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The CO2 molecule is of great interest for astrophysical studies since it can be found in a large variety of astrophysical media where it interacts with the dominant neutral species, such as He, H2, or H2O. The CO2–He collisional system was intensively studied over the last two decades. However, collisional data appear to be very sensitive to the potential energy surface (PES) quality. Thus, we provide, in this study, a new PES of the CO2–He van der Waals complex calculated with the coupled-cluster method and a complete basis set extrapolation in order to provide rotational rate coefficients that are as accurate as possible. The PES accuracy was tested through the calculations of bound state transition frequencies and pressure broadening coefficients that were compared to experimental data. An excellent agreement was globally found. Then, revised collisional data were provided for the 10–300 K temperature range. Rate coefficients were compared to previously computed ones and are found to be up to 50% greater than previously provided ones. These differences can induce non-negligible consequences for the modeling of CO2 abundance in astrophysical media. [ABSTRACT FROM AUTHOR]

Information about molecular collisions is encoded in the shapes of collision-perturbed molecular resonances. This connection between molecular interactions and line shapes is most clearly seen in simple systems, such as the molecular hydrogen perturbed by a noble gas atom. We study the H2–Ar system by means of highly accurate absorption spectroscopy and ab initio calculations. On the one hand, we use the cavity-ring-down-spectroscopy technique to record the shapes of the S(1) 3-0 line of molecular hydrogen perturbed by argon. On the other hand, we simulate the shapes of this line using ab initio quantum-scattering calculations performed on our accurate H2–Ar potential energy surface (PES). In order to validate the PES and the methodology of quantum-scattering calculations separately from the model of velocity-changing collisions, we measured the spectra in experimental conditions in which the influence of the latter is relatively minor. In these conditions, our theoretical collision-perturbed line shapes reproduce the raw experimental spectra at the percent level. However, the collisional shift, δ0, differs from the experimental value by 20%. Compared to other line-shape parameters, collisional shift displays much higher sensitivity to various technical aspects of the computational methodology. We identify the contributors to this large error and find the inaccuracies of the PES to be the dominant factor. With regard to the quantum scattering methodology, we demonstrate that treating the centrifugal distortion in a simple, approximate manner is sufficient to obtain the percent-level accuracy of collisional spectra. [ABSTRACT FROM AUTHOR]

Inelastic H2:He collisions are studied from the experimental and theoretical points of view between 22 and 180 K. State-to-state cross sections and rates are calculated at the converged close-coupling level employing recent potential energy surfaces (PES): The MR-PES [J. Chem. Phys. 100, 4336 (1994)], and the MMR-PES and BMP-PESs [J. Chem. Phys. 119, 3187 (2003)]. The fundamental rates k2→0 and k3→1 for H2:He collisions are assessed experimentally on the basis of a master equation describing the time evolution of rotational populations of H2 in the vibrational ground state. These populations are measured in the paraxial region of supersonic jets of H2+He mixtures by means of high-sensitivity and high spatial resolution Raman spectroscopy. Good agreement between theory and experiment is found for the k2→0 rate derived from the MR-PES, but not for the BMP-PES. For the k3→1 rate, which is about one-third to one-half of k2→0, the result is less conclusive. The experimental k3→1 rate is compatible within experimental error with the values calculated from both PESs. In spite of this uncertainty, the global consistence of experiment and theory in the framework of Boltzmann equation supports the MR-PES and MMR-PESs, and the set of gas-dynamic equations employed to describe the paraxial region of the jet at a molecular level. [ABSTRACT FROM AUTHOR]

