N. N. Filippov

Measurements and empirical modeling of pure CO 2 absorption in the 2.3-μm region at room temperature: far wings, allowed and collision-induced bands

Measurements of pure CO(2) absorption in the 2.3-μm region are presented. The 3800-4700-cm(-1) range has been investigated at room temperature for pressures in the 10-50-atm range by using long optical paths. Phenomena that contribute to absorption are listed and analyzed, including the contribution of far line wings as well as those of the central region of both allowed and collision-induced absorption bands. The presence of simultaneous transitions is also discussed. Simple and practical approaches are proposed for the modeling of absorption, which include a line-shape correction factor χ that extends to approximately 600 cm(-1) from line centers.

Estimation of line parameters under line mixing effects: the ν3 band of CH4 in helium

Spectra of CH4-He mixtures were measured in the P and R branch regions of nu3 vibration-rotation band at lower pressures of 0.26-1.0 bar and for the whole band at higher pressures up to 90 bar. The line broadening coefficients were found from lower pressure data for the lines of the P branch. These coefficients were calculated in the framework of the Robert and Bonamy semi-classical approach. In general, they agree with the experimental ones, but a small J-dependent deviation was observed. Line mixing effects were observed at all pressures. At higher pressures they were interpreted in terms of the adjusted branch-coupling model. At lower pressures line mixing effects were found to be especially pronounced in the region of the relatively weak lines forming the clusters from R(16) to R(19).

Line parameters and shapes of high clusters: R branch of the ν3 band of CH4 in He mixtures

The IR absorption spectra of CH4 in pure gas and in mixture with helium were studied in the region of nu3 band at higher J line clusters R(17)-R(22). The frequencies and intensities of rotation-vibration lines were estimated from the experimental spectra at Doppler shape conditions. The line frequencies and intensities were calculated and used for the attribution of overlapped lines in clusters. The calculated line intensities are close to the experimental values. The calculated frequency structure of the higher J manifolds are somewhat wider than the observed one. The shapes of helium-broadened line clusters were compared with those calculated accounting for line mixing. The relaxation matrix W, which is necessary in shape calculations, was constructed using semiclassical collision rate constants. The calculated shapes are in satisfactory accordance with the measured ones.

Line-mixing effects in the ν3 parallel absorption band of CH3F perturbed by rare gases

Abstract We have measured the absorption profiles of CH3F mixed with argon and helium at room temperature in the interval 940–1110 cm−1 of the ν3 band. The spectra measured at pressures of 0.25–3 bar were used for evaluation of the line-broadening parameters. At higher pressures up to 20 bar significant deviations from the Lorentz bandshape were detected. At the maximum of the Q branch the absorption is super-Lorentzian, the differences reaching up to 60%. In the lower frequency wing of the P branch the observed absorption is up to five times lower than the Lorentzian one. The discrepancies observed are sensitive to the perturbing molecule, being more pronounced in the case of helium. They are explained by line-mixing effects and interpreted by means of a simple model with a single adjustable parameter.

LINE SHAPES IN THE ROTATIONAL SPECTRA OF HF IN Ar GAS: NEW EXPERIMENTAL DATA AND CALCULATIONS OF LINE INTERFERENCE

Abstract The absorption spectra of HFAr mixtures have been registered in the spectral range of 30–410 cm −1 . The line asymmetry has been detected. The widths, shifts and asymmetry parameters for lines R(0)–R(9) have been obtained. The data are compared with the recently calculated lineshape parameters by Green and Hutson. The 20% discrepancy of the experimental and calculated width cross-sections is observed. The agreement of the experimental and calculated shift cross-sections is satisfactory for the lines at lower frequency but is rather poor for the lines at higher frequency. The observed line asymmetry is explained by the line mixing effect in the framework of semiclassical approach.

Communication: Evidence of stable van der Waals CO2 clusters relevant to Venus atmosphere conditions

Non-intrusive spectroscopic probing of weakly bound van der Waals complexes forming in gaseous carbon dioxide is generally performed at low pressures, for instance in supersonic jets, where the low temperature favors dimers, or in few-atmosphere samples, where the signature of dimers varying as the squared gas density is entangled with the dominating collision-induced absorption. We report experimental and theoretical results on CO2 dimers at very high pressures approaching the liquid phase. We observe that the shape of the CO2-dimer bands undergoes a distinctive line-mixing transformation, which reveals an unexpected stability of the dimers despite the collisions with the surrounding particles and negates the common belief that CO2 dimers are short-lived complexes. Our results furnish a deeper insight allowing a better modeling of CO2-rich atmospheres and provide also a new spectroscopic tool for studying the robustness of molecular clusters.

