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New line shape calculations for CO buffered by Ar are compared to high-resolution measurements from a difference-frequency laser spectrometer, over a range of thermodynamic conditions relevant to the atmosphere. The calculations are based on solving the quantum kinetic (i.e. transport/relaxation) equation for the molecules within the impact approximation, and rely on the commonly used MOLSCAT and MOLCOL codes to determine the speed-dependent collisional relaxation rate. Velocity-changing effects are treated classically using a rigid sphere potential. The comparison initially reveals that the experimental profiles exhibit only 10% to 30% of the expected Dicke narrowing, which leads us to reevaluate our understanding of the narrowing process. A more subtle aspect of the disagreement between theory and experiment draws our attention to an assumption implicit in the calculation of the collisional relaxation rate: the assumption of a Maxwellian form for the velocity dependence of the off-diagonal elements of the density matrix (i.e. the optical coherences). This assumption allows for an analytical simplification of the problem, but eliminates velocity-changing effects (so that they must be added back in using a supplementary classical calculation, which is based here on a rigid sphere interaction). We find that the removal of the above-mentioned assumption should allow for accurate and fully quantum mechanical (but numerical) line shape calculations for systems like CO-Ar on existing computers. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

High-resolution measurements of the π ← Π Q-branch of pure N[sub 2]O near 1160 cm[sup -1] were made using a difference-frequency spectrometer with resolution of 5 × 10[sup -5] cm[sup -1] and a signal-to-noise ratio of 2000:1. Lines Q18F through Q12E have been recorded in a single scan, at room temperature and at pressures ranging from 1 to 130 torr. The spectra are analyzed up to 23 torr on a line-by-line basis using a hard collision profile including Dicke narrowing and line mixing. Since the separation of the central lines of this double-sided Qbranch is of the same order of magnitude with the collisional broadening, line mixing is considered in the analysis even at 1 tort and its dependence with pressure is studied. [ABSTRACT FROM AUTHOR]

A practical matrix-based formalism for solving the master equation for a spectral line is applied to the P(2) transition in the fundamental band of carbon monoxide perturbed by argon. The method assumes that the effect of intermolecular collisions on the internal relaxation of the molecules is uncorrelated with the effect of those collisions on the translational motion of the molecules. Comparison with high-resolution line shape measurements reveals that at low pressures, the omission of statistical correlation leads to a miscalculation of the shape of the line. [ABSTRACT FROM AUTHOR]