F. Rachet

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.

Isotropic collision induced light scattering spectra from gaseous SF6

The experimental binary isotropic collision-induced light scattering spectrum of the gaseous sulfur hexafluoride is measured in absolute units in the 30–210 cm−1 frequency range. The contribution of dipole–multipole mechanisms is computed in a semi-classical way. From comparison with experiment, the independent component E of the dipole–octopole polarizability tensor is estimated. This evaluation is compared to a recent theoretical ab initio calculation.

Depolarized collision-induced light scattering by gaseous helium

We have carried out a joint study, experimental and theoretical, of the binary depolarized collision-induced light scattering spectrum by gaseous helium at room temperature. The intensities from helium pairs, calibrated on an absolute scale, have been measured in the much extended, previously unexplored, 5-680 cm-1 spectral domain. A critical analysis of the spectrum has been made by using ab initio anisotropy models available in the literature. Quantum-mechanical computations have enabled us to determine the spectral response of the pair polarizability terms which compose the most up-to-date anisotropy data (Moszynski et al 1996 J. Chem. Phys. 104 6997). The conclusion has been drawn that internuclear distances shorter than the helium atomic diameter are probed via the very far wing of our spectrum.

The isotropic remnant of the CO2 near-fully depolarized Raman 2ν3 overtone

In a recent paper [M. Chrysos, I. A. Verzhbitskiy, F. Rachet, and A. P. Kouzov, J. Chem. Phys. 134, 044318 (2011)], we showed that, in CO(2), the 2ν(3) transition generates a Raman line spectrum that is 98% depolarized, a property in agreement with general symmetry rules. Here, we present an extensive analysis, experimental and theoretical, of the isotropic remnant of this overtone. The isotropic spectrum turned out to be 45 times less intense than its anisotropic counterpart and to have a moment that is 350 times smaller than the moment of the anisotropic spectrum, in excellent agreement with theoretical predictions. Once the measured intensity (along with other data exclusively experimental) was fed back into the formula of the moment, a value for the CO(2) mean-polarizability asymmetric stretch derivative ∂(2)α/∂q(3)(2) was returned that matches the best ab initio prediction to better than 4%. Agreement, in order of magnitude, was found between the intensity reported herein and that reported in the sole prior study of this overtone [G. Tejeda, B. Mat, and S. Montero, J. Chem. Phys. 103, 568 (1995)].

Polarization components of rototranslational light scattering spectra from gaseous SF6

Experimental binary anisotropic and isotropic collision-induced scattering spectra of gaseous sulphur hexafluoride at room temperature are reported on an absolute scale up to 210cm-1. On the basis of a comparison of experiment with theoretical computations of scattering intensities, the value of the independent component E of the dipole-octopole polarizability tensor is discussed for several intermolecular potentials and compared with a recent ab initio theoretical value. A recommended value of E is given.

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.

The depolarized Raman 2ν3 overtone of CO2: A line-mixing shape analysis

In a recent article we showed that the 2ν(3) transition of CO(2) gives rise to a Raman spectrum that is almost entirely depolarized [M. Chrysos, I. A. Verzhbitskiy, F. Rachet, and A. P. Kouzov, J. Chem. Phys. 134, 044318 (2011)]. In the present article, we go further forward in the study of this overtone by reporting a first-principles shape analysis of its depolarized spectrum at room temperature. As a first step in our analysis, a model assuming isolated Lorentzian line shapes was applied, which at low gas densities turns out to be sufficient for qualitative conclusions. As the next step, a sophisticated approach was developed on the basis of the extended strong-collision model in order to properly account for the heavy line mixing between rotational lines. Whereas a marked deviation between model and measured spectra was observed upon application of the simpler model, striking agreement even at the highest CO(2) density was found on applying the sophisticated one. Accurate calculated data were used for the rotational line broadening coefficients without resort to arbitrary parameters. Values for the vibrational shift scaling linearly with the density of the gas are given.

From light-scattering measurements to polarizability derivatives in vibrational Raman spectroscopy: The 2ν5 overtone of SF6

The room-temperature isotropic spectrum of SF6 was recorded at the frequency of the 2ν5 overtone by running high-sensitivity incoherent Raman experiments for two independent polarizations of the incident beam and for gas densities varying from 2 to 27 amagat. Weak yet observable pressure effects were found. A transparent analysis of the Raman cross-section problem along with the first-ever prediction of the value of the mean polarizability second derivative ∂(2)α/∂q5(2) are made and the hitherto underestimated role of the hot bands of SF6 is brought to the wider public. The emergence of an analytic hotband factor is shown whose magnitude is dramatically increased with the order of the overtone and the gas temperature and all the more so upon considering low-frequency molecular vibrations. Our formulas, which in the harmonic approximation are exact, are still applicable to real situations provided certain conditions are fulfilled. For nondegenerated modes, generalization to higher order overtones is made, an issue addressing the much challenging problem of the IR-allowed second overtone bands. The content of this paper is also an invitation towards ab initio derivative-calculations for sulfur hexafluoride, especially given the todays needs in interpreting spectra of significance for greenhouse atmospheric issues.