Collision-induced double transition effects in the 3ν3 CO2 band wing region

IR absorption beyond the head of the 0003–0000 (3ν3) band of CO2 near 7000 cm−1 has been analyzed. This absorption is found to consist of two comparable intensity contributions, namely, the allowed band wing and a collision-induced absorption (CIA) band. The band wing profile has been described by using a non-Markovian theory and the rotational perturbation densities for CO2–CO2 collisions, which was previously calculated from the intensity distribution in the high-frequency wing of the 0001–0000 CO2 band. The CIA component has a typical shape of CO2 CIA bands with the maximum at the double transition (0001+0002)–(0000+0000) frequency. The integrated binary coefficient of this CIA band was estimated to be B2=(1.0±0.6)×10−5 cm−2 Amagat−2. The CIA spectral moment theory has been used for the intensity calculation, which takes into account for the first time the collision-induced vibrational force field in CO2 pairs. By comparing the calculated and measured intensity for the double transition, the polarizabi...

Line-mixing in absorption bands of linear molecules diluted in high-density rare gases : measurements and modeling for OCS-He

Absorption coefficients in the bending ν2 and stretching 2ν3 bands of OCS perturbed by He at high pressures up to 300 atm have been measured and analyzed in the impact-approximation region by two theoretical models accounting for the line mixing. In the first approach, the frequency-independent relaxation operator is treated semi-classically [N. N. Filippov and M. V. Tonkov, J. Quant. Spectrosc. Radiat. Transf. 50, 111 (1993)] with adiabatic corrections. To characterize the strength of collisions and the line mixing efficiency a single parameter G is used and its value is estimated from the intermolecular interaction potential. The adiabaticity of collisions and the corresponding correction to the collisional cross sections amplitude are taken into account by two other parameters ν(corr) and σ deduced from fitting to experimental line widths. In the second approach, the symmetrized non-Markovian relaxation operator of Energy-Corrected Sudden type developed for rototranslational Raman spectra of linear rotators [J. Buldyreva and L. Bonamy, Phys. Rev. A 60, 370 (1999)] and satisfying all the basic properties (in particular, the detailed balance relation and the double-sided sum rules) is adapted to absorption spectra for the first time. Its off-diagonal elements are determined via the common adiabaticity factor and the basic transition rates whose parameters are adjusted on experimental values of isolated line widths. Both models provide a very consistent description of OCS-He 2ν3 and ν2 band intensities up to the maximal experimentally studied pressure, with a slightly more realistic picturing of the inter-branch exchange by the second approach.

Line mixing effect on the pure CO2 absorption in the 15 μm region

Abstract The IR absorption of the pure CO2 gas in the region of 15 μm was examined. A special attention was given to the line mixing effect that influences the spectral shape of the vibration–rotation absorption bands in the Q-branch regions. Two methods in shape description were analyzed. The first method uses the Rosenkranz line shapes with the line mixing parameters, which are found from an empirical rotational relaxation matrix. The second method is based on the strong collision model with adjusted branch coupling (ABC-model). The merits and the demerits of these two methods are discussed, and the results of the corresponding calculations are compared to the measured shapes. It is inferred that the ABC-model for the absorption coefficient calculations can be successfully applied for solving the non-LTE radiative transfer problem in CO2 bands in the atmospheres of Earth-like planets.

Asymptotic behavior of collision-induced line shifts in HF rotational band

Comparison of the experimental1 and theoretical results2 for the collision induced lineshift values in rotational spectra of HF in Ar gas has shown that there is a great systematic difference between these data at higher J values. The theoretical shifts are negative and tend to zero, whereas the experimental ones become positive and increase with line number. Since the lineshift data ofRef. 1 were obtained at rather high density ofAr, we have studied the HF-Ar spectra at lower pressures from 1 to 30 bars to exclude the possible non-linear dependence on gas density.