A. P. Kouzov

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)].

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.

The isotropic spectrum of the CO2 Raman 2ν3 overtone: A line-mixing band shape analysis at pressures up to several tens of atmospheres

A line-mixing shape analysis of the isotropic remnant Raman spectrum of the 2ν(3) overtone of CO(2) is reported at room temperature and for densities, ρ, rising up to tens of amagats. The analysis, experimental and theoretical, employs tools of non-resonant light scattering spectroscopy and uses the extended strong collision model (ESCM) to simulate the strong line mixing effects and to evidence motional narrowing. Excellent agreement at any pressure is observed between the calculated spectra and our experiment, which, along with the easy numerical implementation of the ESCM, makes this model stand out clearly above other semiempirical models for band shape calculations. The hitherto undefined, explicit ρ-dependence of the vibrational relaxation rate is given. Our study intends to improve the understanding of pressure-induced phenomena in a gas that is still in the forefront of the news.

Concentration studies of collision-induced fundamental absorption of hydrogen dissolved in liquid neon.

We report further and more detailed results of our recent investigation [W. A. Herrebout, B. J. van der Veken, and A. P. Kouzov, Phys. Rev. Lett. 101, 093001 (2008)] on the collision-induced fundamental absorption by hydrogen dissolved in liquid neon (T ≈ 25 K). The band shapes were studied in a wide range of concentrations (0.003-0.05 mole fractions) as well as for different ortho/para ratios and at much higher level of accuracy and resolution than before. Due to almost unhindered rotation of the hydrogen molecule and low temperature, an unprecedently rich frequency-domain picture produced by different terms of the interaction-induced polarization was observed. While some of them are conspicuous via fast intracell motion of a light guest (H(2)), others--induced by the electrostatic field of the guest--give rise to lines whose shapes are imprinted by fluctuations of the nearest surrounding. Strong motional narrowing observed on the guest-guest induced lines shows up in their Lorentzian shapes which are signatures of microscopic-scale diffusion. Near-Lorentzian peaks were also detected at the tops of the diffuse lines induced by isolated guests. Their formation may be associated with a long-living defect (vacancy) emerging in the vicinity of the polarization inductor. Altogether, our results give the first unambiguous spectroscopic evidence on the diffusional evolution of isolated binary interactions that emerge in dense chaotic media.

Exact Low‐Order Classical Moments in Collision‐Induced Bands by Linear Rotors: CO2‐CO2

Exact and general analytic expressions are reported for the integrated intensity and the width of collision-induced absorption (CIA) and collision-induced scattering (CIS) bands by gases of centrosymmetric linear molecules. These expressions provide significant insight and allow assignment of partial second moments to the degrees of freedom of the colliding molecules. The expressions are applied to ambient CO(2), whose collisional spectra are reputed to be useful probes for terrestrial and planetary atmospheres. Compelling evidence of the substantial role of hitherto missing polarization and polarizability mechanisms is provided and is in remarkable agreement with experimental observation. Our findings allow the long-overdue simple interpretation of CIA and CIS by CO(2)-CO(2) without the need to resort to short-range interactions to offset the discrepancies between theory and experiment.

Origin of abnormally sharp features in collision-induced spectra of cryosolutions