Line mixing effects have been studied in the ν[sub 2] and ν[sub 1] parallel bands of NH[sub 3] perturbed by He at room temperature. Experiments have been made with a Fourier transform spectrometer covering a wide range of total pressures up to about 400 atm. Analysis of the spectra demonstrates, for the first time, that the spectral shapes of entire ammonia bands can be significantly influenced by line mixing. A theoretical approach based on the energy corrected sudden approximation (ECS) is used to predict and analyze these effects. The model parameters include dynamical factors directly computed from an NH[sub 3]–He potential energy surface and a scaling length which has been determined from a fit of line-broadening data. Comparisons with measurements show that the ECS model leads to surprisingly satisfactory predictions when considering the large spacing between rotational levels. The large effects of line mixing within the Q branches and in the far wing of the absorption bands are analyzed. It is shown that purely Lorentzian calculations can lead to underestimation of the Q branch peak by a factor of up to 4, whereas the overestimation of absorption in the band wing reaches one order of magnitude. On the contrary, the proposed ECS approach leads to much better results and accounts for most of the transfers of intensity among the various spectral components. It is used for the analysis of the shape of Q branches, P and R manifolds and line wings. Remaining discrepancies are discussed in terms of the model approximations and the improper dependence of the basis dynamical factors on quantum numbers induced by uncertainties on the potential energy surface used. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Classical Molecular Dynamics Simulations (MDS) are carried for the calculation of the collision kernels describing the velocity changes of H2 molecules induced by interactions with Ar atoms. These MDS results are first used for the test of a previously proposed three dimensional bi-parametric extension (KS-3D) of Keilson and Storer velocity-changing model. Semi-classical and close-coupling calculations of the speed-dependent collisional-broadening γ coll(v) and -shifting δ coll(v) coefficients are also made for the Q(1) line, using the best available H2–Ar potential energy surface. These data are compared with the corresponding values previously extracted from measured Raman spectra. Furthermore, the calculated values of , γ coll(v) and δ coll(v) are used to calculate the density dependence of the Q(1) line-width from the Doppler to the collisional regime and compare the results with experiments. The conclusions of this exercise are three fold. The first is that the KS-3D approach gives a satisfactory description of the velocity changes. The second is that the procedure used in a previous study in order to extract γ coll(v) and δ coll(v) from measured spectra was correct. The last, consistent with the findings of other independent studies, is that the H2–Ar interaction potential used needs refinements at short distances. [Copyright &y& Elsevier]

A new semiempirical potential energy surface (PES) for the acetylene−hydrogen system has been derived by using the recently introduced bond−bond methodology. The proposed PES, expressed in an analytic form suitable for molecular dynamics simulations, involves a limited number of parameters, each one having a physical meaning and allowing the accurate description of the system also in the less stable configurations. The analysis of novel integral cross sections data, measured with nearly effusive molecular beams, combined with that of available pressure broadening coefficients of isotropic Raman lines at 143 K and IR lines at 173 and 295 K of C2H2in H2, provides a test of the reliability of the proposed PES and suggests also some refinements. An extensive comparison with a recent ab initio potential is also exploited. [ABSTRACT FROM AUTHOR]

Abstract: Theoretical and experimental values have been determined for the pressure broadening of the ν1 +ν3 band of acetylene by hydrogen and deuterium at 195K, and experimental values of the pressure shifts have been determined. Theoretical values have been calculated on the basis of a recent potential energy surface using the close coupling scheme. We discuss the detailed contribution of the various rotational angular momenta of the perturbing gas and the ortho and para contribution to the total pressure broadening cross-sections. We give routes to circumvent the computational cost of such calculations. Experimental values have been measured using a tunable diode laser spectrometer assuming a Voigt line shape. These pressure broadening parameters are compared with measurements performed recently at room temperature and with present measurements performed at 195K in the ν1 +ν3 band of acetylene. A satisfactory agreement is obtained with the present results and available ones at 295K. [Copyright &y& Elsevier]

Abstract: We present measurements of He-broadening parameters for the R(0) and P(2) lines in the fundamental band of 13CO at different temperatures between 12K and room temperature. The broadening parameters are determined, taking into account confinement narrowing, by simultaneous least-squares fitting of spectra recorded using a frequency stabilized diode laser spectrometer. The pressure broadening cross sections are deduced and compared to close-coupling calculations and earlier results obtained for rotational transitions of 12CO. [Copyright &y& Elsevier]

