed data for the incremental mean polarizability. The spectral range previously limited to 170 cm(-1) is now extended to 485 cm(-1) allowing us to successfully solve the inverse-scattering problem toward an analytic model for the mean polarizability that perfectly matches our measurements. We also report the depolarization ratio of the scattering process, lingering over the usefulness of this property for more stringent checks between the various polarizability models.

On the isotropic Raman spectrum of Ar2 and how to benchmark ab initio calculations of small atomic clusters: Paradox lost

This is the long-overdue answer to the discrepancies observed between theory and experiment in Ar2 regarding both the isotropic Raman spectrum and the second refractivity virial coefficient, BR [Gaye et al., Phys. Rev. A 55, 3484 (1997)]. At the origin of this progress is the advent (posterior to 1997) of advanced computational methods for weakly interconnected neutral species at close separations. Here, we report agreement between the previously taken Raman measurements and quantum lineshapes now computed with the employ of large-scale CCSD or smartly constructed MP2 induced-polarizability data. By using these measurements as a benchmark tool, we assess the degree of performance of various other ab initio computed data for the mean polarizability α, and we show that an excellent agreement with the most recently measured value of BR is reached. We propose an even more refined model for α, which is solution of the inverse-scattering problem and whose lineshape matches exactly the measured spectrum over the entire frequency-shift range probed.

Broadband Cavity-Enhanced Absorption Spectroscopy with Incoherent Light

Although broadband incoherent light does not efficiently couple into a high-finesse optical cavity, its transmission is readily detectable and enables applications in cavity-enhanced absorption spectroscopy in the gas phase, liquid phase and on surfaces. This chapter gives an overview of measurement principles and experimental approaches implementing incoherent light sources in cavity-enhanced spectroscopic applications. The general principles of broadband CEAS are outlined and general “pros and cons” discussed, detailing aspects like cavity mirror reflectivity calibration or the establishment of detection limits. Different approaches concerning light sources, cavity design and detection schemes are discussed and a comprehensive overview of the current literature based on a methodological classification scheme is also presented.

On a singularity-free pair-polarizability anisotropy model for atomic gases

We present a collision-induced light scattering spectrum of Ne(2) and analyze it, together with an induced spectrum of Ar(2), in terms of a model for the pair-polarizability anisotropy beta of an atomic gas. This model, which is in essence a slight modification of one introduced by Ceccherini et al. [J. Chem. Phys. 111, 6316 (1999)], is shown to describe the measured spectra much farther into the wings than does the original model. The agreement of our measurements with quantum-mechanical spectra from the most reliable, now available, ab initio computation methods for beta produces evidence in support of the data and counters criticism of our experiment.