A weak, paradoxically narrow resonance feature (shortly, the r-line) near the O2 fundamental frequency in the collision-induced absorption spectrum of oxygen dissolved in liquid argon and liquid nitrogen (T = 89 K) is resolved for the first time. An accurate band shape fitting routine to separate the r-line from the by-far more intense diffuse background and to study its behavior versus the oxygen mole fraction x which ranged from 0.03 up to 0.23 has been elaborated. At small x (≲0.07), the r-line intensity was found to scale as x(2) leaving no doubt that it is due to the solute-solute (O2-O2) interactions. In line with our results on the pH2-LNe cryosystem [Herrebout, Phys. Rev. Lett. 101, 093001 (2008)], the Lorentzian r-line shape and its extraordinary sharpness (half width at half height ≈ 1 cm(-1)) are indicative of the motional narrowing of the relative solute-solute translational spectrum. As x is further raised, ternary solute-solute interactions impede the r-line growth in the O2-LAr spectrum because of the cancellation effect [J. Van Kranendonk, Physica 23, 825 (1957)]. Theoretical arguments are given that multiple interactions between the solutes should finally destroy the solute-solute induced r-line when the mixed solution approaches the limit of the pure liquid (x = 1). Interestingly, the nonbinary effects are too weak to appreciably affect the quadratic r-line scaling in the O2-LN2 cryosystem which persists up to x = 0.23. It is emphasized that studies of the resonant features in the collision-induced spectra of binary cryosolutions open up unique opportunities to spectroscopically trace the microscopic-scale diffusion.

Non-Markovian rotational relaxation matrix for fast collisions between two linear molecules in high-pressure gaseous media. I. General formalism and preliminary testing

Collisional mixing of (vib)rotational lines appearing in spectroscopic signatures of dense planetary atmospheres and combustion environments is rigorously handled for the case of two linear colliders in terms of incomplete (non-Markovian) collisions related to off-energy-shell scattering amplitudes. Contrary to the standard impact-approximation approaches valid solely in band-centre regions, a new uniform broadband spectrum description is developed on the basis of a frequency-dependent rotational relaxation matrix which accurately accounts for the influence of the extra photon energy with respect to the molecular transitions. This matrix is built using a symmetric Liouville-space metric and obeys all known fundamental rules. Its direct calculation from refined potential-energy surfaces and promising modeling methods for forthcoming practical computations are outlined. A simple preliminary test for N2-N2 isotropic Raman line widths argues in favor of considerable effects of the internal perturbers structure on modeled spectral characteristics.

Raman line shape studies of hydrogen cryosolutions

The pure rotational and rotation-vibration Raman transitions in H2 dissolved in liquid argon (LAr, T=87 K) and nitrogen (LN2, T=80 K) are studied at different hydrogen concentrations. Whereas the rotational S0(J) (J=0, 1) shapes of H2-LAr can be accurately fitted by Lorentzian profiles, the wings of the same H2-LN2 lines demonstrate a distinct sub-Lorentzian behaviour. The difference between these patterns is attributed to the change of the time scale of anisotropic interactions which perturb the rotational motion of H2. They should be slower for the H2-LN2 system in which the long-range quadrupole-quadrupole interactions develop. The latter dominate in the formation of the symmetric H2-LN2 S1(J) line shapes. By contrast, the same H2-LAr lines are mainly monitored by the vibrational broadening and their shapes are distinctly asymmetric.

Double Raman Scattering In Gas Mixtures

We report results from two joint experimental and theoretical works that we carried out on double Raman scattering by binary mixtures. The anisotropic spectrum of a band generated by simultaneous vibrational transition in room‐temperature SF6−N2 was recorded along with its extremely weak isotropic counterpart. Successful interpretation of the two spectra was made by means of exhaustive calculations of electrooptical properties allowing one to evaluate the relative contribution of collision‐induced polarization mechanisms.

Collision-induced spectra of hydrogen deuteride solution in liquid neon and their interpretation

First observation of the collision-induced IR-fundamental band of hydrogen deuteride (HD) in liquid neon is described. A developed intracell rattling model yields highly accurate fits to the measured diffuse Q1-branch profiles enabling a detailed shape analysis of the adjacent spectral lines. Strong intracollisional anticorrelation found at the location of the permanent dipole-allowed R1(0) transition is interpreted in the frame of the Fano-Mori theory. A new striking narrowing effect (by 30%) is observed on the sharp Q(q)(1)(0) vibrational line with increasing HD concentration in solutions. The T1(0), U1(0) and a few pair transitions are identified in the spectrum.