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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]

We report an experimental determination of the k00→02 rate coefficient for inelastic H2:H2 collisions in the temperature range from 2 to 110 K based on Raman spectroscopy data in supersonic expansions of para-H2. For this purpose a more accurate method for inverting the master equation of rotational populations is presented. The procedure permits us to reduce the measured k00→02 rate coefficient to the corresponding σ00→02 cross section in the range of precollisional energy from 360 to 600 cm-1. Numerical calculations of σ00→02 carried out in the frame of the coupled channel method are also reported for different intermolecular potentials of H2. A good agreement is found between the experimental cross section and the numerical one derived from Diep and Johnson’s potential [J. Chem. Phys. 112, 4465 (2000)]. [ABSTRACT FROM AUTHOR]

Using a high resolution Raman spectrometer, we have measured Ar-broadening coefficients in the ν2 Q branch of C2H2 for 22 lines at 295 K, 20 lines at 174 K, and 16 lines at 134 K. These lines with J values ranging from 1 to 23 are located in the spectral range 1970.9–1974.3 cm−1. The collisional widths are obtained by fitting each spectral line with a Rautian profile. The resulting broadening coefficients are compared with theoretical values arising from close coupling and coupled states calculations. A satisfactory agreement is obtained at room as well as at low temperatures, especially for odd J lines. By comparing broadening coefficients at 295, 174, and 134 K from a simple power law, the temperature dependence of these broadenings has been determined both experimentally, and theoretically. [Copyright &y& Elsevier]