We present measurements of Ar-broadening parameters for the R(0) and R(7) lines in the fundamental band of 13CO at eight temperatures from 80 to 297 K. The broadening parameters are determined by simultaneous least-squares fitting of spectra recorded using a frequency stabilized diode laser spectrometer. The comparison of the broadening parameter values for R(7) derived at room temperature and different pressures from different line profiles shows that an empirical line profile, which takes into account narrowing effects (Dicke narrowing and absorber speed dependence) but neglects any correlation between collisions, is able to describe the observed lines with constant values of the narrowing and broadening parameters over a 10–500 Torr pressure range. Starting from a recent ab initio potential energy surface, theoretical thermally averaged close coupling values of the Ar broadening parameter are calculated for the same temperatures. The comparison between experimental and calculated values shows an overall agreement of 1.5%. [Copyright &y& Elsevier]

Theory and experiment are combined in a novel approach aimed at establishing a set of two-body state-to-state rates for elementary processes ij→ℓm in low temperature N2:N2 collisions involving the rotational states i,j,ℓ,m. First, a set of 148 collision cross sections is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent potential energy surface for N2–N2. Then, the corresponding rates for the range of 2≤=T≤=50 K are derived from the cross sections. The link between theory and experiment, aimed at assessing the calculated rates, is a master equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (≈2×10-3 mm3) of N2 traveling along the axis of a supersonic jet. The calculated collisional rates are assessed experimentally in the range of 4≤=T≤=35 K by means of the master equation, and then are scaled by averaging over a large set of experimental data. The scaled rates account accurately for the evolution of the rotational populations measured in a wide range of conditions. Accuracy of 10% is estimated for the main scaled rates. [ABSTRACT FROM AUTHOR]

Abstract: The collisional shifts and widths of several P-branch spectral lines in the fundamental band of CO–Ar have been measured at temperatures between 214 and 324K and pressures between 0.025 and 1atm. The widths have been determined using a line shape model based on the solution of the transport/relaxation equation for the appropriate off-diagonal element of the density matrix. The model uses a realistic molecular potential energy surface to calculate the speed dependence of the collisional broadening, and a rigid sphere potential to calculate the translational motion. It is found that both the shifting and broadening coefficients follow a power law dependence on the temperature. Additionally, it is demonstrated that studies have tended to overestimate the accuracy of collisional widths when the line shape model used to obtain the widths involves multiple fitted line shape parameters or fails to fit the measured spectra within the experimental noise. [Copyright &y& Elsevier]

• The R(0) line of CO perturbed by Ar is studied experimentally in a wide pressure range. • Line shape parameters are calculated ab initio using two potential energy surfaces for a wide temperature range. • Theoretical model reproduces experimental spectra at a sub percent level of uncertainty. • Significance of the line-mixing effect for accurate modeling of spectra is demonstrated. We use three independent spectroscopic techniques, operating in the millimeter-wavelength range, to study molecule-atom collisions, and validate our quantum-scattering calculations on two recent potential energy surfaces. We study the first pure rotational transition in a CO molecule perturbed by Ar. This molecular system is a good prototype of atmospherically relevant cases. It is, on the one hand, affordable for calculation of the line shape parameters by modern ab initio methods, and on the other hand, is very convenient for experimental studies because of its regular, well spaced rotational spectrum having a moderate intensity. We show that the simulated collision-perturbed spectra, which are based on our fully ab initio calculations, agree with the experimental line profiles at sub-percent level over a wide range (more than four orders of magnitude) of pressures. We demonstrate that the agreement between theory and experiment can be further improved if the model accounts for the collisional transfer of an optical coherence between different rotational transitions (the line-mixing effect). We show that the two surfaces tested in this work lead to a very similar agreement with the experiment. Capability of calculating line shape parameters in a broad range of temperatures is demonstrated. [ABSTRACT FROM AUTHOR]