The recently proven multichannel spectral theorem for time delay is explored with the goal of providing a useful expression for ’’lifetimes’’ of bimolecular collisions. The theorem is reduced to an approximate relationship between the classical time delay and the classical excess density of states. Numerical studies on collinear atom–diatom systems confirm the accuracy of the classical relationship and a simple geometric interpretation of the excess density of states for these systems is provided. Results on the sensitivity of collisional time delays to variations in potential surface well depths and system masses are presented. the time delays being simply obtained from the excess density of states.

A semiclassical theory of the width and shift of molecular spectral lines is developed for gases. Overlapping and nonoverlapping lines are considered, within the framework of the impact approximation. Use is made of ``exact'' semiclassical theory of molecular collisions, recently developed by Miller and by Marcus, and of developments in the quantum mechanical theory of spectral line shapes, by introducing the former into the latter. Comparison is made with a classical-like approach.

The coupled states approximation is extended to treat combined fine structure and rotational state transitions in 2P atom–1Sg+ diatom scattering. CS calculations of opacities and degeneracy averaged integral cross sections for F(2P)+H2(1Sg+) nonreactive collisions agree well with the CC results of Lester and Rebentrost. The present, nonunitarized form of the CS approximation leads to a selection rule which forbids j′1 j′2 j12m12→j1 j2  j12-m12 transitions when j12 = half-odd integer values. This selection rule is not present in an exact treatment.

Trajectory studies on a variety of collinear collisions systems using a potential surface with a deep well are described. Two distinctly different modes of complex formation, dependent on system masses, are observed. One mode is limited to low translational energies whereas the other persists to high energies. Trajectory plots and simple geometric models are combined to provide a consistent interpretation of these mechanisms and of the observed oscillatory dependence, on system masses, of the reaction and complex formation probabilities.

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